[Winston O'Boogie]

Not sure if I already posted this schem before so, uploading now as I came across it looking for something else.
It was the standard gain block used in the desks.
Excuse the strange way in which I depicted the transformer, naturally it's one transformer with four windings, not two seperate pairs, you get the idea if you squint a bit.
The transformer is ungapped since the dc, ideally, cancels.  Radio-metal core.

No hard copy of the full channel anymore, may have scanned it but, not found it yet.
Passive EQ very similar to Helios.  Use Helios but scaled down in impedance and it'd be close enough. 

[Winston O'Boogie]

Oops, wrong section maybe, meant to post in preamplifiers although it applies here too as makeup gain.
Maybe one of our esteemed mods could move it if that's a  better place?

[pahstah]

Hey John, thank you for posting this,
 seems like soundtechniques schematics are impossible to find, but for me is the holy grail of all the mixers, I think that a good 40% of all my favourite records were cut through the Sound Techinques A-range: Nick Drake albums, Lou Reed Transformer, Led Zeppelin IV, The Beatles Hey Jude, The Doors, Space Oddity and others David Bowie records to name a few!
Some five or six years ago Geoff Frost produced a rackmounted A-range channelstrip but it didn't last long and i wasn't able to buy one...

[gyraf]

Moved.

Thanks for this - do we know anything about the output transformer?

Looks kinda like the Neumann PEV and PV76 architecture..

Jakob E.

[Winston O'Boogie]

Hey John, thank you for posting this,
 seems like soundtechniques schematics are impossible to find, but for me is the holy grail of all the mixers, I think that a good 40% of all my favourite records were cut through the Sound Techinques A-range: Nick Drake albums, Lou Reed Transformer, Led Zeppelin IV, The Beatles Hey Jude, The Doors, Space Oddity and others David Bowie records to name a few!
Some five or six years ago Geoff Frost produced a rackmounted A-range channelstrip but it didn't last long and i wasn't able to buy one...

Yes I think those records sound fantastic too, and I'd include the early Elton John stuff cut at Trident in that mix, even if you don't like Elton. 
The Sound Techniques A Range was pretty much unknown about for years I think.
I got a good peek inside some modules about 12 years ago that were in one of the desks sent to the U.S., possibly the Elektra board but not sure exactly if these were those or not.
I did have the full schematics, one a big photocopy of the original blueprint showing the EQ, and then other bits which I had to trace out including the amp I've posted.   Sadly I lost all my documents and library 10years ago except anything I had emailed to anyone or anywhere so...
But the EQ is basically a Pultec passive type except with an inductor peaking low end boost like on the Helios.  Only 2 frequencies on the low end if I recall, both with the same L but a switched C.  I'm thinking it was 80Hz and 120Hz but it could've been a bit higher?

The amp was the same as I posted for pretty much everything, the Mic input transformer had multiple primary taps for impedance switching and secondary pads @, I think, -5 and -10dB.
I could ask a friend in L.A. who does own two Elektra modules for particulars on freqs etc.

With the blessing of Geoff Frost, Sound Techniques has been picked up again and there is at least one fully built new desk that incorporates the passive A Range eq with the active mid of the System 12.  They did have plans to release modules but I don't know if that's still so or if they're just interested in complete desks.

Anyway, have a peek around online for that new product info, I think it's the same website address as Geoff had.

Cheers dude.

[Winston O'Boogie]

Moved.

Thanks for this - do we know anything about the output transformer?

Looks kinda like the Neumann PEV and PV76 architecture..

Jakob E.

Hey Jacob,
thanks for moving this 

The transformer was a radio-metal core as originally specified by Geoff Frost so, highish Ni rather than steel.
He did think that one of the Sowter's was a good sub., and I would think a quad-filar as a 600:600 would work.  The DC should cancel if transistors are balanced.
I show a different value for one of the output transistors of 6K8 in parenthesis on the schematic which was what I found on 3 of the dozen+ cards I examined more closely.  I assume this was to balance things more for DC etc.  but could've been a cock-up back at Sound Techniques I suppose 

[hitchhiker]

So cool to see this!  About ten years ago I reversed a scheme from pics but I never tried bread boarding it. Too many question marks on my drawing. I labelled it Class A Complimentary Symmetry on the output section, which I got from a book I own. That too was a guess.  At the time I gleaned most of the pinout numbers from basic test drawings posted on the old Sound Techniques web site.

I'll share this not quite correct schematic just for fun. You can see John that the cards in the pics I had employed the 6K8 you mention. Also the preamp section had different transistors.

[Winston O'Boogie]

Hey cool!  Thanks for posting Hitchhiker  .   Considering you traced it out from some online pics, good job.
Your drawing is less convoluted than mine for sure and there is additional info regarding gain setting etc.

And you show a Sowter replacement, was that info on the website?  I heard it second hand through the friend with the Elektra modules who'd been in direct contact with Geoff since forever.


Wonder if wayback machine has the pics you used?

[hitchhiker]

Yes I have at least some of the pics but for the life of me I can't see how I ever was able to get it done from what I have. It is possible that the high quality monitor I had then was so much better for judging colours than my cheap modern flat screen ? Or,  I may have lost some better pics over the years? I do know I spent days on it and was very determined. Even as it was I had quite a number of question marks with the resistor values.  Interesting that your scheme has different values for a number of the lytics. I would have been reading the labels on those. That makes me wonder if my pics are of earlier versions?  I think some of the pics were from an ebay auction but I believe most I got off the old ST site.

[Winston O'Boogie]

If you think they're of any use for additional info, post what pics you have Hitch.  I'd like to see them.

I didn't see anything worthwhile using wayback machine unfortunately.

I think we'd probably err on going up in value with the electrolytic these days.  Mine, or the ones you copied, could've been changed out at some point between 1969 and 2008 too I suppose.



 

[hitchhiker]

OK, I'll post a few of the parts side.

[hitchhiker]

same module

[hitchhiker]

same again from same auction

[hitchhiker]

different module from old ST site

[hitchhiker]

another module

[hitchhiker]

test jig drawing from old ST site

[hitchhiker]

last one for now

[Winston O'Boogie]

Very cool, thank you
The first 3 pictures are the same type of input module I examined, even down to the low frequency inductor just laced up to the cabling, hanging there.  I bet this is from the same desk.  Looks the same as my mate's other 2 modules too.

There were no faders in the batch of stuff I saw but were some buss amps.  The last module in your pictures with the fader looks like a fader with an adjustable amplifier, possibly it's the mic pre, I think this is from a different type of A range desk tbh.

The other module that's more neatly assembled, wonder what that is?  Looks like few shielded inductors on a daughter sub-assembly to the top right but...  ?

Anyway, great to see these, thanks for saving and posting.

[hitchhiker]

And you show a Sowter replacement, was that info on the website?  I heard it second hand through the friend with the Elektra modules who'd been in direct contact with Geoff since forever.

Yes, the transformer info was from the ST website.  I thought I also had info on the transistors used but I can't seem to find that right now.

[Winston O'Boogie]

Yes, the transformer info was from the ST website.  I thought I also had info on the transistors used but I can't seem to find that right now.

OK thanks, got it.  Sowter site says that part is steel rather than what Geoff said were originally radiometal core transformers.
I wonder if one of the Cinemag higher Ni or pinstripe lams quad-fil. might be closer?
Anyway, good to know what he'd tried and said worked.
Cheers

Sound Techniques - EDIT: Piccies of innards

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Sound Techniques Studio/Desks, Chelsea - UPDATE

Started by The Lab

Trident A-Range PCB, bom, etc

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