[Garbz]

Ok in 100000 words or less name all the sources of noise you can. GO.

I'm trying to locate the source of a noise in my current balanced amp. The noise extends back to my main amp but I don't think it's a ground loop.

Cases for ground loop:
Main amp only makes ground loop type noise when headphone amp is plugged in to it's tape outs.
Both amps dead silent when headphone amp is disconnected.

Cases against ground loop:
Signal is quieter when connected to my amp's single ended inputs. WHAT THE!
The signal is going through a transformer with the ground lift on

Lifting the mains earth wire from the chassis of the headphone amp does nothing.
The noise is still there (but a bit quieter) when using my ipod instead of the main amp's tape.

Something interesting is when i lift the ground loop breaker (two 1N4002s back to back) to disconnect the signal ground from mains earth in the only place they are connected, the noise changes in tonality, but not in volume.

FYI the system is built according to Rane's Notes on system interconnection. Earth to chassis, Pin 1 of balanced cable to chassis, and all other signal grounds star grounded to psu and then to chassis via a ground loop breaker.

 

[GIGANTOID]

The kids next door, I've thought of using a 'cable upgrade' on them.

 

[Francis_Vaughan]

Inductive pickup into the balanced line transformer? Try moving it, especially changing its orientation. This would fit most of your symptoms.

 

[Garbz]

What's it picking up from where? The transformers are in a separate chassis. I'll test this this arvo by using the pre-outs from my amp which are balanced, but unfortunatly volume dependant :S

 

[Garbz]

Hit it on the head. Definitly the transformers. Will play with them a bit more. The wiring inside got rather grotty, maybe it's time to re-work that.

 

[morsel]

Noise ≠ hum. Hum is usually due to a ground loop problem, which is usually due to connecting signal ground and AC ground in multiple places. It is optimal for safety to tie AC ground to cases. It is optimal for sound quality to tie signal ground to cases and not use AC ground at all. If cases with different grounds are touching, that's a ground loop. If signal ground is not tied to the case, that's no good either. You will have to put on your detective cap and sleuth it out. Try a 10Ω 1W resistor in parallel with a .1µF high voltage film cap instead of the diodes for ground loop breakers.

A balanced power system allows for use of miserably grounded gear without hum.

edit: Looks like you found the problem, noisy transformers, not ground loops?

 

[Garbz]

well my guess is it's a mix of dodgy transformer wiring and possibly how they are aligned.

I've followed all the correct methods to ground the gear. Both cases are kept at identical potentials through the shield in the balanced cable. Both amps (well not sure about the main amp but definitly my amp) is star grounded to the psu and then connected to the signal via a ground loop breaker. Rane Note 151: http://www.rane.com/note151.html Figure 4 is similar to my setup except no direct connection between the chassis and circuit. In any event playing with the ground loop breaker and the earth connection did bugger all.

Even with both amps connected via a balanced interface but not via the transformers there was still slight audible hum but it was below the noise floor of the psu. I.e. heard only at absolute max volume at the dead of night.

 

[Garbz]

Well the transformers are now hooked up as follows:

The humming is less so all i can assume is it's the spare wiring i've pulled out. Input ground and output ground doesn't give a consistent reading for any continuity so the final result coudln't posibly be a ground loop.

Incidently lifting the ground switch does create horrible humming.

/Edit: The symptoms change completely now.
I get horrible hum when i disconnect mains earth.
Disconnecting the ground loop breaker doesn't change the tonality at all now.
But the humming noise (also has a bit of a wowing sound too like an engine room or a crap turntable) is still only present when the amp is plugged in. My only other thing to test would be capacitors on the transformer inputs. DC on the amp side of the xformers may have an effect here.

 

[Garbz]

Ok something is still generating a loop, and it's not inductive pickup in the xformers.

Using ipod:
Perectly silent through active Balacned line driver.
Very slight non noticable hum through transformers.
With ground switch lifted perfectly silent.

Using amp's output:
Hum and pulsating noise identical though active BLD and xformers
With ground switch lifted hum is horrible. Completely opposite to the ipod!!!!

There is still hum comming from somewhere