Puzzling computer noise showing up in Crack. Also, separate soft hum.

[pkb]

Thanks so much for your help this far.

I have a few hours on the Crack and a few more questions to pose to the fantastic community here:

1) I'm planning on using this amp primarily if not exclusively with my computer. The computer makes very high-pitched processing noise that I can hear from across the room if it's quiet. This noise--very similar to the sound of tinnitus but distinguishable because it modulates and disappears based on whether or not the computer is processing (aka, playing music or running audio or video software). Certain programs will stop and start this noise, certain ones won't trigger it, and certain ones won't stop. This noise shows up, almost painfully after a few minutes, in the Crack.

At first, I thought the culprit was my line-out from my DAC (M-Audio Firewire Audiophile) because when I pulled my phones out of the crack and put them into the headphone jack on the DAC, the sound disappeared.

I checked this by plugging the crack into the headphone jack of the DAC. The sound was in fact slightly more pronounced from the headphone jack.

I then tried re-arranging how the active components (of which there are many--monitors, (audio) monitors, computer, Crack, other junk) to see whether I could find a configuration where the noise went away. No good.

I tried swapping the Crack to a different plug in the same room. No good.

I tried running an extension cord from the kitchen to the computer and plugging the Crack into that. Voila! Sound is gone.

Extension cords running hither and thither is not a sustainable solution because the lovely lady who lives with me a) lives with me and b) bought me the Crack kit to begin with. I don't want her to regret the fantastic gift. Has this problem come up for other Crack users? Has is been solved before? I couldn't find an account of it.

Second, and much less distracting:

When I turn the amp on and plug the headphones in, there's a very soft hum, lower frequency, which sounds my '68 Fender Deluxe when I turn it on and have my head stuck down there near the tubes and transformers. The manual mentions "tweaking the hum pot", but I'm pretty sure there's no hum pot in the Crack. This is way less of an issue than the computer noise.

[mosshorn]

I had the same issue, or at least I think I did. Does your sound modulate when you move the mouse around? Mine was a grounding issue, if your PC is built, try tightening all the screws (that includes motherboard mounts, PSU, drives, etc). This eliminated my problem.

[pkb]

I'll try tightening my screws.

In the meantime, this seems to have fixed the problem: http://www.radioshack.com/product/index.jsp?productId=2062214

It was suggested by this fairly comprehensive article: http://support.apple.com/kb/HT2743

The amp still has a soft low freq hum.

[Paul Joppa]

Yeah, computers often have noisy ground lines, and the Crack chassis/safety ground drains that noise through the interconnect grounds - which are not quite zero resistance, so the ground noise gets into the audio. Sometimes it makes noise even if the audio system is not connected to the computer at all.

Just to be sure, turn the volume control all the way down and disconnect the computer interconnect, to see if the hum is still there. And, if it is, move the Crack around (away from other electrical devices, especially any that have transformers or motors). These experiments will help identify the source of the hum - which is often a pretty messy process!

[Laudanum]

Im really amazed how quiet Crack is.  I mean, literally dead quiet.   So, I would be considering source ... I know computers can be troublesome for noise.  And if noise/hum persists not related to computer, then maybe check tubes, cables or solder joints.   There has been a lot of feedback regarding how black the background is with Crack.

Regarding the Rat Shack gizmo, it has been mentioned here before.  I used one years ago, it helps but basically destroyed low end response.  Another member here recently tried one and found it to be a sonic mess as well.  If it works for you with no degrading of the sound, then great.  But just a heads up, give it a good listen with and without.  The negatives arent subtle IME.

[pkb]

I tried totally blacking out the room. Everything turned off.

The hum remained. It goes away when I turn the amp off (completely unconnected, the exposed RCA jacks seem to act as antenna), but comes back as soon as the tubes warm up and start transmitting signal. The hum is loudest when the tubes have just begun amplifying and the sound tapers off a bit from there.

I've walked around a room and pointed the amp at (and away from) everything I could reach. The sound didn't change at any point.

The sound also doesn't change in relation to the volume setting. It's the same with the knob completely off and completely maxed.

It's a nice soft friendly hum, but still a hum.

[Doc B.]

What headphones are you using?

[Grainger49]

I tried totally blacking out the room. Everything turned off.

The hum remained. It goes away when I turn the amp off (completely unconnected, the exposed RCA jacks seem to act as antenna)   .  .  .  .   

Exactly, the proper way to test is with shorting plugs.  If you don't have any, you can make some.  In the mean time you can take a jumper (if you don't have any buy some, you always need some)  and short the center of each input RCA jack to the outer conductor of the RCA jack.  This will tell you if the "open" inputs cause hum or if it is inside the amp. 

If it turns out it is inside the amp post back and there is a list of points you need to check.

[pkb]

I'm using HD-650's.

What headphones are you using?

[Doc B.]

OK. I was wondering if maybe you had some uber-sensitive cans that might be magnifying the noise floor, but that is not the case. Soft hum generally indicates 60 Hz or 120Hz pickup of the mains AC somewhere, either via magnetic coupling from the transformer - which we have designed to eliminate - or from some other kind of coupling or leakage. Possibly one of the tubes has a bit of heater-cathode leakage, which can impart a bit of the AC heater voltage onto the signal via the tube's cathode. It might be worth trying different tubes if you have any spares. It could also be that the tubes just need to cook for several hours and form the cathodes to work out the bug.

[pkb]

Swapping the output tubes didn't have an effect on the hum.

What are the problem spots I can start investigating if we proceed with the assumption that the source of the hum is the amp?

Thanks!

[Doc B.]

The input tube could be making the noise. Do you have a spare to try there? I suppose another possibility might be a cold solder joint on one of the filter caps. You might try reflowing those connections.

[pkb]

I'll do as you suggest and report back.

Where do y'all get your tubes? Are there general or fundamental rules in tube purchasing, such as "avoid ___" or "I've always had good results when I _______"?

What a great resource, community, and product. Thank you!

[Grainger49]

Check this thread:

http://www.bottlehead.com/smf/index.php/topic,1864.0.html

[Yoder]

I was having a very similar problem with a tube DAC/Preamp I have. Like yours I could hear it across the room, and it seemed to be worse when things got hot. I tried all kinds of stuff, and then noticed that when I moved the power cord it stopped. Consequently, I took a small sharp metal object and scrapped the pins on the receptor and it was resolved.