DC entering my amp?

Thanks everyone. Will work to do some DC measurements and see what it turns up.

@itball Pay attention to what @gs5556 says- he is giving you really good advice for getting to the bottom of this. I would add that if the preamp isn't connected and the DC Offset at the speaker terminals goes up, then the amp is at fault.

@cleeds I only know because the amp has a particular protection for DC and that is what lights up. It has a separate overheating protection and that is not being triggered. Speaker wires are all banana terminated and all clear of strays (I checked them).

What I would do is check for DC offset at the speaker terminal outputs. Set a DMM to DC Volts and measure across the speaker cables at the amplifier binding posts with the amp on (for at least 5 minutes) and everything connected as normal, all on. Then turn off everything, disconnect the interconnect from the preamp. Turn on the amp, wait five minutes and measure again. If there is more DC with the preamp connected, that may be the issue. If the DC is the same in both cases then it's the amp.

If there is DC offset, it should not be more than a few millivolts with today's designs. If there is more, say 50mV or more, that is an indicator of a problem with the amp. Protection circuits should not activate if there is no problem with the amp so you may be looking at sending it to the manufacturer for diagnosis.

Once every week or two that function gets tripped, not necessarily while playing music. I use good quality cables and the connections are tight. I have a tube preamp ahead of the SPL so thinking that could be it.

@itball The amp could also have a noisy transistor or opamp at its input. If this is the case, disconnecting the input cables could make the problem worse. DC is a rare problem to encounter with a tube preamp!

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This device is to block DC on an AC line. That’s not the OP’s problem, which is ostensibly DC on the output.

If an amp’s DC protection is engaged, I’d check its output caps first. But how do you know it’s a DC fault that’s triggering protection? It could be overheating or a stray strand of speaker wire touching the wrong terminal, for example.

Maybe something like this would help?