tubed preamp off and ss amp left on?

Sounds like some type of grounding problem to me...Ir would be interesting to see what would happen if you lifted the ground...Just a thought

12-17-06: Warrenh
Funny, I had posted a similar question a few weeks ago, having come back to seperates after over 20 years. If the volume is down on the preamp, should it matter which you shut off first? Nowhere in my amp or preamp manual does it say to be certain to shut off first or last. I think volume down on the pre is the answer?

When the power switch is turned off a voltage transient spike may pass thru the preamp outputs thru the ics to the Amp. Result, a loud pop. This will happen even if the volume pot is all the way down.

My Sonic Frontiers line One preamp, when placed in standby before shutting down the preamp, apparently shunts the outputs of the preamp thus preventing the voltage transients. That is the reason in my earlier post I suggested trying the mute function if the preamp has one. The mute circuit in the preamp may shunt the outputs, thus, no transient spike leaving the preamp thru the ics to the amp.....

Thanks for the responses. I just rechecked and the loud hum when pre is off and amp on is gone. Why? I do not know. I have made many changes lately trying to get rid of a low level hum in speakers when nothing is playing. This hum is still present. I have tried cheater plugs on two of the 3 components and the low level hum persists. Audible out to 2-3 ft from speakers. I have one more pair of interconnects to change. I doubt it helps but hopeful it will. If not I am going to change back to my old amp and see. BTW, the pre is lector zoe and amp is belles 350a reference. Other amp is aloia st15.01. What are the symptoms of an impedence mismatch? Can tubes create hum? I don't remember having this problem with stock tubes. May put them back in also. Thanks again.

Twc, if your cable tv is nearby, you should also try disconnecting the coax cable tv wire at its first point of entry and then turn your rig on and see if the hum is gone. If there's no more hum, you need a ground loop isolator which will take care of this issue for you.

Do you still have the hum when the preamp is disconnected entirely (with no cable attached to the amp inputs)? Are the pre and power-amps connected to the same outlet ? Just ruling out possible sources of ground loops.

also how old is the amp ? If it's old it could be failing power supply capacitors ? Just a wild guess.

The low frequency hum is almost certainly mains frequency (60Hz), so it's either coming through the amp's power supply, or via poor grounding somewhere.
cheers