Crack with oscillating fuzzy scratchy noise

[grocerylist]

Hey Bottleheads,

I just finished building my Crack (no speedball yet). I'm noticing an oscillating scratchy fuzzy noise (maybe ~2 seconds of very fast static followed by a longer steady static of ~.25s not sure if that makes any sense but that's the only way I can think to describe it) at low volumes (can't be heard over audio when the volume is louder), when audio is paused and when no audio source is connected.  I've moved the amp to another outlet (with no audio source connected) where I don't have as many electronics and still hear this noise, however it seemed to be more pronounced when the volume was at zero or very low and as the volume increased it went away slightly. I did forgo the stock pot for an Alps blue velvet rk27 but after reading the forums here I bought the lugged version off ebay which I'm now realizing is a counterfeit. Not sure if the fake alps pot could add this noise? I've since ordered an alps pot from a reliable supplier and hope to replace it once I complete my speedball upgrade.

Also, if I move my long old cheapo radio shack rca cables around a bit and hold them steady I can get the noise to go away. I'm not sure if this would indicate the cables are poor (as I've not got another device to test them on) or that something in my RCA inputs needs to be looked at closer? I'm awaiting delivery on some better RCA cables but they won't arrive for a little while.

Does any of this sound like a common issue with construction and any ideas what I might ought to look into? I'm happy to take some pictures of my connections if it might help troubleshooting.

BTW all resistance and voltages measurements were in line with expected values.

Appreciate any input!

Thanks,
grocerylist

[grocerylist]

This is interesting to me...

I just noticed that if I put my hand over the vent holes and between the two tubes that the sound goes away, without touching anything. And if I put one hand above the vent between the two tubes while moving the rca cables I don't hear the noise. Does that make any sense?

[Doc B.]

Sounds like you are picking up noise thru the cable shields. Unplug the cables from the Crack and listen again. If the noise goes away you might need cables with better shielding. If the noise does not go away you can try shorting the inputs by connecting the centerpin of each RCA jack to the ground lug of the jack. If it goes way then, you are definitely picking up noise from elsewhere and cabling with a good shield will most likely solve the problem.

The other part of the solution is to determine the source of the noise. Computers, phones, appliances, light dimmers can all create the kind of noise you describe. If you have any of those in your listening environment try turning them off to see if the noise changes.

[grocerylist]

Even with no audio source connected to the rca inputs I was getting the noise.
I think I've traced the noise source to a noisy network switch, I unplugged this and it went away. I also experienced the noise when that switch was powered up but with my crack in a different room. I'll have to try other outlets around the house to see if I can find another outlet not on the same circuit breaker.

It's not extremely intrusive but if I could get rid of it I would. Unfortunately my listening area is near my computer and a couple other servers and network devices, which I'm not really able to turn off. I also use a powerline network adapter nearby which could cause some noise on this circuit breaker.  I don't suppose the Speedball upgrade would by chance as a side effect remove noises like this?

[Paul Birkeland]

I also use a powerline network adapter nearby which could cause some noise on this circuit breaker.

Yes, these put noise on the AC line.  The Speedball will not get rid of it.  A line condition just might though!

[grocerylist]

Thanks for the tips Doc & Paul, I'm loving it loving it so far!

[Paul Birkeland]

I have determined that a simple ground breaker can be installed in the Crack to prevent this noise from being an issue.  It's just two antiphase diodes that take the place of the buss wire between the IEC ground lug and the chassis grounding tab. 

If you're going to buy diodes to do this, I would recommend a 2A rated diode that is not a Schottky diode.  This is also a place where you don't want to accidentally use zener diodes.  STTH2R06 looks like a good choice for this.

[fromnowon]

This is great!
Really simple and cheap too.
Thanks for continuing to think about it and work on it.

[Paul Birkeland]

Thanks for being patient.  I've had this thread open in a separate browser tab for quite a while, and honestly I was really dreading having to tell everybody to go blow a bunch of money on an isolation transformer.