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High impedance inputs - open circuit

For some unknown reason, I recently decided to read the Owner’s Manual for my tube mono-blocks that have been in my system years: CR Developments Artemis. The manual states:
This model has high impedance signal inputs. It is important that the inputs are not left ‘open circuit’ or unconnected. Open inputs may cause instability or spurious oscillations. Do no insert or remove signal connections while the amplifier is on.
Ooops! I’ve done both. While the amps were on, I have been swapping out the input IC’s (balanced) when A/B’ing cables for comparison. I’ve also removed the IC’s when manually biasing the tubes. At least, I always did leave the speaker cables connected.

How can I tell if there’s any damage. The amps seem to work and sound fine. There’s no distortion of any kind. One issue that I’ve noticed is that sometimes they don’t go as loud as I recall. Some CD’s still go very loud and other not so much. Then again, it might be my new preamp. Any thoughts? Thx!
steakster

Showing 1 response by almarg

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almarg
9,670 posts
 
While it's never a good idea to connect or disconnect cables to an amp while it is powered up, I suspect that you've been lucky in this case. Doing those things with XLR inputs is far less likely to cause problems than with RCA inputs, in part because XLR connectors make the ground connection before the signal connections when connecting a cable, and break the ground connection after the signal connections when disconnecting a cable, while RCA connectors do the reverse. And perhaps also because when a balanced input is left open the design might be such that a small voltage differential exists between the inverting and non-inverting inputs in the balanced signal pair, which would help keep the circuits in the amp in a stable state.

Also, since the speakers were connected, if a problem had occurred you probably would have heard it. And when you were biasing the tubes you might also have seen the meter or other bias indicator doing strange things, if a problem had occurred.

So given also that "the amps seem to work and sound fine," I wouldn't worry about it. But I also wouldn't do those things any more.

Regards,
-- Al
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