Valley People Transamp Schematic

Hello,

recently I read about the Transamp Mic amplifier. Does anybody have schematics of the circuit? I would like to know how such a thing is made.
I searched already here, found some links (eg by CJ) but they do not work anymore.
If anybody has the schematic can you post it or send it to me via email?

Raphael

http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=12&t=114

Here are some links that have Valley apps notes. There is a schematic in note 103, but my recollection is original TA used a three opamp approach (two gain stages followed by differential to SE) not the two opamp version with differential performed by the two..

I have also seen a schematic around of different valley rack product with a preamp in it that was very similar. I don't think Pau Buff ever formally published the actual potted TA schematic.

There's enough info here to get you close. You probably want to dial it in for whatever transistors and opamps you use

JR

Is there really nobody who can provide me with the schematics?  :-[
I know the app notes but don't find the helpful. because you get no detailed informationwhat is really inside this module.

Raphael

I thought I remembered CJ cracking one of these open and reversing it a few years ago.  Maybe ping him and ask...

what do you want to know about it?

8 transistors, matched and thermally coupled, thats it.

That must've been the VCA (202?), not the TransAmp.

Dan, is right, I forgot about the current source,

here are 5 pages of stuff,



http://www.groupdiy.com/index.php?topic=6926.0



this was in my binder, i think it works,

an output transformer defeats the whole purpose of this amp, but a 600:600 makes it stable.

ok, alright, we know know what you Really want, right?

jeeezx, first the transformer stock gets wiped out, now they are digging into my opamps! 

so we have 6 transistors, mps4355, this means a 2 transistor current source, which is legal,

and an opamp i can not make out, so closee, but yet...

any help?

i'm gonna bust down with a huge jpg to piss Pat from sactown off, 

The opamp in the original TA was TL072 (IIRC).

That schematic looks close but is not for what's actually inside the potted shell. So despite being for something else there appears to be at least one error of omission, and parts not in the old TA.

For one obvious omission, there needs to be another 100k to ground. on the other input transistor base.

I sure don't recall a 470 pf across the gain pot, if 470 pf there you would surely need more than 10 pf across the 3M  to keep opamp from screaming.

Yes, the current source was only two transistors in TA ( IIRC). 

Those transistors FWIW were not considered low noise, but GP medium power devices. Valley selected them for low process noise, and used these same parts in their VCAs matched for Vbe.  They had relatively low Rbb due to their medium power handling.

JR

JohnRoberts said:

I sure don't recall a 470 pf across the gain pot, if 470 pf there you would surely need more than 10 pf across the 3M  to keep opamp from screaming.

Click to expand...

I was thinking the same thing. This is one place where you'd normally try to avoid stray capacitance.

Whatever that Note in the lower right may say, you wouldn't want to have such large input resistors on the 5532 or any other BJT-input op-amp.

That Phantom injection network might work with some mics, but it sure isn't within spec.

JDB.
[drafting errors or deliberate mistakes?]

The first TL opamp has misplaced/mislabeled +/- inputs.

There actually IS the "missing" 100K resistor in the schem: right above the "LOW GAIN CMRR" trimmer.

It's quite possible that they used "divided by 10", i.e. 6,8uF/330K combos if BJT opamps were used - a 5532 works exceptionally nice with f.e. 1Meg biasing resistors, and the way they used them, would cancel/null the DC offset if BJT's would be used instead of TL072.

The "tranny around an opamp" is best sounding opamp amp... for me. Especially for instrument inputs. The second best, after a real single-ended CFB gain-block. (in fact, a transamp IS a CFB amp made with a VFB opamp. So a poor guy can get by with buying a slightly more expensive input tranny and slightly more expensive opamp and make it sound __almost__ as good as a very expensive discrete opamp. Clever, clever.)

good catch on the opamp polarity, i remember that error on the fix it list.

remember, i said " i think it works" did ii know something?    ;D


the 470 pf cap was for tone,

without an output transformer to screw things up,this is as close to a "strait wire" amp as you will find.

so the 470 pf was a substitute for a HS 66, which was added later with gusto. 8)

certainly the following signal chain will roll things off, but i was going straight into an OTL headphone amp, into the Sony Pro phones which have this high end thing going on already,  oh, and a transformerless SM 57 getting barked into by the dog, can you imagine the eq needed for this signal chain? 
(i wanted to hear the incredible detail, in order to appreciate this amp, your support gear has to be as clean or better, or else, ...?)

the transistors have the old tropical fish color codes for matching or id.

i built my DIY valley pre with i believe 3 transistors in parallel instead of 2,
i had four on each side for ultra low noise, but the trade off was not worth it, noise elsewhere, (can you say opamp, or pwr supply?) so why bother. (thanks 2002 vca )
plus, the matching was greatly simplified going from 4 to 3 pairs.
and the current source trimmed out easier you can get this back and forh zener thing going on, especially if your meter does not have a high input Z.

they were stuck together with thermal pads and grease, better thermal than the epoxy.

i was shocked to find no coupling foil or shaved cases, like the DBX stuff.

this HP vacuum tube meter goes down to 1 mv AC Full Scale. just sits there, even with the cover off.
no shield around the wafer sw or tube or nothing. good for this type of matching.

one of the draw backs of this circuit is no tone control.
certainly , something could be kludged in, but i do not think that would be in the spirit of the original engineers.

did we not have the original engineer post up here, or we had a letter that he had written to one of the forum members? it was uhh....rather....."provacative", yes, that it. ;D

ok, looks like i opted out for some fancy smancy state of the art Burr Brown op amp,

some German stacked film caps, and ran the I/O Langevin style, passing the traces over each other for stability C.

you can see the gray thermal pad, just put a roach clip ocross the pairs, contact cement and Go.

looks like i shaved the current source triples.

optional 1/4 inch output jack so i don't have to F with silly patch bays.
UTC 66 equiv, external pwr supply and we are rockin.