Humming and static noise, Plinius SA 100 MkIII

This amp (I have had two of them) does not behave as well in bridged mono as in regular 2-channel stereo. I couldn't use mine in mono at all because of distortion problems.
Try running the amps in stereo, instead, and see what happens.

Thank you for your input. I'll try some of the suggestions including using one amp as stereo and post the results. Thanks again.

Using the same, in bridged mono on 91 dB speakers. And that's along with a Genesis/NHT active x'over/1600W bass amp. Never heard anything more than a slight pop, very rarely. I've had a stethoscope on them. I'm also using a 5KVA transformer coming off 240V stepped down to balanced 120V just for the SA100's. Balanced AC doesn't really bring anything to the Plinius amps as I've tried it as a isolation transformer as well with no discernible difference, and that does have the advantage of not derating the transformer by half. Has generally been more beneficial for sources and preamps than amplifiers.

Strictly using an isolation transformer will eliminate DC offset but that may also just mean relocating the noise from the amps to the iso. There are cheaper ways to eliminate DC, like PS Audio Humbusters but transformers have other advantages. Some have used Topaz/Xentech iso's near the breaker panel along with/without dedicated lines. The Plinius draws 500W each but an iso transformer should be at least double that capacity.

The Plinius is very high gain (38 dB, where "normal" is 25 to 29) in bridged mode with XLR cables, which accounts for the noise at a high volume setting. I'm using a passive TVC "preamp". Not only is no preamp gain required, the TVC provides galvanic isolation.

For myself, the difference between stereo and bridged mono was in the nuance. It's possible, with lower impedance/more reactive speaker loads or mismatched components, bridged mode might not be the best choice. In my case, the speakers are a fairly benign, mostly resistive load but only 4 ohms, which a bridged amp "sees" more like 2 ohms. Do your own research on that.

Before the MkIII, Plinius had a reputation for mechanical hum from the large internal transformer, particularly after one magazine review. The MkIII addressed this by using 2 smaller transformers. It's something that can affect any amp but bigger amps more so.

All the above is assuming you've already gone through all the ground loop/ cable organizing/ dimmer switch/ etc. protocols. Also assuming that you're using XLR cables and taking advantage of differential operation.

The way I did it... cheaply.
http://mysite.verizon.net/vze22yzp/id10.html

Thank you all for excellent advice although some of it just went over my head. I did one more experiment and changed amp on one side. A VAC tubed amp. The hissing noise was still there at high volumes without any input from source. So somewher, I am picking up the noise where than is amplified by the amps.

BTW, I am using a Richard Grey Power Station and RCA connectors since my preamps don't have XLR connection.

OK. I'm going to assume that my comments were the confusing ones. Here is a simple method for locating your noise.

1) Turn on your amps only, but have them connected to the pre-amp. Any noise?

2) Assuming no, now turn on your pre-amp with volume minimized. Any noise now?

3) Assuming no, now with the source (like a CDP) turned on, but with Volume control minimized is there any noise now?

4)Assuming no, now with source turned on but not playing anything increase your volume control. You hear the noise now?

If the answer doesn't become obvious to you then, feed back with results and I'll tell you what I think might be involved.