This one's a bugger, I think?

[larhinds]

The left channel of a stereo amp system in an old analoge synth exhibits hiss. The strange thing is the hiss is not present when the volume fader is all the way up [0 attenuation]. As you pull the fader down @ about a 10 of the way down the hiss appears and continues to increase until it's at it's loudest around 3 tenths of the way down. As you continue fading the hiss fades off eventually to nothing. Some of the details of the circuit are as follows. Output of preamp from a MC1439G op amp to the fader, through a resistor and a .1 coupling cap into the amp driver, another MC1439G. Out of the driver to 2 power transistors in what appears to be a push pull configuration. The output of the transistor set go to the speaker, but also to a paralleled resistor, capacitor combo, back into the input of the driver Op Amp. Feedback I suppose. I've replaced the amp driver op amp, coupling cap and the feedback cap. One curious thing is there appears to be 275mv DC at the out of the misbehaving preamp Op Amp. .07mv DC on the good side.

I would appreciate any help.

 

[KrisBlueNZ]

Hi larhinds and welcome to the forums

The strange behaviour you describe regarding the volume control position could be due to noise coupled to the input of the stage that follows the volume control (or generated within the op-amp of that stage).

The volume control is a variable voltage divider. At the top and bottom ends, the wiper is shorted to the output of the previous stage, or ground, respectively, so there is a low-impedance signal presented to the input of the following stage, which will "overpower" any other signal that's being weakly coupled into that input. Towards the middle of the volume control range, the impedance of the signal is higher, since there are two resistances involved (the resistance between the top of the volume pot and the wiper, and the resistance between the wiper and ground).

On the assumption that the noise is probably being generated inside the op-amp, you could replace the op-amp in the following stage, or swap the two identical devices over. The noise could also be getting in from some other source but I'd suspect the op-amp first.

 

[larhinds]

Thanks KrisBlueNZ for the fast and comprehensive reply. What you explained definitely clears up a few things. Since I already replaced the driver IC I think the next order of business is to replace the preamp IC. Wish me luck.