Looking for curator and info dissemination guru for Sound Techniques A range. Zero work, all glory, pro bono.

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Winston OBoogie

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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. 



Edit:  Attached document removed
 
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 said:
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  :D
 
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.
 

Attachments

  • ST reverse guesswork Hitchhiker.pdf
    648.3 KB · Views: 159
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 said:
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.
 
Thanks guys. This info has been invaluable, though I unfortunately only saw this when I was half done reverse engineering it!!

I have 4 of these on my bench at the moment if you need any pics or info.

Ken
 
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