Got Noise?

I played around a little last night and here is where we are:

I confirmed that the noise is ONLY present when the amp is on. I can turn on any/all other components and don't hear the hissing sound. Disconnecting the amp from the rest of the components (ala Mofi) eliminated the noise. That leaves the IC's as a prime suspect and me with a possible answer to the problem. I'm using balanced/unshielded Empirical Design 422 interconnects AND they are (by necessity) routed very close to the 4 gang outlet box where all of the components are plugged into the wall. In fact, one of the wires is literally resting against the power cord for the amp about an inch away from the XLR connector. (BTW, I wasn't oblivious to the potential problem here but given my room/setup and the way the XLR connector wants to "sit" when correctly rotated and seated into the amp receptical it is difficult to avoid this result). So, this morning I moved the power cables around as much as possible to get them away from the IC in question and things improved--somewhat (I'm still not happy). Options? I could stay with balanced IC's but go with a cable with better shielding OR go with a pair of single ended IC's which not only move the cable away from the power cables (slightly) but also allow rotation which should give me (slightly) more wiggle room OR move the rack away from the wall a few inches to gain more leeway OR a combination of the above OR one of your ideas which I have not yet thought of. Thoughts? Many thanks as always.

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

Sorry, Al, I forgot to include answers to 2 and 3 in my reply. 2) Flipping the ground switch introduced an additional hum so I moved it back to the original position. 3)Muting the preamp eliminated the hissing noise. Let me know if this alters your suggestion for a cure and thanks.

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: Dodgealum
I played around a little last night and here is where we are:

I confirmed that the noise is ONLY present when the amp is on. I can turn on any/all other components and don't hear the hissing sound.
.....

According to me, you'll only hear the hiss/noise once the amplifier is on 'cuz the amp is the only piece of electronics driving the speaker. Amp not on, no noise from the speakers.
Of course, this does not necessarily mean that the amp itself has a problem. 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?

So, this morning I moved the power cables around as much as possible to get them away from the IC in question and things improved--somewhat (I'm still not happy). ....

to me this points to unshielded interconnect cables & to the preamp (since Dodgealum is not fully satisfied with the reduction in noise indicating that the interconnects are not the full source of the problem).
I believe that Almarg already stated this in his latest post.

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.

yeah, this would be a good test to see if the cables are acting like antenna picking up RF noise.
My gut feeling is that balanced interconnects should not be the issue. Afterall balanced cables are used for the very fact that they are balanced hence noise cancelling. They perform remarkably well in (RF) noisy environments such as pro studios. If a balanced interconnect (even tho' unshielded) is being a noise antenna, something does not smell right to me.....

My brother once had a similar issue - the single-ended cables in his system were picking up signals from a FM station & creating a low level signal output from his speakers. Once he switched to balanced interconnects (I forgot shielded or not) the problem was fixed. Here Dodgealum is already using balanced cables.....

I have a strong hunch that it's the preamp. To me all fingers point to that device. As Almarg wrote:

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.

each time the preamp signal is not allowed to reach the power amp, there is no noise.

IMHO. FWIW.