Unusual amp from 1987

[stanislav1957]

Found service manual for real Revox 285.
Different transistors. Rails 52V

I guess original printout did not show transistor types or values, and someone added it manually.
Luckily, Q109 is not BC327

add a repeater

 

[stanislav1957]

I measured the loop gain

 

[minek123]

This is my latest results.

I also tried Jfet input - it looks like it's a drop-in replacement.
Sim attached (for both: BJT and JefFet input)
LSK170A/B work just fine, same results overall as with BJTs.
I checked my stash of Jfets - I have LSK170 grade D.
Unfortunately grade C and D does not work in this simulation.
I thought that since we have LTP coscoded here, this is great opportunity to use Jfets..
I also tried Revox voltage shifter with a resistor - bad idea.

 

[stanislav1957]

That's what I meant

 

[minek123]

??

 

[stanislav1957]

wrong file.
R23. I changed the inclusion of R23 .

 

[minek123]

OK. Thanks Maxim!


Otherwise, everything else look OK?
Any idea why LSK170 Jfets grade C/D don't work, but grade A are OK?

 

[stanislav1957]

All right. That's the question. Common mode voltage. Ignore or fight ?

 

[minek123]

>All right. That's the question. Common mode voltage. Ignore or fight ?

I'm going to assume this is rhetorical question
If you don't know the answer, how would I know?

 

[stanislav1957]

>All right. That's the question. Common mode voltage. Ignore or fight ?

I'm going to assume this is rhetorical question
If you don't know the answer, how would I know?

2 ways.
1) enter feedback for tracking.
2 . to fix the problem.Switch scheme the amplifier to the inverting option.
1 option will complicate the scheme of the amplifier.
3 just ignore the problem.

 

[minek123]

I went back to posts Hayk (kokoriantz) hinted about few days ago.
Eg. post #75

Here is the sim of it. It's simple, symmetrical (that's disputable if this is good or bad ), perhaps something can be made of it?
What you think of it?
2 * CCS can be added on top and bottom of LTP, and maybe 2 buffers
between LTP and VAS?
As is, distortions are little bit too high, I guess, but it's simple.

 

[stanislav1957]

I experimented with the famous Otala scheme in MS-9. The principle is the same

 

[minek123]

Slew Rate is the same as LMK/Wiederhold (~ 15V/us).
So no improvement...

 

[stanislav1957]

I'm certainly not a Pro, just an Amateur.But in my opinion it is necessary to make amplifiers with this frequency response

 

[moschfet]

I have a similar circuit, an inverted amp, ~ 0,006 THD, but a little to high DC:

 

[minek123]

This my attempt to make do with low-current LTP (1.2 mA each transistor).
Seems comparable to high-current LTP with cascode - but perhaps I don't look at all the numbers I should be looking...
SlewRate is over 30V/us.

 

[minek123]

I have a similar circuit, an inverted amp, ~ 0,006 THD, but a little to high DC:

This is CFA ?
I guess not...

 

[stanislav1957]

A simple amplifier with a high slew rate voltage

 

[moschfet]

Yes, I think so.

 

[minek123]

>A simple amplifier with a high slew rate voltage
This one is CFA amp!
That's why it needs servo... notorious DC offset issues.