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Studers A779 & C279 circuits
Old 12th January 2015
Studers A779 & C279 circuits

Hi all!
Not sure if high end forum is a right place for this… Rather than sound of Studers discussed elsewhere on GS, I'll go to circuits here.

I happened to have both boards since recently, which are generally considered to be the same.
As I checked all the info on GS and else, and spoke to some people, I figured there is some contradiction regarding crucial part of boards, the preamps. So I decided to take a day, open the boards and follow the schematics.

As I understand, C279 model came earlier, from mid '80, and is made in Regensdorf (Switzerland). Some circuits in my 279 board have dates like 03-87, with serial no. above 4000. My A779 with low serial (around 1100) was produced later, into '90, and is made in Loeffingen (Germany).
(Paradoxically, my 779 was quite used at some local radio station, while older 279 is still mint.)

Did anything change over those 5-6 years, within same board design?
Both boards have 1 : 3,16 transformers at the input, and NE5534 right behind them, so I understand this combo works as a preamp. Transformers are Studer branded, but some people say they are manufactured by Beyerdinamics (dunno if thats true), and are responsible for characteristic "Studer sound".

In C279 model, after this comes 5532 chip, one for stereo signal, and 2 in series for mono signal, if I'm reading the schematics properly. Btw, XLR balanced line inputs are *stereo* with both boards.
In A779 board, 5532 chips were changed for far less known MC33078 (both made by Texas Instruments). Did later chips show better characteristics??? I've no idea. I guess 5532 chips were still available in early '90, during production run of A779?

From there, the signal goes to EQ stage, and there are several MC4559 in serial. My understanding of electronics is not enough to see what exactly 4559 are doing. They are the same in both boards, some are TI and some JRC manufactured.
Finally there is dbx 2150A VCA chip, to which fader is connected to, instead of typical slider potentiometer. At first I thought this is inferior design, but in fact dbx 2150A are high-end vintage VCA chips, used in top compression gear, so no worries letting signal go thru them. With this design, pots never get scratchy, and faders are totally clean indeed.

Nice surprise comes in extra in-out section of A779, the one of the bottom of the device. Stereo master out signal is going to another 5532 and transformer, before getting to XLR output. And that's per channel.
Similarly, aux return has one studer 1 : 0,62 transformer on each channel. Should I say discrete preamp on aux returns, on a board of this size?
All together, A779 has 10 I/O transformers.
Oh, and interior of both devices are stunning - neat, tidy and symmetrical.. well it's Swiss! Compared to this, synths and pedals inside look like a mess.

Now, take this description with a grain of salt, I'm just a electronics newbie. Or please correct me if I went wrong. That's why I still have some questions, if you care for some discussion on this boards:

1) Ok, this is silly newbie question: only mic input has nice transformer & 5534 preamps, while line inputs have only 5332.
Would I benefit for line signal juice, if I plug instruments in mic input, with attenuated levels?

2) What about 4559 chips? Some people swopped them for something else (TL072.. etc), with explanation that "after 30 years there are better chips". That might be truth, but I think - designers in fact HAD other choices even back then, but opted for 4559. Who am I to question decisions of Studer engineers?

3) Some chips in both input and output stages are in sockets. How come? Meant to be swapped? Is there a hidden message from engineers after all? (But not 4559 chips!)

4) Any ideas on MC33078 preamp chips? There is zero opinions on them on the net!

Cheers, Greg
Old 15th February 2017
Here for the gear

Great info... Thanks greg. I just got my revox Mr 8 back from a full re capping and it's sounding sweet. Pretty sure it shares most of these characteristics. Such a cool desk. So far piano, drums, trumpet and vocals sound great through this desk. Had a play around with distorting the press too and although I didn't record it I reckon there's something there too. Good fun.... Thanks to plush too for info on another thread..
Unfortunately I can't answer any of your questions.. Soz..
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