RCA input hum [solved]

[jabbr]

Just built my crack. Sounds great. When RCAs not connected, no hum even at high gain (using valab 23 stepped attenuator and valab rca connectors). When I connect cables there is a hum at high gain. It differs depending on cables but present. Probably not audible when music playing but this bugs me nonetheless. I'm using a Sound Devices USBpre2 as a DAC, and when using its outputs, there is no hum at similar gain.

So... planning on adding a balanced input.  I have a couple of Jensen 10KB-D transformers that should do the trick.

Any suggestions? Any better solutions?

[JamieMcC]

You could try winding the rca cables that run from your dac to your Crack together so they have a few twists round each other see if that helps. It worked for me.

[Doc B.]

An alternative would be to get better shielded single ended cables, and figure out what is causing the hum pickup.

[Grainger49]

There is no mention of shorting the inputs to determine that the hum is external to the Crack.  If that is what you find, the Crack is silent with the inputs shorted, then find a double shielded cable.  Unbalanced shielded cables use the shield as the signal return.  With a second shield you can dedicate the outer shield for noise abatement.

[Doc B.]

He said there is no hum with open inputs and no cabes attached, so I think we can already conclude that there would be no hum with the inputs shorted.

[jabbr]

When I connect each end of the same RCA cable (~1m) to the right and left RCA inputs there is a faint hum ... no hum at all without anything connected even at max volume.

The hum is only audible with the attenuator turned up about 75% -- the hum does vary with different cables.

I've tried a commercial monoprice cable.
I've tried a shielded silver plated copper twisted pair (22g) (M27500-22rc2s06)
I've tried a solid shielded microwave cable.
I've tried unshielded litz braided quad 26 awg silver plated copper.

I've resoldered RCA inputs.
I can buy some Blue Jeans/Belden 1800F cables?

[Grainger49]

It still sounds like the cable is picking something up in the air.  Look 2 posts up.  The double shield might help.

[Doc B.]

Maybe you should try moving gear around to see if you can get whatever the cables are picking up hum from positioned a little father away from the cables.

[jabbr]

I've ordered a cable based on Belden 1505F (double braided). Will try.

The crack is about 2 feet above my computer and 18" above my DAC. It is aside my 4K monitor.

I think I also need to make shorted RCA plugs to properly test this out. I want to be sure that its nothing inside the Crack ... I haven't built a kit in several decades so theres a good chance I've messed something up. I notice that there is a soft hum and a soft tick about once per sec (like a clock).

Assuming my cables really are picking up something, isn't the proper way to go balanced? Just trying to understand.

[Doc B.]

The tick tends to indicate a digital source is creating some of your interference. Could be the computer. Have you tried moving the gear around a little? Not trying to be repetitive, but I have found a fairly common theme on the forum these days that we make a suggestion to try something but we don't hear back if the OP tried it. It helps a lot if we can use a logical flow of tests to get to the desired result.

[jabbr]

Thanks, I tried moving the DAC and Crack around but it doesn't modulate the sound.

[Doc B.]

Is that 75% volume level setting louder than you can listen or is it in the useful listening range? Sometimes the hum can be hard to isolate because it is coming from more than one place. You might want to check to see if the hum changes character when the DAC is connected or disconnected to the input cables. This doesn't always work because sometimes the cables will be super noisy if disconnected at the input end. But if the noise gets a little louder when the DAC is connected to the cables than when it is disconnected it means a little of the DAC noise floor might be contributing to the noise.

[jabbr]

With the cables disconnected from the DAC the noise is much less but still audible at the two highest steps of the attenuator.
The noise varies with positioning of the cables and reduces when the cables are directed away from the digital devices and electronics.
When I connect my headphone directly to the DAC output it is silent even the output of the DAC turned all the way up.

[Doc B.]

The Crack is amplifying the output of the DAC, so yes, you will hear the DAC noise floor more through the Crack turned way up than directly from the DAC line level output. If the DAC has separate gain stage for the headphone output then it might still have less gain into your headphones than the Crack.

 "highest steps of the attenuator" - are those steps ones that you find a useable listening level, or are they too loud for listening? I'm trying to get a feel for whether this noise is super intrusive, mildy annoying at higher volume settings, or only there when you crank it to the top with no signal present.

[jabbr]

I resoldered everything. The noise with only free floating cables attached seems less, and when I connect both ends of the same cable to right and left, only very faint at highest step and barely at second highest.

The highest steps of the attenuator are way higher than my usable listening level. Noise is only audible when no signal present at 6 highest steps.