I just installed a remote controled stepped shunt attenuator (http://www.welbornelabs.com/remote.htm) at the output of my active xo (between the xo and the 2 amps) and a loud hum/buzz of power line freq. (50Hz modulated) appeared. No hum before installing this attenuator.

The hum stays constant regardless of the volume settings of the attenuator. It is reaching both amps (biamping) and disappears when the amps inputs are disconnected (so, the amps are ok).

The hum is present even when the XO and attenuator power supplies are off, as long as there is connection between the attenuator and the amp. The hum is not affected by removing/replacing the connection of a source (so no sourse issue).

At the XO and the amps I have signal ground and power grond connected to cabinets, which are all earthed via the power cables (all equipment using same power outlet).

I asked welborne for support as the attenuator does not operate adequately (mute not functioning and the volume is either zero or steady - each step up keeps the volume the same, only at zero, there is no sound). However, no reply yet, and probably the ground loop / hum issue is on top of the rest of trouble.

Please help fixing this nightmare.

a new finding: the hum stays when I unplug the interconnect at the XO (preMP) side but disappears if I unplug at the amp side. this happens in each channel, each of the 2 amps. Seems the cable works as an antenna. But it is shielded and is not long (1m)! No power cables around, very strange, I'm lost...

Hi traf:

It appears that the problem is with the attenuator, since unplugging the connection between the attenuator and the amps removes the hum, while unplugging the connection between the xo and the attenuator (but keeping the connection to the amp intact) does nothing to alleviate it.

But I have more basic questions, if you will:

The output from the active XO probably consists of a high-pass and a low-pass signal (split at the xo frequency). The high-pass and the low-pass signals probably share a common ground. Is this correct?
Is the attenuator applied on the high-pass signal, or the low-pass signal, or do you have separate attenuators on each?
What happens if you connect the xo outputs directly to your amps without any attenuator in the middle?


If you can make a quick sketch of your connections, and scan and post an image, it might help in troubleshooting.

Regards,

Ashok

Many thanks for the effort.

As I noted in my second post, the hum remains even when I disconnect everything from the amp BUT the cable (no XO, no attenuator connected).

When I remove the 1m cable from the amp (only cable at the input of the amp, nothing on the other end of the cable, tried several types of cables) the hum is gone.

I also experimented with the cable connected to the amp and a CDP (not AC powered/plugged) and the hum is back!

as to the setup: yes, it is a low/high pass active XO and the 4 channel attenuator is at the OUTPUT of the XO (between XO and 2 amps).

I also connected to the amp a cable with another attenuator and nothing else. It humms and the attenuator reduces the hum only slightly, when turned to min.

Both amps hum, so it's not them. Furthermore, I used so far these 2 amps, one by one, w/o active XO, w/o any hum (same ic cables).

more results:

Made several tests with both amps and various cables/options. The second amp humms w/o cable, the hum could be reduced to almost acceptable levels when a particular cable with a pot at 12 o'clock is placed (nothing else). When I plugged the other end of the cable into a dvd output (not powered, just to use its shield/cabinet) the hum increased!!! but was well below the other channel where there was no cable at the input of the amp.

summary: each amp humms but responds differently to the same conditions, one being dead quiet w/o any cable at input and noisy when a cable is attached; the other amp acted opposite (no cable, more hum).

seems that any minor change in the cabling has rather unpredicted but big impact.

My high-end ic is 3 twisted silver wires, one used as a shield. The shield wire is soldered to the 0 signal wire at the XO output but NOT at the attenuator input. Likewise, the shield wire between the attenuator and the amp is connected to the 0 (signal ground) only at the amp side. So, the shield is not attached to the attenuator, but only to the xo and the amp, and is interrupted at the point of the attenuator. Should I modify those one end connections of the shield?

forgot to answer your Q on the low/high pass grounds. Yes, they are shared at the XO as the signal grounds are connected to power/shassi ground.

I did not connect the XO directly (w/o the attenuator) as this would require a lot of de/soldering and hope it is not needed, given the latest mixed resuts I reported, indicating that the issue is about wiring rather than the xo or the attenuator.

Latsest results:

Once I separated the audio/power ground from the earth/cabinet, the loop between one of the amps and the xo dissapeared, and so is thu hum. My concern is if this separation is ok, having in mind that i power those units by a balanced power transformer with its center tap connected to the wall receptacle's earth.

As to the amp No 2 (active XO and 2 amps) there is no loop, as the hum is there immediately after I attach any cable (only cable) to its input!!??? RFI?
Here I am lost.

I hoped for more help, although it is diffficult to cure loops at a distance.

Latest results: Almost all hum gone after several experiments with lifts and cables. Seems my solution is the single ground option, found here: http://www.trinitysoundcompany.com/grounding.html

My intention is to have the single earthing of the audio/power gronds at the active XO unit, as it is in the middle (CDP-XO-2AMPS). Does it really matter which unit?

After a week of strugle, I found the origin of the hum at one of the amps. It is a DIY Thel Accuphase100 2 monoblocks amp, dual power supply. In addition to the DC powe supply, the amp is fed by ac from the transformer secondaries, to operate in A class, as far as I remember.

The hum slightly varies in amplitude/freq, just in sinc with the audible accoustic buzz of the big tranny. Not affected by the pot and stays there w/o any interconnect attached. Louder in one channel.

I searched the forums and found only one very similar issue, but no solution, at:

http://www.audiokarma.org/forums/showthread.php?t=108181&highlight=transformer+hum&page=2

When I touch the insulation of the internal input cable, or the ac cable feeding the amp board (comes from the secondaries), or bring those 2 cables closer to each other, the hum is up.

Then I shielded (shield to ground) both the signal input cable and the ac wire at the louder-hum channel and each of those 2 steps significantly reduced the hum, to the relatively low level humming of the other channel.

I tested many other options (chassis ground lifts incl.) but w/o difference. Any ideas?

P.S. Feeding the amp directly or via my monster mains balanced power supply does not make any difference (so it should not be a mains DC issue, right?)

I found the problem: a solder leak, created a very short "antenna" at the input terminal. This antenna picked-up the hum from the ac/power/transformers.