Differential amp in the Sony CDP-911.
 

Hi all...I have been posting at the tubes section about converting what I suspect is a ne5532 used as a differential amp..into a tube differential amp..

Can anyone confirm that it is a differential configuration..or better even post or mail me a schematic of said cdplayer?..


Cheers,
Bas

Hi Bas,

Many of japanese low-fi (or mid-fi) CDP uses NE5532 for the OP-Amp. The higher levels usually use OPA.

These CDPs have the same circuit topology. Right after the Digital To Analog converter, you will found the OP-amp, after the OP-amp there are 2 transistors configured almost similar like differential pairs. These transistors are for muting purposes (to avoid unwanted pop during turn on and off, in place of a relay for expensive CDPs). Removing these transistors will improve the sound performance.

May be this is what you were talking about? :idea:

OK, I’ve skimmed your thread on tube forum.

I noticed that you want to replace the amplifier section of your CDP with a tube (PF86).

I’m not an expert, so I will not try to answer your question regarding how to implement the upgrade. Rather, I will comment and give my opinion on your post.

I think there had been a mis-analogy between OP-amp circuit and tube circuit or whatsoever (grid stopper, differential, balanced terms). As far as I can remember, NE 5532 is a dual (gain stage) OP-amp with 8 pins. 2 pins for power supply, 3 pins for each gain stage. In each gain stage, 1 pin is the output, 2 pins are the input, of which 1 is reffered to ground (depend on the phase invertion wanted) and 1 for incoming signal.

I think what you saw as 19K9 on pin 2 and 3 are the resistors connecting each stage’s input to ground.

If you want to replace the OP-amp with a tube amplification, that will be a total upgrade! Then, why you have to be bothered with the OP-amp circuit? To me, this is just like creating a new amplifier (you may want to arrange your tube in a single ended arrangement)

Personally, I don’t think that replacing an OP-amp with a single tube is an easy task, unless improvement is not a necessary constraint. But you can ask for other's work and do a "plug and play" with a little modification.

Bas, I think I made a mistake. You must be talking about the input to the first gain stage of the OP-amp! If anyone else don't post a better picture, later I will.

But this is my simple drawing:

Hi Jay,

Yep, I think that is what is happening. That is a differential amp right? If so I should be able to build a tube version for it.

Thanks.

Regards,
Bas

Yes, the circuit is a diferential amplifier, although I'm not so sure what is the real function of the differential amplifier here (I only know feedback and "vocal cutting" implementation).

I have modified my CDP only starting from the OP-amp, so I don't know what the DAC really do with the input signal. But I have seen in the net (I have tried to find it again with no success) the modification with a tube so you are right that you can do it. But, whether you HAVE TO build a differential amp also (with the tube), that is what I don't understand. I seen only 1 tube (in the picture, not in the schematic), may be it was a dual triode.

Regards,
Jay

Hi Jay,

It is very likely that it is indeed a dual triode...ie 1 tube.

Regards,
Bas

As you have a differential output from the DAC, you will need a differential circuit. Have a look at Thorsten's site:

http://thunderstoneaudio.nav.to/

And look for:

Thermionic Valve Analogue Stages for Digital Audio

There he explains how to proceed with DACs having differential outputs and provides a tube circuit.


Carlos

Obrigado!