Humming and static noise, Plinius SA 100 MkIII

I'm not sure the source of the hiss you hear is actually from your amp.

Consider that when turned on an amp is at full power all of the time. It's noise floor, to the extent it is evident, will be constant. It will not be affected by the position of the volume control if your preamp is off. What you hear with the pre-amp off probably represents the noise floor of your amp.

When you turn your preamp on it is also at full power as seen by your amp, even with the volume control minimized. The amplifying stage of a pre-amp is NOT affected by the volume control which preceeds it in the signal path. If you hear a hiss now, that wasn't there before, you are hearing the noise floor of the preamp.

If you have no noise until you turn on (and select) a source and you then note that the noise increases with rotation of the volume control, you are hearing the noise floor of the source (or the recording playing on the source). All your volume control affects is the level of the gain from the source before that signal reaches the amplification section of the preamp. It does not actually control the preamp's output gain which is constant.

That might explain why your hiss varies with movements of the volume control and exists even when you are using different preamps.

Something to think about..........

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.
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