STILLWALKERpd » Gyraf

1176LN

admin — Mon, 23 Nov 2009 12:13:55 +0000

Gyraf Do-It-Yourself 1176LN rev#F

This pages contain information about building your own version of the classic 1176 compressor.

I will try to update this page reguarly if anyone shows interest in its topics. Comments and corrections are welcome, but I can’t promise to reply to all mail. If you do a project and describe it on your webpage, I’ll be more than happy to add a link here, so others can benefit from your experiences.

Disclaimer: Notice that all information, schematics, layouts etc. are supplied “as is”, and that we can in no way be held responsible for its acurateness, functionality or even safety.

Gyraf Audio shall not be responsible and disclaims all liability for any loss, liability, damage (whether direct or consequential) or expense of any nature whatsoever which may be suffered as a result of or which may be attributable, directly or indirectly, to the use of or reliance upon any information, links or service provided through this website.

The G1176:

A classic FET compressor. Not tubes, but a nice do-it-yourself project. Click the picture to see a series of pics of the first one being made.

This is an adaption of the original UREI 1176 LN rev#F to components and parts that is available (hopefully) everywhere. Adaption includes changing to standard european transistors, component values, Lundall output transformer and rotary switches for easier enclosure layout.

Click picture to watch the first unit being made…

The Schematic:

The Schematic for the 1176 compressor

We have updated the schematic to show approximate expected voltages around the transistors for easier error-tracing.

The PCB Layout:

The PCB Layout of the 10×16cm pcb board for the 1176 (300KB PDF-file). Rightclick the link and select “save as..”
NOTE: The PCB drawing is MIRROR’ED for easier printing, see the Pultec-DIY-page for details.

Gerber-format files for the PCB.
Thanks to Mikkel C. Simonsen for the conversion!

The parts list for the 1176

Here you can find a PDF- version of the complete 1176 manual, including adjustment procedure and all. Observe, that this is a VERY large file - 7.67MB. Rightclick the link and select “save as..” to save this on your own machine. Better get this while it’s still there…

http://classes.berklee.edu/mpe/pdf_files/manuals_pdf/urei_1176ln.pdf

Front panel layout example…

On building the unit…

Building should be straight forward. Quite a lot of people has succeeded till now. But there are some common errors that you should avoid:

- Check component values before soldering in. Check specially resistors with an Ohm-meter.

- If using NTE substitutes for semiconductors, make absolutely sure that they are actually also pin-compatible. Some DIY’ers has had problems with this.

- Use a heat sink for the 7824 voltage regulator. A small one will often do.

- Check and double check the PCB for shorts and solder blobs before powering up.

- Remember that transistors aren’t very forgiving. If they have been mounted wrong and powere’d up once, there’s a good chance they are destroyed. Replace on suspicion.

- Make sure that the centre tap of the 2×24V at the power transformer is connected to ground

- Connect 0V/Ground to chassis at - and only at - the input XLR(F) connector. Here you can also connect the mains power ground from the IEC power connector if you need that.

- Note that the LL1540 input transformer is OPTIONAL. The “real” 1176 LN rev#F uses the electronically balanced input. The input transformer option is there simply because I like what it does for the sound. But it’s not used in the original.

On adjusting the unit…

- Once the unit is built and checked, proceed with the adjustment procedure. You could use the procedure from the original .pdf manual, but that could be a bit tricky because we changed the bypass to be on the meter switch in stead of on the attack switch. So when they say “turn attack fully counterclockwise to bypass”, it could translate into something like “bypass the unit with the meter switch”. But that is not easy to follow, so here’s a simplified adjustment procedure:

There is four adjustments.

1: “Q Bias”. This is set by bypassing the unit, putting through a signal, adjusting in- and outlevel so output is around +1dB, and adjusting the trimmer untill a level drop of 1dB is acheived. This sets the FET start point in the beginning of it’s conducting range.

2: “Dist. trim”. Bypass the unit, input a ~1kHz, ~0dB signal, set unit gain to unity (by input- and output controls), and set trimmer for minimum distortion. If you have no dist.meter, simply set at middle position. Somehow this is not an important adjustment soundwise.

3: “GR meter Zero”. No input signal. Meter switch in GR mode. Adjust untill meter reads 0dBVU.

4: “GR meter Trck” (tracking). Set ratio to 1:20, set Meter to bypass. Input a 1kHz, 50mV signal. Set input level to ~12′oclock. Set output level so you get 50mV at the output also (now you have unity gain). Now set the meter switch to “GR”. Slowly turn up the level of your input signal (at the signal source, not with the input level control) while you monitor both input- and output levels with a voltmeter. When you reach a point where output level is half the input level, you have 6dB of gain reduction in the unit. Now set the “GR meter Trck” trimmer so the VU-meter reads -6dBVU. Remove input signal and (re)set “GR meter Zero” for a reading of 0dBVU. This calibrates the gr indication.

TroubleShooting:

- If you have problems, Please read the (rather long) Recording.org 1176 post before asking for help (also in that thread preferably, please):

Forum title: FEEDBACK FOR GYRAF 1176LN (repost) - LINK BROKEN

..no, sorry. That collection of information was shut down. Now we’re reconstructing as much as this as we can on our new hangout:

http://www.groupdiy.com/

Orson Whitfield from GroupDIY has written a very good, compressed version of the thread about getting the G1176 running and fixing possible flaws. This is a great resource if you have problems during the process. Actually also worth a read before you are getting started:

Orson’s G1176 DIY audio page for beginners

—————-

The SLAM mode…

By popular demand, here’s information about how to include the hyped “Slam” or “Nuke” mode in this version of the 1176:Slam/Nuke Mode Switch

If problems arises, errata and changes will be posted on this page.

ERRATA:..

28-04-2002: Peter Cornell mailed me with an error he’d found in the 1176 layout: The BF245 FET for metering gain reduction is connected mistakenly. This is corrected on PCB REV#6, 04-2002.

01-07-2002: Steffen Mueller mailed me, reminding me to mention that the VCA FETs - the BF 245’s - must be the “A” type, the cutoff voltage being around -2V.

And the BC107C’s does’nt really have to be “C” types - a lot of people have had problems getting hold on these. Go with the “B” type, it has plenty of Hfe for our purposes.

18-07-2002: Frank Rollen points out, that the BC107’s on the main PCB, rev#6 are wierdly mounted, making assembleying hard.Actually these TO18-housings are by error wieved from the pcb-side. No harm should be done if you use the rev#6 board, and just bend the middle transistor leg (the base) across to the hole. Remember to have the small metal edge on the transistor facing the “E”. Also updated the orientation of Q8, BD517, shown the wrong way around.

The “EBC” markings on the PCB were all correct though.

New version is PCB rev#7

21-09-2002: I’ve recompiled the PCB artwork PDF-files because of some printing problems on 600DPI printers.

23-10-2002: Updated schematic to show static DC voltages. This can be very usefull when tracking down errors..

06-11-2002: UPDATED THE COMPONENT LIST. NOW BETTER SIMILARITY WITH THE PCB. 21-01-2004: Gerber files added, thanks to Mikkel C. Simonsen!

If you let me know how you’re managing this project - and if you have information that could interest other diy-people - I could link to it, or put it on this page.

In the meantime, check out the GroupDIY/Kev DIY-Page on Recording.org, with a lot of good links for this type of projects:

KEV’S DIY GROUP PAGE ON RECORDING.ORG

MNATS’s 1176 PAGE - one of the best webpages covering this project - there even is an alternative PCB board layout:

MNATS’s 1176 PAGE

And NRG-Recording, Germany, also takes a shot at this:

http://www.nrgrecording.de/diy.html

On the “LAB” forum at www.groupdiy.org there is a thread about correcting possible errors you may encount in the G1176:

http://www.groupdiy.com/

…there’s a good chance you will find me hanging around there…

Also check out Juergen Haible’s webpage about making your own 1176:

www.oldcrows.net/~jhaible/compressor

Gyraf Audio’s Obscure Schematics

admin — Wed, 19 Aug 2009 11:58:28 +0000

Here’s some schematics and information that can be hard to find elsewhere. If you have some schematics that you think belongs among these, please do not hesitate to contact me.. I hope to be able to extend this little collection with time, so check back for updates.

NOTE: Some of the larger .gif-files dosen’t show correctly in some browsers. If you have that problem, instead “right-click” on the link and select “save target as..” to save to your harddisk - and view it from there.

Mic Preamps:

Neumann_PV76 Mic Preamp

Neumann V476 Mic Preamp

Disa 91b21 Microphone preamp (!!)

Tube-Tec MP1A Preamp

NTP 179-270 Mic pre and Compressor

Amek M-2500 Mic pre stage

Calrec UA8000 mic/line input stage

Calrec 1061 input channel w/ EQ.

Equalizers:

Neumann PEV Equalizer

Ear 825 “Passive” tube eq

Sontec 430C EQ

Sontec MEP250a

Pultec PE1A Equalizer

Pultec HLF3 Passive High/Low-cut Filter

Pultec MEQ5 EQ Section

Amek M-2500 EQ-stage

Langevin EQ251A Passive - Thanks to mr. Rick Chinn

Klein & Hummel UE-400 EQ

Klein & Hummel UE-100 Tube EQ:

KH_UE100_equalizer.pdf - Thanks to Zebra50!

Klein & Hummel UE-1000 Discrete EQ:

KH_UE-1000.pdf

Compressors:

Siemens U273 Diode-based Limiter

CBS_Audimax_440.gif

DISA 91N02 Limiter - inspiration source for Manley’s Vari-Mu

CCA LA-1D limiter/AGC w/ 6386 tube

Drawmer 1960 (original version) (Removed by request from the manufacturer)

NTP_179-140_Manual.pdf (1.8Mb)

NTP_179-160_Compressor.PDF (1MB)

NTP_179-300_Limiter_card.pdf (700Kb)

Drawmer DS-201 Gate (temporarily removed)

Drawmer DS-202 Gate (temporarily removed)

Microphones:

AKG SolidTube schematic (Electrect!)

AKG C414 (The old version)

AKG C535EB (Eltrect!)

Neumann Handheld Microphone

Neumann KM74i

Neumann KM73 - KM74 - KM75

Radiometer Microphone (Danish)

Schoeps N-221-B w/AC701

Senheisser MD421 mechanical drawing

Sony_C37P.pdf

Sony_C38A_C37FET.pdf

Sony_C38B.pdf

Sony_DC-DC_Converter.pdf

A Japanese article about the Sony C800 Microphone:

Sony C800G Schematic - with component values..

Mixers:

SSL 4000E Mixer:

SSL 4000E Input amplifiers - early version, 1979

SSL 4000E Input amplifiers - recent version

SSL 4000E Quad VCA, inserts, Compressor time constants

SSL 4000E Bus compressor time constants, quad fader, autofade

SSL 4000E talkback compressor

Neotek Elite Mixer:

Input channel “A” board

Input channel “B” board

Mixbus amplifiers

Aux mixbusses

Controlroom, Studio and Solo

Other devices of interest:

Neumann PV15 Isolation Amplifier

Neumann PV46 Line Amplifier

Neumann W444 active fader module

Neumann 472 Isolation amplifier

Neumann 482 Distribution amplifier

Neumann V475 Mixbus amplifiers

Neumann WE66 RIAA playback equalizer

Bridged attenuators layout

NTP paper describing NTP 179-120, 179-160, 179-230, 179-400, 182-100, 182-200, and old Pricelist

VCA’s and discrete opamps:

Aphex_1537A_VCA.pdf

Aphex_VCA_505_card.pdf

VCAs_Ben_Duncan.pdf (an article)

DBX 2150 Application notes

Dbx 202 VCA - schematic and data

NTP M-100 Discrete opamp, 1978

More versions of NTP M100 Schematics.pdf

NTP M-100 datasheet page 1

NTP M-100 datasheet page 2

RadioDesigners Handbook - RDH:

RadioDesigners Handbook - Ch.5 - Transformers

RadioDesigners Handbook - Ch.1 and 2 - Introduction to the radio valve

Other stuff:

Voltage multipliers with CMOS gates.pdf

Aphex_Exiter_II.PDF

RTW1227E PPM schematics

Behringer ADA-8000 8-ch AD/DA converter analouge sections

Westlake BBSM-10

Revox G36 tubed tape recorder Schematic

Beyer Dynamics catalouge page - audio transformers, 1969

Calrec PQ1549 Equalizer

admin — Wed, 19 Aug 2009 11:57:31 +0000

Welcome to the Gyraf Audio Calrec PQ1549-page…..

This page - like the rest of this site - is in a continuous process of ever lasting construction.

12. March. 2003

Gyraf Do-It-Yourself Calrec PQ1549 EQ

This pages contain information about building your own version of the Calrec PQ1549 Equalizer, the one found in the Calrec “Polar” desk. This EQ is one of my absolute favorites when it comes to standard IC-based types, so I made this project to make it possible for more people to experience these qualities.

I will try to update this page reguarly if anyone shows interest in its topics. Comments and corrections are welcome, but I can’t promise to reply to all mail. If you do this project and describe the process on your webpage, I’ll be more than happy to add a link here, so others can benefit from your experiences.

Disclaimer:

Notice that all information, schematics, layouts etc. are supplied “as is”, and that we can in no way be held responsible for its acurateness, functionality or even safety.

Also you should take extreme caution when working with mains voltages. These voltages are lethal, and the smallest error will be chatastrophic. And we like you to stay alive and well, so you can help other people sharing our bizarre interest for building retro-pro-audio-equipment.

Basic safety rules:

- NEVER work with live voltage switched on. Switch off, discharge, work, connect measuring equipment and power up.

- Always keep your mains connector in plain sight when working, so you can assure yourself that it really is disconnected.

- Always tidy up your working area before connecting your project to the mains. This gives you some time for second thoughts about what you are doing.

The Circuit:

The Schematic for the original PQ1549 EQ section

The Redrawn Schematic for our version of the PQ1549

The Calrec PQ1549:

The Calrec PQ1549 is a classic no-nonsense parametric equalizer from the famous (and huge!) Calrec “Polar” desk, that were only manufactured in very small numbers. I believe that there’s less than 15 desks like this left worldwide.

This is an adaption of the original Calrec PQ1549 module to components and parts that are available (hopefully) everywhere. Adaption includes removal of the mic preamp, filters and switching, changing to newer and even better opamps, adding electronically balanced input and output and a hardwired bypass switch. No doubt that some of you would have liked the Micamp and all, but these imply quite a few not-so-easy-to-get components. So I’m going for the raw EQ section.

Looking on the redrawn schematic, we first have a standard debalancing input buffer, converting our balanced input signal to unbalanced, and feeding a series of four amplifiers connected one to each’ own filter section, and finally balanced again for the output.

The four inverting add/subtract amplifiers - working in series - has the filters’ inputs connected to either the stage input (at max. boost) or to the inverted stage output (at max cut). Keeping in mind, that the output from each stage is a inverted replica of the input, it’s easy to see that when the dB+/- pots are in their centre positions, the normal and inverted signals cancels out, and no signal is applied to the filter section input. The output from the filter sections are added in with the inverting inputs of the amplifier stages, effectively boosting or cutting any frequency that the filter sectionlets through.

The four filter stages are - more or less - standard state variable filters. Their frequency is controlled by a 2×100K negative logaritmic potentiometer, which is the only “hard to get” component in this design. The “Q” factor is switchable high or low, the switching also controlling filter gain to acheive a constant gain-versus-Q. The “Shelving” switches on the low and high end also bypasses the “Q” switch.

The Artwork:

The PCB set for the PQ1549 consists of two 100×160mm “eurocard” pcb’s, that are interconnected. The PCB’s are single-sided, and carry all switches and pots, limiting wireing to input, output and PSU.

Note, that a +/-18V, ~100mA power supply is needed, and not included here. If it seems that there’s a need for it, I will do a pcb for that also..

The PCB Layout for the two 10×16cm pcb boards needed (604KB PDF-file).
Rightclick the link and select “save as..”

Small PSU for this EQSmall psu for +/-15V or +/-18V. (300KB PDF-file).

Use a 2×15V transformer and 7815/7915 for +/-15V or a 2×18V transformer and 7818/7918 for +/-18V. This should be able to drive about four EQ’s with a 30 VA transformer.

Gerber files of the PCB’s:
The Gerber files for the EQ - thanks to MCS..(224kb .zip file)

The Racking:

I do not have pictures of the finished unit yet, but Magnus Johansson is working on a detailed construction description, that will also include stuff like what rack to mount the EQ in. An initial idea would be to have two EQ’s in one 19″2u box

The Parts List:

This is a parts list compiled by Magnus Johansson:Parts list
Thanks Magnus..

The Problematic Parts:

Well, every project seems to have it’s own problematic part. In this case, it’s the 2×100kC (dual, negative logaritmic) potentiometers for the frequency adjustment. The negative logaritmic scale is needed to get an even distribution of the frequencies when turning the knobs. A dual linear COULD work, but will give unevenly distributed frequencies, making it harder to set the EQ correctly on the lower end of the scale.

The 2×100kc pots are widely used for frequency selection in parametric equalizers, so the easiest way to get hold on them would be to buy them as spare parts for some existing device. E.G. TC electronic uses them in their 1140 parametrics, and they are good at supplying spare parts worldwide.

If you have access to “Monacor”, they have a 2×100kc pot as a sparepart for their PMX600 desk (pos. “VR106″), at a very good price.

Also http://www.electrovalue.co.uk seems to carry the Neg. Log. Dual Pot. (Thanks Nat!)

And, for US; http://www.smallbearelec.com (Thanks Ed Anderson)

If you’re out of luck regarding sourcing the neg. log. pots, they can be made this way:

You take a standard 2×100K log pot, of the type where the two tracks are facing each other - that is, the wiper of the pots are in the center, between the two carbon tracks. It has to be this kind of pots, and it’s naturally also imperative that it is a type that can be taken apart and assembled again..

This kind of pots consist of a log section and a neg.log. section..

When you take this apart and change the position of the two tracks, so the back one goes to the front and vice versa, they will now act like a neg.log. potentiometer. We did this in the good old days, before we could afford having pots built to specs..

NEW!! Magnus Johansson has done a description of how to make a Calrec eq:

Magnus’ Beginner’s guide to the Calrec Equalizer..

..Including an explanation of how to rebuild stereo log. pots to stereo neg.log..!

Tim Campbell’s EQ:

PCB “A”

PCB “B”

Front

Front all

Open

From back

The Original:

Feedback Recording’s Calrec UA-8000 “Polar” board

If problems arises, errata and changes will be posted on this page.

ERRATA:..

31-10-2002: Original drawings

If you let me know how you’re managing this project - and if you have information that could interest other diy-people - I could link to it, or put it on this page.

In the meantime, check out Kev Ross’ DIY-Page on Recording.org, with a lot of good links for this type of projects:

KEV’S DIY GROUP PAGE ON RECORDING.ORG

The GroupDIY forum would be a good place to ask questions about this project:

http://www.groupdiy.com/

…there’s a good chance you will find me hanging around there…

G7 microphone

admin — Wed, 19 Aug 2009 11:55:52 +0000

The G7 (Gic) microphone is designed in the proud European tradition of tube microphones. The spources of inspiration were the Neumann U67 and U47, two legendary microphones that I was lucky enough to have around for reference and comparison when I did this project.

Schematic for the Gyraf G7 Microphone

Here’s a selection of schematics, showing my inspiration sources - classic broadcast and studio microphones - they are all German designs dating from 1940 to 1960.

Neumann U47
Neumann U67
Neumann M49
Neumann KM56

As you can see, my design is somewhat like a combination of the circuits in M49 and U47, but mounted with the tube used in U67. There’s really not a lot of complicated electronics here.

The Gyraf G7 Microphone
This is a true variable-pattern microphone, capable of doing omni, cardiod and figure-of-eight. This is acheived by polarizing the front and back diagphram of the capsule in different ways with respect to the center electrode.

To act like a microphone, we need a voltage charge across the capsule. When the capsule changes capacitance (that is: the distance between the electrodes are changed by sound pressure) is changed, so does the amount of energy that can be held stored in this capacitor. But as we are charging/discharging with a hell of a small current - the 1GigaOhm resistor - the current really has nowhere to go, and so results in a varying voltage potential across the capsule.

The polarization scheme can be a little difficult to understand at first. To acheive simple remote switching of the polar patterns, a single variable voltage is used for this, only changing the charge of the back part of the capsule.

We want to keep the front electrode of the capsule at ground potential - 0V - at all times, both to act as a shield for incoming electrical disturbance and to avoid electrostatically attracting too much dust from the environment. So to keep a voltage charge across the capsule, we bias the center electrode by the means of two 470K resistors dividing our 160V supply voltage in half - resulting in +80V.

Now we have -80 volts at the front electrode, referred to the center electrode. If we now bias the back electrode with the same (-80V ref. Center = 0V polarization voltage), a positive sound pressure on the back capsule will have the same voltage-potential effect on the center as when applied on the front capsule. This sensitivity pattern is then OMNI directional.

If we polarize the back electrode at +80 Volts, no voltage difference will exist between this and the center electrode, already offset at +80V. This in effect mutes the back capsule, resulting in the CARDIOD directionality.

At last, if we polarize the back capsule at +160 Volts, we’ll have a charge of +80 Volts relative to the center electrode. Now a positive sound pressure applied to the back electrode will produce an 180 degrees out-of-phase signal compared to the front capsule. When applying a sound pressure from the side of the microphone, so both capsules sees the same sound pressure, the signals coming from the two capsules will be in opposite phases, effectively canceling each other. This is the FIGURE-OF-EIGHT directionality.

The changing voltage potential on the center electrode of the capsule is picked up and amplified by the EF86 Pentode, which is wired in triode mode - just like the VF14 pentode in the U47. It’s input resistance has to be kept VERY high not to disturb the charge/discharge of the capsule, so we use a 1G resistor to bias the grid to 0VDC.

From the anode of the EF86 we take the signal thru’ a 2u2 DC blocking capacitor to the output transformer, a Lundahl LL1538 microphone input transformer, here used the “wrong” way around, converting the 15K output impedance of the tube into more useable 600R, and so driving the cable and your microphone preamp with low impedance.

On the PCB board there’s an option for adding a trimmer and a capacitor in order to introduce cathode feedback to the tube amplifier stage. This lowers both distortion figures and output impedance, but cancels out a lot of the “tubey” sound - the reason we bother to make a tube microphone in the first place. The reason I have’nt removed this option, is for purely educational reasons - if you want to do some experimenting yourself. This is also where you can put in various equalizations if you like that. But for the best sound - at least to me - leave the tube undisturbed by feedback and other dirty tricks.

The PCB Layout
The pcb layout for the G7 Mic.
This is an about 420Kbyte PDF file - rightclick and select “save as”. The tracks side of the pcb is drawn MIRROR’ED to make closest contact between print and pcb. Think about this when doing the pcb’s. When the text on the board reads the right way around seen from tracks side, then you’re on right track. Note that a lot of the components mounts on the track side of the board, only the transformer and the angled tube socket goes on the “right” side. This is done in order to minimize total diameter of the assembley, fitting in smaller microphone body tubes.

The power supply
This is a very simple power supply. You only need a couple of mA at 160V for the anode, and about 200mA at 6.3V for the heater. The PSU is based on standard transformers to make it easier for you get hold on them. First there’s a mains-to-9V about 5VA transformer, that is rectified and regulated to 6.3V DC by a LM317T regulator. This is sent to pin 6(+) and 7(0) on the XLR, to power the heater in the microphone.

The 9VAC from the first transformer is also taken to the secondary(”the wrong way around”) of a 220:15V transformer. Now we have about 135V AC on the primary of this second transformer. We rectify this and remove ripple with a 220uF/200V (or maybe larger)electrolytic. We take this DC thru another ripple filter consisting of a 10K resistor and another 220uF/200V (or maybe larger)electrolytic. Now we have our 160VDC for the anode (HT) voltage of our EF86 tube, and run this to pin 2 on the XLR, the ground goes to pin 1.

Now we need the polarizing voltages: 0V, 80V and 160V. The 0 and the 160 are easy, we have them already from the HT supply. We do the 80V simply by dividing the 160V by two 100K resistors, that will also work as “bleeders” ensuring that when you power off the mic, it should be safe to open after 10 to 15 minutes.

BUT ALWAYS CHECK YOUR VOLTAGES BEFORE TOUCHING ANYWAY!!! ( Burnt out bleeders often results in burned out tecnicians. And we dont want that!)

OK - the three different polarization voltages are taken to a 3-pole switch that selects the pattern you want, and goes to XLR pin 5.

The pcb layout for the power supply.

This is an about 220Kbyte PDF file - rightclick and select “save as”. This is the PCB for the power supply for the microphone. Made mostly because Steve Cole asked me. Maybe his notes on building the mic will appear here in the future? The power supply connects to the mic by a 7-pin XLR cable, and has the normal 3-pin XLR that carries the output from the mic.

Gerber files of the PCB’s:
The Gerber files for the G7 Mic. (41kb .zip file)
The Gerber files for the G7 PSU. (32kb .zip file)

Thanks to Mikkel C. Simonsen for the conversion!

Mechanics:

I have’nt made up mechanical drawings for the microphone yet, and maybe it’s really not necessary. The microphone body is a 240mm long, 50mm diameter brass tube with a 50 mm cut-out for the capsule and a bottom plate to close it. In this bottom plate, where the 7-pin XLR-connector also mounts, two 5×5mm metal rods are mounted, holding the PCB and protruding all the way to the silicone rubber holding the capsule. The grille protecting the capsule is formed wire mesh made of brass or steel, and glued in place with epoxy. To get the top part of the mesh (the end of the microphone) in right shape, you take a rounded piece of wood, slightly smaller than the inside diameter of the mic body tube, and hammer the mesh gently into shape.

Tim Campbell’s microphone building page - 1
Tim Campbell’s microphone building page - 2

Further constructional hints about the housing can be found at Stewart’s page - see below.

Pictures: Prototyping the G7 Mic.

Components List:

Microphone:

1 pcs. Large-diagphram capsule, e.g. AKG CK12 (from AKG414)
1 pcs. EF86 Audio Pentode
1 pcs. Noval pcb socket (for EF86)
1 pcs. Lundahl LL1538 5:1+1 Microphone input transformer.

2 pcs. 1G (yes, one GigaOhm, 1.000.000.000 Ohms) resistor - RS Components
1 pcs. 33M resistor
2 pcs. 470K resistors
1 pcs. 100K resistor
2 pcs. 10K resistors
1 pcs. 1K6 resistor
1 pcs. 220R resistor

1 pcs. 100u/35V Highgrade Eletrolytic capacitor
2 pcs. 2u2/160V Polyester capacitors
1 pcs. 1u/63V Polyester capacitors
2 pcs. 100nF/160V Polyester capacitor
1 pcs. 10nF/160V Styroflex capacitor
1 pcs. 1nF/160V Styroflex capacitor

1 pcs. 7-pole XLR connector, Male, Chassis mount

And the other stuff:

PCB
Microphone body
Harden’ed silicone glue for capsule shockmount
Wire mesh for the grille
Thin wire for mounting

Psu:

(220V Model)
1 pcs. Transformer 220:9V, ~5VA
1 pcs. Transformer 220:15V, ~3VA

(110V Model)
1 pcs. Transformer 110:9V, ~5VA
1 pcs. Transformer 220:15V, ~3VA

1 pcs. IC-Regulator, LM317.
2 pcs. Bridge Rectifiers, 250V/1A

2 pcs. Electrolytic capacitors, 470uF/200V
1 pcs. Electrolytic capacitor, 2200uF/25V
1 pcs. Electrolytic capacitor, 10uF/35V

1 pcs. Resistor, 1K8 1/4W
OR Trimmer, 4K7
1 pcs. Resistor, 470R 1/4W
1 pcs. Resistor, 10K Ohm, 1W
2 pcs. Resistors, 100K Ohm, ½W

1 pcs. Metal box for the PSU

1 pcs. 7-pole XLR connector, Female, Chassis mount

1 pcs. 3-pole XLR connector, Male, Chassis mount (for output)

1 pcs. Rotary switch, 1 pole/3way (polar pattern select)

1 pcs. IEC Fused power inlet + T315mA fuse

1 pcs. Mains Power switch
1 pcs. Mains Power indicator light (optional)

Cable:

1 pcs. XLR, 7pole, Male, Cable.
1 pcs. XLR, 7pole, Female, Cable.
5M Cable, 7 or 8 conductors, shielded. Native Tube microphone cable is available from e.g. Mogami. Expensive, but good.

The Capsule:

The capsule is the heart of the microphone - and your G7 won’t be any better than the capsule you put in there. Originally, the G7 was designed for use with the AKG CK12 capsules, but many-many other capsule types has been tried and found good by various diy’ers over the years. If you have a capsule laying around, just try it; if the membrane gets “sucked in” to the backplate and little sound comes out, it isn’t fit for the rather high polarisation voltage we give it (the 80V). This, however should not damage the capsule (if it’s not left in-circuit and powered too high for a very long time). See GroupDIY: Modify g7 mic for 60v polarisation voltage? for notes on how to reduce the polarisation voltage to a easier-managable 60V

Other than the AKG CK12 capsule, I have built a few G7’s with a M7 capsule built by Dale Ulan - see GroupDIY: Official M7 Line-Up Thread - this capsule works very well in the G7 also, giving a rather different but still sweet result. With Dale’s M7, you’d probably want to do the lower-voltage-modification mentioned above

Lately I’ve been told on several occations that it is getting harder to get to buy AKG’s CK12 as a standard spare part - and I get the question “what other capsule could I use?” or “what would you recommend?”. The answer is that there is a couple of sources that has turned up lately, making it easier to buy good condenser microphone capsules.

The inexpensive option:
Peluso of “Peluso Microphone Lab” in the US http://www.pelusomicrophonelab.com/capsnmnts.html has a capsule named “CEK-12″ at USD195 that works rather well in the G7. Peluso’s capsules are chinese manufactured parts, supposedly assembled in the US - with better quality control than you’d expect from the OEM. Quite a few G7-builders have used this, and a great many finds the sound very good - specially for the price

The hard option:
The German company MBHO http://www.mbho.de/ manufactures a really good variant of the CK12 - but it is notoriously hard to get them to sell you a capsule, as they prefer to be OEM for other microphone companies - and they can already sell everything they produce.

The nice option:
Tim Campbell of “Campbell Transmitter” has recently managed to hand-make a recreation of my favorite brass “pre-plastic” CK12, named “CT12″. There are many “flavours” of the brass CK12 - and even though Tim is capable of recreating just about every variation on the CK12-theme, he’s selected this to be his “standard” capsule. Pricing starts (for the time being, May 2009) at USD365 for a capsule without mount (and you’ll want the mount!) or shipping - and though this may seem rather expensive it really isn’t for the amount work and craftmanship that’s put into it. This capsule is spot-on what I originally wanted the G7 to be. Highly recommended - not only because I know Tim, but because that capsule seems to bring out the best in the G7 (and no - I don’t even get any cash-back for saying so).

Second opinions:
The GroupDIY forum would be a good place to ask questions about this project:

http://www.groupdiy.com/

…there’s a good chance you will find me hanging around there…

Michael Krusch’s version of this mic, based on a scrap Rode NT2 and OEP transformer:

http://www.digital-synthologie.de/rode_nt2/index.html

If problems arises, errata and changes will be posted on this page.

Jakob Erland
11-05-2009

REVISION HISTORY:

07-07-2002: Pcb Rev# 2, Schematic Rev# 1: Original
20-07-2002: PSU pcb Rev#1. Schematic redrawn. PDF-files for pcb’s.
27-01-2003: Components list changed to support the LM317 heater supply.
05-2009: Capsule info added.

pultec eq

admin — Wed, 19 Aug 2009 11:55:11 +0000

The G-PULTEC

The Original schematics:
Schematic for an Pultec PEQ1A type EQThis is a piece of reverse-engeineering I did because at the time nobody seemed to have access to the schematics. Note that the transformer data are guesswork and dc-resistances. Recent correction includes potentiometer values. Thanks to Chris Jenrick for sharing the “real” filter schematics for reference.

Schematic for the Pultec MEQ5This is the schematic for the Pultec MEQ5 midrange-equalizer. Combine this with the Pultec PEQ1A, and you have just about everything you need EQ-wise. The schematic shows the filter sections only, but amplifier section seems cross-compatible with the PEQ1A. I have’nt built a prototype yet, but with a bit of luck we can reuse the amp-pcb from the PEQ1A. There are currently no PCB-layout available for this filter.

Our version: The G-Pultec:
Schematic for our SRPP-based version PEQ1A EQ
This is an example of an updated and easier-to-do version of the Pultec PEQ1A. The changes include:

- “No feedback” SRPP gainstage, avoiding the interstage- and the esoteric output trafo, but preserving all that we really like in tube audio.
- Standard rotary switches - 2×6 in stead of 2×7 and 2×3.
- PCB-mounted switches for easier assembley.
- More low freq cut/boost frequencies.
- Unbalanced input with an option for transformer balancing.
- Stabilized 6,3V heater voltage.
- PSU based on two off-the-shelf power transformers, so you dont have to run around trying to get an obscure 220V secondary transformer.

PE1A Front panel layout example

This is to give you an idea about how you can cram this eq into an 19″2U standard rack-case:

The PCB Layout
This is the long promised pcb layout for the diy-Pultec PE1A equalizer. This first PCB is for the passive equalizer, and the second is for the amplifier and power supply board. The filter pcb, carrying all the needed front panel switches and pots, is for convenience made as one 100×160mm pcb, as this size (eurocard) is a standard size of photoresist-pcb. You’ll have to cut it thru’ and connect the two pcb’s with the five jumpers.
The amplifier/psu pcb is made as one 100×160mm pcb, carrying all the rest of the components, excluding the two power-transformers.

The pcb files are 600dpi gif files and PDF-files, and you will have to print them in some scale to get them to the needed exactly 100×160mm. If anyone has an idea of how this can be done more web-friendly, please don’t hesitate to contact me.. The easiest way of doing the pcb’s is to print it on a transparent or semi-transparent film on a laserprinter, and use this directly on the photoresist-pcb.

NOTE: The tracks side of the pcb is - as always - drawn MIRROR’ED to make closest contact between print and pcb. Think about this when doing the pcb’s. When the text on the board reads the right way around seen from tracks side, then you’re on right track.

While working on the prototype, I found that the gain in the originally intended ECC82 SRPP stage was too low. I thus changed to a different tube - the ECC88/6DJ8/6922 - which also provides lower output impedance and thus better low-frequency performance when driving the LL5402 output transformer. The changed tube means a different heater voltage - 6.3V - which is obtained by a LM317 voltage regulator.

The PCB layout and the Component placement for both the amplifier and the filter section (350 Kb pdf-file)

———————

Steffen has made a nice introduction to winding your own inductors for this kind of stuff. Check it out:

http://www.diygallery.de/DIYsites/inductor.html

Kev also has a page about making inductors:

http://recording.org/users/kev/Inductors.htm

The GroupDIY forum would be a good place to ask questions about this project…there’s a good chance you will find me hanging around there…:

http://www.groupdiy.com/

Here’s a link to the original Pultec EQP1A manual (PDF file):

Purple Audio’s original schematic for the amplifier section.

ERRATA
If problems arises, errata and changes will be posted on this page.

03-03-2006: Note that the PCB silkscreen says “10K Lin” at the low-boost potentiometer - this should be 10K Log, like it’s shown in the schematic.

Jakob Erland
03-03-2006

REVISION HISTORY:

11-2001: Amplifier changed to ECC88-tube for more gain and lower output impedance.
02-2002:1K Ohm resistor at sw3b changed to 10K Ohms - this fixes gain-loss when bypassing.
09-2002: PCB Artwork collected into one PDF-file for scaling reasons, gy_pd.pdf

The SSL Clone

admin — Wed, 19 Aug 2009 11:52:54 +0000

The SSL Clone

The SSL Mixbus Compressor Clone
10-05-2006

This is a design I did way back in my early years, so you’ll have to bear with the ugly pcb layouts. And sorry, this design is absolutely free from tubes and transformers. But it has a nice moving coil meter at least.

My reason for doing this clone is that the SSL is one of the worlds widely recognized stereo mix compressors - it has a well known sound, and is lusted after by many recording engenieers. But the SSL4000 desk is prized as to be out of reach for most small studio owners. This design is - as you’ll see - quite simple, easy and economically reachable, so I thought it would make a fine DIY-project for people that know their soldering iron and wants to start building pro quality DIY audio equipment. The beauty of this design is that - contrary to a lot of other esoteric projects - the components are (relatively) easy available anywhere, and no special knowledge, tools or adjustments are needed.

The compressor is cloned from our good old SSL4044E desk, dating back from ‘85. The SSL mixbus compressor is an integral part of the mixer’s main quad fader and autofade system - cards 82E26 and 82E27, and as you can see if you compare these to my schematic, a series of changes have been made for rehousing the circuit. The main changes concerns the electronically balanced input and output and the removal of unneded components regarding the autofade and main fader circuits. Another change is that the sidechain is common to both VCA’s, making better tracking abilities - your mix wont’ “tilt” so easy when you’re compressing heavily. We did things like that to the SSL in the old days. This might seem like a big change, but actually the sonics are preserved very close to the original, and tracking errors are cancelled.

THE CIRCUIT:

Schematic for The SSL Clone

The anatomy of this compressor is surprisingly simple:

Input signals are de-balanced by a couple of NE5534’s, who in turn are driving the VCA in current mode via 27K resistors - the THAT VCA’s are current in - current out devices (note that on the old component overlay, these resistors are marked 15K look here). From the VCA’s the signal is taken to a current-to-voltage converter combined with a simple, yet proven, balanced output drive stage, consisting of two NE5532’s. This concludes the story of the audio path. Not much electronics to mess up your precious audio signal here.

The signals from the input opamps are also summed via 47k’s and sent to the input of the sidechain VCA. The purpose of this VCA is to act like a tracking “dummy” VCA, paralleling the GR action of the main VCA’s, and thereby making it possible (in the original design, the 4000E console) to use the single set of main VCA’s for compression, fader, computer and autofader at the same time - without having to resort to less predictable feed-forward compression schemes. So this is a combination of a feed-forward and a feed-back arcitechture, acting mostly as a feed-back compressor

The sidechain signal that is obtained after the VCA is then full-wave rectified by two TL074-stages, and at this point an offset is added - and signal gain is controlled - by the ratio switch. The ratio switch actually changes the threshold a bit to compensate for level flukturations caused by different ratio settings. This is the only part of the design that in any way differs from a direct off-the-application-note design.

The rectified sidechain DC is now fed thru the bypass switch and one of the bank of resistors selected with the “attack time” switch, to charge a tantalum capacitor selected with the “release time” switch. These release tantalums are bypassed with different sized discharge resistors, each delivering the desired release time. The fifth step on the release is an “Auto” setting, combining two release time constants (91k+6u8 and 750k+u47). This will give short time constants for short programme peaks, but if compression is going on for a longer time, the slow time constant will set in. Right after the book.

This rectified - and now timed - DC sidechain signal is buffered, and sent to three different circuits:

First of all it’s summed with a DC voltage coming from the “makeup gain” pot, and used to control the main VCA’s. On the original SSL compressor the makeup gain pot is active all the time, so when bypassed there’s excess gain. On the control PCB I’ve made an option for disconnecting the makeup gain when bypassed, leaving the makeup gain more useable for in/out comparison. This can be shorted if you prefer the original scheme - but I don’t think you will.

The DC sidechain control signal is also summed with another DC voltage - coming from the “threshold” pot - and used to control the sidechain VCA. In this way the added gain in the sidechain VCA looks to the rectifier like there’s more signal coming in, changing the threshold this way. The “real” threshold to be overcome before charging the attack/release caps, is the 0.6V AK voltage across the diode placed between the fullwave rectifier and the A/R timing. Note that, for use with modern-day levels, the threshold potentiometer can seem to be too sensitive; in this case, mount a 47K resistor between the control PCB and the +side of the threshold potentiometer - lookhere for details

The last use of the rectified, timed and and buffered signal is driving the 1mA GR-meter for monitoring of the ongoing compression. This is linear scale, at about 50uA/dB, making a 1mA meter showing 20dB full-scale. If you want to have a meter reading of 0-10dB full scale, replace the 2K resistor (on the main board, right next to the connector to the control board) with a 1K. You can use just about any linear-scale meter you like, just changing the 2K resistor in series with it. For a 100uA meter, the simplest way is to shunt a 330R resistor across the meter. Another option is to mount a trimmer potentiometer right at the meter, and adjust to taste

The power supply part should be pretty much self-explanatory, the only weird part being that it’s both +-12V and +-15V. The 12V’s are used in the board as subregulators for obtaining predictable gains in case the main (+-15V) supply fluctuates.

For Reference: The involved cards from the SSL4000E main mix section:

SSL 82E26 Card Schematic
SSL 82E27 Card Schematic

THE COMPONENTS:

The DBX 202XT VCA can be very hard to get these days, so I designed a substitution circuit based on the THAT2180/2181 type VCA - a recent version of the DBX2150 that SSL also uses for their channel compressors. This substitute circuit is based on reverse-engeineering a 202XT VCA - it turned out to consist of ten paralleled 2150’s with a common low-impedance buffer for the control inputs. So this is the substitute, but done with only one THAT2180 (or 2181).

I’ve built the compressor with both the 202, 2150 and 2181, and the “sound” dosent seem to be that different, only the 202 may tend to be a little more transparant than the 2150 - but that’s not always a good thing in compressors. Btw, DBX chip production is long gone now, but “Thats” are more than decent substitutes. The “That 2180″ is a pre-trimmed replacement for the 2150, the 2181 has external trim. If using the pre-trimmed THAT2180 VCA’s, simply cut off (or bend to the side) pin4, which is the distortion trim connection. The THAT VCA’s are available - not everywhere, but still - from several web-based sources nowadays - my favorite supplier is ProFusion in the UK - they will sell small amounts, and will ship to all over the world

As this compressor was originally designed for using the - now obsolete - DBX2150 VCA’s, there is a change in one resistor that is necessary to obtain the correct ratios - look here to see the component in question - the 100K resistor marked has to be replaced by a 127K resistor to compensate for higher input current sensitivity on the new chips.

Data for THAT2180:
http://www.thatcorp.com/datashts/2180data.pdf

Data for THAT2181:
http://www.thatcorp.com/datashts/2181data.pdf

PDF file: rightclick and select “save as”..

For the THD adjustment of the 2181 VCA’s, dont worry too much if you dont have access to a distortion-meter - just leave the trimmers about centered. Actually I know of a guy who manufactures compressors based on this design, and even though had a distortion-meter, he used to trim the VCA’s a bit off-center - just to get a little more “sound”. It should be noted here, that the distortion doesnt really have an annoying character: it tends to be almost exclusively second harmonic as far as I can measure… and hear.

If you use the 2180 you can just leave out the distortion trimmers, as these are pre-trimmed from factory - or you can simply cut off pin4 from the chip (or, if you’re not the violent type, just bend it sideways so it don’t reach the PCB).

The VCA’s are the only pieces used in this design that can be relatively hard to get hold on - all other are kept to industrial standards.

Components list

THE PCB’S:

The PCB are in two parts. One is the mainboard, carrying most of the electronic and the power supply. There’s space for a on-board power transformer, but depending on the quality and size of this, you might want to mount a transformer - preferably a toroid type (2×15V, say 10VA or more) - off the pcb. Always take care when handling mains voltages!!. The main board is 100×160mm “eurocard”, a pcb size easy to get as readily photo-sensitized, and also easy to expose evenly. When mounting components onto the PCB, use IC sockets - the “turned pin” types are the best. For the THAT VCA’s (that are single-in-line types) just cut a 16-pin socket in halves.

Observe that one of the three VCA’s (the SC. VCA) is oriented reversed compared to the others, and two NE5534’s are also mounted differently. Also take care that the housings of the 7815/7915 voltage regulators dont touch each other, one carries 0V, the other -15V. No cooling should be needed for the regulators. Note that if you use off-PCB mounted trafo you put in the upper bridge rectifier, and with onboard trafo the lower. Around the output stage the PCB gets a little overcrowded, so be stratetic in the order you mount the components. Sorry about this, but - as said - this is a very early layout, and “if it works - dont fix it”. If you run completely out of room, you could always mount come of the smaller capacitors from the underside of the PCB

The 100nF bypass caps are standard 5mm polyester types - marked “.1″ on the component overlay - and 100pF and downward are all ceramics. The signal carrying 22u and 100u electrolytics are high quality, low ESR types, the PSU decoupling are standards. Mount the PCB in the case using 5-10mm threaded stand-off spacers, and remember to use insulating washers under the PCB to avoid possible ground loops and other trouble.

Fuse the primary of the transformator with about 25 watts worth of fuse, current depending on your local voltage. The easiest way is to use an IEC power inlet with integral fuseholder. For connections between the boards, and to and from the in/outputs, you can use pcb connectors like I did, or just solder the wires directly onto the PCB (there’s not much space to do so - you may want to solder the wires onto the underside of the PCB).

The chassis should be connected to 0V at - and only at - the ground (pin1) of one of the input XLR’s. In case you choose a IEC power inlet connector with a power ground, you should connect it to this point also.

For the power switch I prefer a rotary type, but any type heavy enough to safely carry mains voltages will do.

The second PCB is 30×160mm (one third of a 100×160 piece), connects to the mainboard via a 10 pole cable, and carries the controls - the switches for ratio, attack and release. Here you also attach the makeup gain and threshold pots, the bypass switch, as well as the meter and the occational power-on led.

The size of this PCB is made to fit behind the front of a 19″ 1u case, making wireing quite simple. Note that, depending on your 19″1u cabinet, there might not be room for an angled PCB connector. Then instead solder directly to the trackside of the PCB (see pic).

For the switches you need the “Lorlin” type - it’s manufactured by a lot of different companys under different names. The attack and release switches are 2×6-types, the release should be programmed to 2×5 positions, and the bypass and ratio are 4×3-types, the bypass programmed to 4×2. Programming is done by taking off the nut and relocating the washer/locking pin beneath it. The value of the two pots is 50K lin.

The pots dont have to be of any particular quality, as they wont pass audio, just carry dc: Instead focus on getting a good damped feeling - I dont know why, but quality and feel never seems to meet but in esotericly-prized hifi.

The artwork files are in PDF format to maintain scale. Please note that the PCB (copper) side is drawn MIRROR’ED - enabling closer printout-to-photoresist contact when making the PCB. If in doubt, use the text on the board as reference. The easiest way to handle PDF-files on Internet is to right-click on the link and select “save target as”. This is because most browsers don’t allow saving PDF documents.

The Schematics, Component placement, and the (reversed) PCB layout for The Clone (650Kb pdf-file, PCB Rev#9, SCH Rev#7)

MCS’s Gerber files for The Clone (200Kb .zip-file, REV#7)

..And oh, by the way - I hate to have to mention this, but it seems to be necessary: Please read (and understand) the disclaimer on our main DIY-page. Even though you may like to, you are not allowed to use the projects described in these pages for any kind of commercial purpose - including building units for others where that involves money and such.. All this is ment for free, educative, purposes - please don’t distort that..

PCB RESOURCES:

For those of you that are unsure and don’t want to get into “rolling your own” PCB’s already, here’s a couple of ready-made options:

Gustav has a PCB-service for all of these projects, available in most parts of the world: Gustav’s operation at the GroupDIY forums

Ben has an Eastern-world PCB-service for the SSL, G9, and G1176 projects, easily available in Australia, Asia, and New Zealand areas: Beesneez studio, Australia

Chef in the UK has G9-diy and G-ssl pcb’s also: AudioKitchen, Chef’s PCB operation

THE EXTERIOR:

SSL use Sifam (AL29, eg. Farnell:969-746) moving-coil meters and Sifam knobs (eg. RS:225-704) - types that you can get from RS, Farnell, or Canford if you want to look like the originals. Personally I find them pretty ugly, but your taste can be different, and some people might be more comfortable putting their valued mix thru’ a homebrew machine, if it at least looks a little like something they know. Look at the Aliases and webpages at the end of this page if you need inspiration. You can actually use just about any meter that you like, if it is 5mA FSD or less, standard linear scale. But remember to change the (2K) resistor in series with the meter, if other than a 1mA meter is used.

My Prototype’s Front Panel Layout

Exterior design is a pretty important factor in how people will use - and thereby how they will judge - the unit. The pictures shows how I’ve chosen to do it. But then again, I’m really more into tubes, transformers and bakelite than solid state designs like this - though I must admit that I love The Clone’s sonic qualities for a lot of mix types and for group compression on drums. Often, if you do your mixes entirely inside the computer, you’ll be ending up with material that is hard to get to blend together - making individual track level setting very hard to do. This little box makes things much easier.

Pictures from various construction stages

PROBLEMS WHEN BUILDING:

If you read through the related error-fixing thread at Groupdiy.org, you’ll see that the errors that the average DIY’er encounters when taking this problem on is limited to a few categories:

- Solder blobs, shorting PCB traces. The layout is very tight at places, so do yourself a favour and check and double-check - preferably with a magnifying glass.
- Bad soldering, not giving proper contact between component and PCB trace.
- Wrong component types or values. Specially error-of-magnitude on resistors.
- Wrong orientation of components. Including capacitors, IC’s and diodes.
- Wrong wireing.
- Last, it’s (for reasons unknown) common to forget mounting the wire-link at the control pcb, right beside the meter connection. This results in gross distortion in both channels.

DISCUSSION FORUMS AND RESOURCES:

A Detailed description of the SSL Clone project and posts about possible problems and solutions can be found at the GroupDIY forums. This is the first place you should check if you experience problems while building:

GroupDIY.org’s SSL-clone META thread

Gyraf Audio G9

admin — Mon, 13 Jul 2009 22:13:16 +0000

Welcome to the Gyraf Audio G9 DIY Page….

08. Jan. 2007

Do-It-Yourself G9 Tube Microphone preamplifier
The G9 project is an adaptation of the Gyratec IX dual microphone/line/DI preamplifier to suit the DIY’ers demands. Searching the net, I’ve never come across a complete tube microphone preamp design, so I decided to share this version of our very popular design, the Gyratec IX. It is a classic, conservative design, that easily matches the performance of even the most expensive and esoteric units I’ve been able to compare it to.

I wish to thank Kev and Byron from DIY-Factory for test-piloting this project, and for their changes and comments on the original design. Without those guys this level of DIY-friendlyness would’nt have been even remotely possible.

The G9 contains two channels of real-tube, high-gain, transformer-balanced microphone preamplifiers with additional line and instrument inputs and transformer balanced output. It also features switchable phantom power, high-pass filter and phase reverse, as well as independent controls for input gain and output level - giving the user some creative options to work with.
The signal path contains only tubes, transformers and passive components, to preserve your signal integrity. VERY different from many “tubed” consumer products. But as we are not purists, we incorporate modern semiconductors in the powersupply sections - simply because this is way the easiest, cheapest and best solution available. For power amplifiers there’s quite an audible reason to use tubed power supplies, but for preamps I haven’t been able to spot any advantage so far.
I will try to update this page regularly if anyone shows interest in its topics. Comments and corrections are extremely welcome, but I can’t promise to reply to all mail I receive. In “The Lab” - a forum on GroupDIY.org - there will surely be people that can and will answer most of your questions regarding the design, construction, and sourcing of parts. If you build this project and describe elements of the process, I’ll be more than happy to add it - or a link to it - here, so others can benefit from your experiences.
Disclaimer:

Notice that all information, schematics, layouts etc. are supplied “as is”, and that we can in no way be held responsible for its accurateness, functionality or even safety. Gyraf Audio shall not be responsible and disclaims all liability for any loss, liability, damage (whether direct or consequential) or expense of any nature whatsoever, which may be suffered as a result of, or which may be attributable, directly or indirectly, to the use of or reliance upon any information, links or service provided through this website. Now you know that..
Basic safety rules:

You have to take extreme caution when working with Mains and High Voltages. These voltages are lethal, and even the smallest error will be catastrophic. And we like you to stay alive and well, so you can help other people sharing our bizarre interest for building retro-pro-audio-equipment.
- NEVER work with live voltage switched on. Switch off, discharge, work, connect measuring equipment and power up. The G9 powersupply takes an average of 15 minutes to come down to non-dangerous voltages, providing that the tubes has had a chance to get hot. But if there’s something really wrong, the lethal HT can stay for weeks! TAKE CARE!! ALWAYS positively measure HT voltage on the PSU caps before touching ANYTHING. A good idea is to have a resistor, like 10K/2W, on a cable with two clips that you can attach to the caps for discharging. But please. Remember. Take care.
- Always keep your mains connector in plain sight when working, so you can assure yourself that it really is disconnected.
- Always tidy up your working area before connecting your project to the mains. This gives you some time for second thoughts about what you are doing.

The Schematic:

As you can see from the schematic, the G9 is really a VERY simple, no-nonsense tube mic pre. Click on the schematic for a high-resolution version, suitable for printing

Circuit description:

The input is taken to Phantom resistors and the line input relay. The line relay - as well as the Phantom power - is controlled by the front panel rotary switch, SW1: (line - mic - mic+p48). When in “line” mode, we set the input attenuation to -26dB, the Zin to~10KOhm - but still Z-matched to the transformer, with an option of either OEP A262-A3E or Lundall 1528.
Note that OEP’s HAS to be in screening cans, including the output trafos (that are not canned on the pictures, sorry). Without it, I’ve had oscillation problems because of the high gains and bandwidths involved.
The secondary of the input trafo is taken through a switched “instrument” jack plug on the front of the unit. Very nice for guitars, basses and keyboards (and fantastic for a Mellotron if you happen to come across one..) - and then to the input gain stage, based on an ECC82 (12AU7, 5814 etc..).
The input gain stage is pretty much a standard feedback gain-controlled circuit like the ones used in all old tube audio gear. It’s gain is controlled by shunting the fed-back AC signal on V1a’s cathode to ground - think of it as a opamp gain block with 47K (R10) from output to inverting input. You can now control the circuit’s gain by shunting the inverted input to ground - through a cap, C7, because we don’t want to disturb the DC cathode potential, made by R6 and R7, setting bias in V1a. C7 also protect our gain switch, SW2, from destruction by DC..
The only tricky part in this is the gain switch, normally divided into one feedback switch and one attenuation switch.. But - you know I like the simplicity, availability and easy mounting of the “Lorlin” type switches - so I had to think a little here.. and came up with this “grounded common” switching scheme, actually performing two different tasks:
On the first 4 steps, the lowest gains of the gain switch, the system gain is kept to minimum - only R12 (1M) shunting to ground to avoid DC build up in C7, our 10uF/63V polyester AC cathode shunt cap - and the output of the gain stage is attenuated by R26 (47K) and either of R19-22 (5K6-82K).
On the highest gains - the steps 5 through 11 on SW2 - attenuation resistors R19-22 are taken out of circuit, and AC cathode-to-ground shunt resistors R13-18 (1K-100K) becomes active, rising the gain.
The now gain-controlled signal is taken to the high pass filter, a simple passive 6dB one around C8,9 and SW3 - and then to the “output gain”, a 47K log pot feeding the output driver stage.
The output driver is a 18dB gain SRPP stage, also using an ECC82 tube. It drives the output transformer - either OEP A262-A2E or Lundall 5402 - through a 4u7/250V polyester DC blocking output cap.
The output trafo is wired 2:1 to get the 2400 Ohms Zout of the SRPP stage down to around 600 Ohms. The 2:1 ratio means that when using the OEP 1+1:2+2 transformer, it has to be reversed. I’ve had very good results using this output stage topology in all sorts of designs - it’s simply my favorite tube circuit. Also used in the Gyraf Pultec amplifier.
The output of the transformer is taken to SW4, the phase reverse switch, that has 10K resistors across it to control transformer load when switching. And then to the output XLR…..
The Power Supply:

The power supply is right off the book.
The 245v HT is made from 220V/50mA ac, rectified and regulated by IC1, a TL783 high voltage regulator. The TL783 is limited to max. 120V in-to-out differential voltage, so the three 39V zener diodes (D3-5) protect it from voltages higher than this. Also the 1N4004 (D2) protects it from lower voltage on the input than on the output, a condition it hates.
Note that for space, the two 100u or 220u/350V (C14-15) HT reservoir capacitors are mounted off-board, in chassis mount collars, and can be anything from 100 to 470uF, minimum 350V DC.
The output voltage of the regulator is set by the ratio of R34 and R36+36, to about 245VDC, keeping us within the range of 250V capacitors. The HT directly drives both output stages, and is filtered by C25/R31 for the input stage.
The 12V for the heaters is derived from 15V/1A ac, rectified, and regulated by IC2, a 78S12 1.5A voltage regulator. The heater voltage is also used for controlling the line relay, RY1. Note that the 78S12 regulator dissipates quite a lot of power - around 5W - and MUST be mounted (electrically isolated) on a good heat sink, preferably with thermal contact to the chassis. I use a heavy aluminum angled profile that is mounted to the bottom of chassis (see pics). It would also be possible to mount the 78S12 on three (short, heavy) wires, and bolt it to the chassis back - electrically isolated. Failing to heatsink the 78S12 appropriately will result in thermal shutdown - you’ll loose the 12V lines whenever the unit heats up.
The 48V phantom is also relatively straight-forward: It’s a zener regulated design, adjustable with PR1, and fed from a voltage tripler running off the 15V that goes from the first transformer to the second.
The PSU Transformers:

You need two 220:15+15V 1A (30VA or larger) toroid transformers, the first converting 220v to 15v - for the heater and through the tripler to P48 - and the other one converting the 15V also used by the tripler back to 220v for the HT.
For 110V mains, you’ll need two 110+110:15+15V (30VA or larger) toroid transformers. The two first primaries - for mains input - should be coupled in parallel, for 110V. The two primaries of the second transformer should be connected in series, to bring voltage up to around 220Vac.
Note that:
· Toroid transformers should definitely be used here, as the mic preamp is a quite sensitive unit.

· Main PCB size is 150×209 mm, one half of a standard 21×30 pre-sensitized PCB.

· Control PCB size is 100×160mm standard size pre-sensitized PCB, that - after etching and drilling - has to be cut into three pieces. Two control boards, and a spare psu. The spare PSU is just because I had the extra space anyway, and I find that having extra HT+ Heater PSU’s laying around greatly helps tube creativity.

· PCB is - as always - drawn reversed, i.e. as seen from component side. Refer to PCB-side text if you’re in doubt.

· On the PCB layout, component numbers larger than 100 denotes components for channel 2.
All components should be easily available, and the unit requires no other trimming than the phantom voltage set to 48V.

The Components list:

Here’s the latest updated shopping list. However, there’s no guarantee of correctness (yet..), so you better check out for yourself if it’s sensible..
g9_components.txt - the shopping list.

The PCB Artwork:

These are the PCB layouts for the G9 dual micpre. They’re contained in a 510KB PDF-file to maintain scale - just remember NOT to use the “fit to page” option in acrobat when printing.
As always, the track side is shown mirrored - as seen from component side - for better copying onto photoresist PCB’s. The set consist of two pcb’s, one 15×21cm - the main board, and one 10×16cm, the control board.
The main board is straight-forward, but the smaller control PCB has to be cut into three sections; two sets of front-panel controls and one (BONUS!) spare HT+Heater voltage PSU for your further tube experiments. This is both because the space was there anyway, and because I think that the hard thing getting started with when working with tubes is - the powersupply.
510KB .PDF document with layouts and component placement (rev#5).

Mikkel C. Simonsen of TechTalk has helped me with the conversion to Gerber files - for those of you who don’t want to make the pcb’s yourself:
377KB .ZIP-file with layouts and drill-files in Gerber format.

It’s easiest to save this to your hard disk by “right-clicking” the link, and selecting “save target as..”

The construction process:

First of all a warning: This is not a project that is suitable for the
beginner. If you haven’t worked with tubes and high voltages before, I’d
recommend that you start out with something simpler than this preamp.
I’ll leave up to you how to mechanically construct the unit, but there’s a few
points that are important:
- When mounting the Toroid power transformers, make sure that the center bolt
that holds them is only touching the chassis in one end. This is really
important, because if the center bolt touches chassis in both ends, you are in
fact adding an extra, shorted winding to the transformers..
- Keep the PSU transformers as far away as possible from the audio
transformers - specially the input transformers. Also keep your distance from
the front panel, as the “output gain” is somewhat high-ohmic and hence
sensitive. A good way of working is to leave transformer mounting to the very
last step in construction - after all other is working and can be checked.
With the preamp running, move the PSU transformers around in the box (WATCH
WHERE YOU PUT YOUR HANDS!) to find the spot where they’re interfering the
least. Mark the spot, and mount the transformers here..
- Connect 0V/Gnd to chassis at one - and only one - point: At the input XLR’s.
- Connect the power ground from the power inlet to the ground at the input XLS’s also.
- If you experience oscillation at extreme gain settings, try using shielded wire to/from the instrument input jack on the front. This is the first wires of the group of five that connects main board to control board.

The Mechanics:

This project is designed to be mounted in a 19″ 2U rack case. Because of the way the control PCB is laid out, you’ll need to follow a front panel layout like this:

The G9 project is designed to fit into a 19″ 2U rack case, and laid out to fit
a 19″2U frontplate. Off course it would be possible to use all sorts of
enclosures with some creativity, but you’ll have to change quite a lot of the layouts to do so. I’d
recommend that you go with the original solution.
The frontplate drawing shows where to drill the holes for the controls -
remember to measure twice and drill once, not the other way around.
You’ll get the most precise results if you start with small drills and
gradually work your way up to the right size. Drilling with a large drill-bit
from start doesn’t work! For the XLR’s you’ll need either a tapered “multicut”
stepped drill, or a set of manual hole punches. The most used sizes for XLR’s
are 20mm for XLR M and 24mm for XLR F.
For the Power IEC-connector it’s a bit more complicated. First mark up clearly
the needed contour on your back plate, drill a row of closely-spaced 3mm holes
all way around inside this perimeter, take a hand-held drill and - carefully -
move from side to side to cut the material between the holes. Once you’ve
removed the material from the center, the rest is a matter of filing down to
the needed size. Hard work, but not complicated…
For front panel marking, I’ve always liked engraving - but that’s easy for me
as I happen to have an engraving machine in my workshop.
Other options could be having a frontplate engraved from an engraver, or
ordering from “Shaeffer Apparatebau” that makes custom frontplates from CAD
drawings. If you happen to make up a S.A. CAD-file for the G9 - and you’re
willing to share it - I’d very much like to have a copy here for other
constructors. But please have it made first, so we’re sure that it is working alright.
A simpler and cheaper solution is to mark the front with white “Letraset”
transfers and covering with lacquer.
Or marking the front with a pencil - in handwriting - and using a hand-held
engraver like a “Dremel” to engrave it. I did this before I got my Gravograph.
Still another solution is using a blank frontplate in stead of the black one.
With this you could simply laser-print the needed frontplate marking (in
reverse!) on to a semi-transparent plastic film, and stick the plastic film to
the frontplate using a sheet of double-sided adhesive tape. With your printed
text and markings on the inside of the plastic film, this is very long-lasting
compared to the Letraset solution. I think Farnell has the double-sided
adhesive type of sheet/tape.

Pictures of the G9:
NOTE: This is an early unit, with minute differences from the current DIY-version

The G9 as seen from the front..

G9 top view..

The main PCB and one control PCB..

G9 overview. The mu-metal plate at ch.2 is for shielding. It should be unnecessary when using toroid transformers.

Closeup of one of the control PCB’s.

If problems arises, errata and changes will be posted on this page.

ERRATA:..

02-02-2003: Original drawings
28. March. 2003: R9/109 changed from 220K to 47K to prevent oscillation problems
25. May 2003: D15/D16 was missing a connection on PCB rev#3. New PCB’s are now rev#5.
09. mar. 2004: MCS’s Gerber files added.
08. Jan. 2007: If you run into problems with oscillation on the highest gain settings, try running the connections to/from the front panel High-Z jack connector “directly” from the relevant points on the PCB - by cutting the PCB traces, and running a length of shielded cable directly to/from the jack. See this image for reference!!

If you let me know how you’re managing this project - and if you have information that could interest other diy-people - I could link to it, or I could put it on this page. Share your information. It really wants to be free.

The GroupDIY forum “The Lab” is the right place to ask questions about this project, as I’m not able of supplying one-to-one help for the projects:

http://www.groupdiy.org/

…there’s a good chance you will find me hanging around there…

G9-Specific forum at “The Lab”

Paul S. has built a nice “retro look” version of this project:

http://home.houston.rr.com/lpstephan/g9_mic_pre.htm

Some other relevant links:

RafaelFreddy’s Pultec MB1 project

Tubetech MP1A - schematic

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02-2003 - 04-2010