Got Noise?

Hi Mark,

If you can hear hiss at the listening position while music is playing at any volume, something is definitely wrong. Notwithstanding the fact that the high sensitivity of your Daedalus speakers will reproduce any noise that is generated or introduced downstream of the volume control to higher levels than would result with less sensitive speakers. With my preamp muted, the hiss level from my Ulysses is almost imperceptible with my ear less than two inches from the tweeters. I do have a very slight ground loop-related hum, though.

Some thoughts and questions:

1)My guess is that the problem is unrelated to AC power quality.

2)Have you tried both positions of the "Gnd" switch on the back of the amplifier? N.B.: To prevent the possibility of loud transients, I would strongly suggest not changing the position of that switch while the amp is on, and waiting a minute or so after turning the amp off before changing its position.

3)What happens to the noise level if the preamp is turned on but is muted?

4)Are your interconnects between preamp and power amp shielded or unshielded, and balanced or unbalanced? Shielded and balanced would provide the best noise rejection. If that is not what you have, you might want to try some inexpensive Mogami Gold Studio cables, which are known for their excellent noise rejection.

5)If they are not already, route the interconnects with as much separation as possible from power cords or other possible sources of electrical noise.

6)Good suggestion by Mofi about disconnecting everything but the amp. For the results to be totally conclusive, though, you should put shorting plugs such as these on the amp inputs.

Hope that helps. Best regards,
-- Al

Hi Mark,

Before finalizing any further suggestions, I'd still like to know the answers to items 2 and 3 in my earlier post.

My tentative thought, pending those answers, is to suggest that you try the Mogami shielded and balanced cables I mentioned earlier. Changing to unbalanced cables might alter the sonics you presently have; would increase susceptibility to ground loop hum or other ground loop-related problems; and very conceivably might not help the hiss problem.

Best regards,
-- Al

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

05-10-12: Bombaywalla
Merely saying that unless the amp is on, the speakers are not driven thus any noise generated in the electronics will not be heard. This is obvious to me but no one else including Almarg stated it so, what am I missing here?

Yes, I noticed that too, and you're not missing anything :-)

Good comments in your post.

Best regards,
-- Al

Mofi,

Note that Mark indicated that "the level of noise is audible when I am playing music only at very low volumes." That would appear to indicate that the noise level is insensitive to the volume control setting, which would mean that it is not being caused by anything upstream of the volume control.

Best regards,
-- Al

amp on + preamp off = noise

OK. So that rules out the preamp as the source of the noise, leaving the cables as what should be focused on, as you are doing. If those efforts aren't successful, the Mogami's (or other shielded balanced cables) would seem to be the logical next step.

Best regards,
-- Al