Quadral PA 03 Phonologue

[Upupa Epops]

I am searching for schematic of this old " excavation ". BTW, designer of this equipment was horrible goop...

[bobodioulasso]

I reverse engineered this module.
You know what?
It sounds nice, dead quiet and stable.
It sounds a tad like an dh220 hafler but a lot more silent and with a better bottom..
I tried it on 50v rails. (Ithink 2x60V would be the right value.)
R8 and R9 set quiescent and offset. Actually 100ma per device, offset is 100mV with R8=R9.
All resistors are 1%
Q1 to Q10 are BC550/560
Video transistors are used as drivers.
I do not understand why C7 has a so large value, though.
What are C10 and P1 for?
Any comments on this schematic?

[bobodioulasso]

I realised i might be wrong about Iq and offset trimming.
Advice needed.
R8 and R9 are fixed values on this board. I (wrongly?) draw them variable as i empirically moved the offset by trimming one of them..

Cdoms were ceramics, i replaced them using silver micas.
Under 47pf Cdom and with no output zobel neither output self, it oscillates.
I forgot to say: No turn on thump.

[bobodioulasso]

I have to apologise about the mistakes in the previous schematic.
After a deeper looking at the pcb , i finally draw a coherent schematic.
P1 sets the output bias.
I added a little circuit to set the offset: R4, C4, P2 and the two 1n4148.

[bobodioulasso]

Still a big error.
The loading resistors are ,of course, in the other collectors -positive input side- of the LTPs.

After readings about inverting amps, i realise the added offset setting circuit is unnecessary.

Adjusting R15 should do it.

I'll try it after replacing the LTP pairs by matched transistors.

...Still are missing resistors 47R inserted in the rails for supplying the front end and relative decoupling electrolytics caps...
See the first schematic.

[bobodioulasso]

Here it is. I Hope so.

[Tekko]

Feedback looks weird.

[wahab]

Originally Posted by Tekko

Feedback looks weird.

It s an inverting amp...

[bobodioulasso]

It is an inverting amp.
So, R1 seems unnecessary.

What do yo think about the little tail current?
Shouldn'it be higher?

[bobodioulasso]

This amp is built on the basis of the Motorola application note AN 1308.
Plus
Front end cascoded.
Inverting.
Use of video transistors in vas and drivers.
Lateral mosfets outputs.
Vas more lightly loaded.

I am tempted to try Jfets in the input. (cascoding at 24v.) adjusting the tail current at 6ma and adjusting R10 and R11 to keep the same voltage drop on them.