Crack switch conundrum

[Paul Birkeland]

It becomes audible around 50% volume. Is that just a case of "normal" noise (i.e. within design tolerance) or should I be hunting for ground loops? (If so, how do I do that?  )

Depending a bit on the headphones, 50% of the travel on the volume control may yield an unbearably loud amount of output with headphones connected, which would indicate that you are just hearing the nosie floor.

[Loquah]

I'm using HD650s. At about 50-55% seems to be my sweet spot and I'm not a loud listener. My source was an LOD from an iPod Video. Does this sound like I'm not getting full output or is that normal listening level for HD650s?

[Paul Joppa]

If it goes up with the volume control, then it must be coming in before the volume control. Ground current noise adds to the signal and does not change with the volume setting, so it must be on the input signal line. It could of course be the source, or (more likely I would guess) it's picked up by the circuitry or interconnects.

To check, first use shorting plugs on the input and see if that kills it. Try at all volume settings; if it is gone at zero and at full volume but loudest around 80% of the volume control then it is picked up at the volume control output.

If it is gone with shorting plugs, try again with nothing plugged into the inputs. If it is quiet at zero volume and maximum at max volume, then it it picked up by the wiring between the input jacks and the volume control.

[Loquah]

Thanks PJ.

I tried disconnecting my braided wiring from the RCAs to the pot and the sound went.

I've replaced the braid with a high quality shielded interconnect cable and the hum is completely gone. I also made a direct connection from the RCA grounding pins to the main earth on the chassis just to be sure.

There does still seem to be the slightest hint of hum. It's not altered by the volume control and seems to just be system hum. Any thoughts on removing this or is a tiny bit of hum (i.e. only just within audible limits) normal? I can only really tell it's there by switching the amp off and back on again (i.e. I notice it by subtraction more so than by addition).