Hi Mark,
So my understanding is that:
(a)Amp on + preamp off = no noise.
(b)Amp on + amp inputs disconnected = no noise.
(c)Amp on + preamp on and muted = no noise.
(d)Amp on + preamp on and unmuted = noise. The noise level is independent of the volume control setting.
(e)The noise was present both before and after the recent upgrade of the amp. Dan checked the amp for any noise issues when he did the upgrade. So presumably the problem is not some defect in the amp that is degrading the common mode noise rejection that its balanced inputs should be providing, such as would occur if one of the two signals in the balanced signal pair for each channel were not being responded to.
If all of that is correct there are two possibilities that occur to me. Your guess is as good as mine as to which is most likely:
1)The unshielded cables are picking up the noise, in which case going to shielded balanced cables such as the Mogami's I suggested could be a good solution.
If that diagnosis is correct, the reason the noise disappears when the preamp is muted or turned off is probably that the preamp shunts its outputs to ground through a low impedance when it is muted or turned off. The low impedance essentially shorts out the inputs to the amp, reducing the amount of noise that is picked up and able to enter the amp.
2)The preamp has a problem in its circuitry somewhere between the volume control and the output, that affects both channels. Possibly a tube, as Michael (Uru975) suggested.
One more thing to try, if you haven't already, would be to disconnect the balanced interconnects at the preamp end, while leaving them connected to the amp, and then turning on all of the components so that they are generating whatever noise or interference they normally generate. If that results in a similar amount of noise through the speakers it would pretty much confirm that noise being picked up by the cables is the problem.
Best regards,
-- Al
So my understanding is that:
(a)Amp on + preamp off = no noise.
(b)Amp on + amp inputs disconnected = no noise.
(c)Amp on + preamp on and muted = no noise.
(d)Amp on + preamp on and unmuted = noise. The noise level is independent of the volume control setting.
(e)The noise was present both before and after the recent upgrade of the amp. Dan checked the amp for any noise issues when he did the upgrade. So presumably the problem is not some defect in the amp that is degrading the common mode noise rejection that its balanced inputs should be providing, such as would occur if one of the two signals in the balanced signal pair for each channel were not being responded to.
If all of that is correct there are two possibilities that occur to me. Your guess is as good as mine as to which is most likely:
1)The unshielded cables are picking up the noise, in which case going to shielded balanced cables such as the Mogami's I suggested could be a good solution.
If that diagnosis is correct, the reason the noise disappears when the preamp is muted or turned off is probably that the preamp shunts its outputs to ground through a low impedance when it is muted or turned off. The low impedance essentially shorts out the inputs to the amp, reducing the amount of noise that is picked up and able to enter the amp.
2)The preamp has a problem in its circuitry somewhere between the volume control and the output, that affects both channels. Possibly a tube, as Michael (Uru975) suggested.
One more thing to try, if you haven't already, would be to disconnect the balanced interconnects at the preamp end, while leaving them connected to the amp, and then turning on all of the components so that they are generating whatever noise or interference they normally generate. If that results in a similar amount of noise through the speakers it would pretty much confirm that noise being picked up by the cables is the problem.
Best regards,
-- Al