Sounds like a grounding or wire routing issue. Can you post some pictures?

Thanks for reply. When I connected inputs to GND (i.e. connected all three together) there is still the disturbing noise, maybe weaker. It is very unstable; the first time I connected them, it was almost silent. When I tried connecting two and two, the noise was quite strong, and connecting all three produced a strong noise again. Connecting all three to cabinet ground does not help.

Here are two photos, one of the whole setup with Buff and Ivy disconnected. The other is a close-up of the noise-generating Sympatico.

As you see, I have changed the capacitors to EPCOS B41456, 40V, 22000 mF because I experienced the sound to be better with them.

Later today: I disconnected both Sympaticos and listened to Buff+Ivy through phones. Strangely enough the same pulsating noise is there even with no Sympatico connected. The transformers feeding the Sympaticos were also turned off. I measured the voltage: 5.50V to Buff and 15.06V to Ivy.

Does this suggest the grounding/wiring hypothesis?

Very thankful for any further help!

Edited by user Monday, May 2, 2011 4:12:04 AM(UTC)  | Reason: Not specified

It it most likely a wiring issue.

I would recommend, if you can, moving the small trafos for the digital section into the main chassis so they can get nice short runs to the PSUs. Running long AC lines all mixed together is a recipe for noise.

If you need to keep them remote, try twisting the secondaries (each pair) together and try to keep them separate.

The various signal lines need to be neatened up and kept apart as much as possible.

Thanks for suggestions. Sorry for the long text, but I need a few word to describe this.

I have now tested the Sympaticos alone and the Buff+Ivy alone, and experiences that both units produce this noise independently. Then I took everything upstairs and moved the small trafos into the main chassis as suggested by Brian. I also simplified the AC wiring as much as possible. The other wiring is more difficult to change because it is silver, - inflexible end expensive, but I did my best also here. Well, the result was anyway very good, no audible noise neither in speakers nor in headphones.

But, alas, when I moved it down to the living room, the noise came back. I then, by chance, noticed exactly the same background noise on the telephone. Well, I turned off all other electricity in my apartment; no wireless, no computers, etc., but without effect; noise persisted. So I must conclude that there probably is some electromagnetic radiation influencing my gear. I have another amplifier nearby which is completely silent, so in some way the Buff+Ivy+Sympatico is more sensitive.

Is there any way to shield it or make it less sensitive to such influences? Or can it still be something else?

Thanks for reply.

No, using the same outlet did not remove the noise. I also tried connecting the other amplifier to the same outlet in my living room, and it did not produce any noise. But when I connected it to the IVY's single ended output, the one I use for phones, the same noise came through this amplifier also. I have a dedicated 16amp circuit with one outlet for my stereo. The strange thing is that this particular noise suddenly arrived some days ago, and has stayed since then.

I have a KE Power Source Mains Conditioner (http://www.kempelektroniks.com/Line-Conditioners/Power-Source.aspx) which I use for the digital parts. The noise is not changing when I try all possible combinations with or without this filter, also including the Sympaticos.

Edited by user Wednesday, May 4, 2011 8:19:24 AM(UTC)  | Reason: Not specified