Caps going out?? Whats this noise...

Just to give all an idea of what goes on when the Lightspeed Attenuator has no power connected, the amps inputs will see an open circuit.
I think that one of the channels of the amp is not quite behaving itself when it sees this open circuit and goes noisy.
Which to me says it could be the input loading resistor to ground has gone open circuit, and the base of the input transistor/s is/are seeing the universe, no big deal as when the Lightspeed is powered up it becomes the input loading resistor anyway, which will be lower than the one in the amp.

Cheers George

Thanks Al! I knew you'd know what was going on. I appreciate you always being so willing to share your time and knowledge. Thanks!

ZD, he's using a Lightspeed, which attenuates via light-controlled resistors. When he turns off the preamp, power is removed from the LEDs which control the resistors.

Bruce (B_limo), what I would suggest is that you verify that no significant noise is heard for all settings of the Lightspeed's volume control when the Lightspeed and the source component are on but no audio signal is present. If that is the case, I don't see that there is a problem.

It sounds like the noise, whether it is being generated by the input circuit of the amp for that channel, or is being picked up by the interface cable, or both, is eliminated when a relatively low impedance is connected to the amp input (relatively low with respect to what it may be when the Lightspeed is turned off). That's not surprising. Everything else being equal, higher impedance circuit points are more susceptible to noise issues than lower impedance circuit points.

Since the proper turn-on/turn-off sequence for any system is amp on last and off first (to prevent "transients" that may be generated at turn-on or turn-off by the preamp or other upstream components from being amplified and applied to the speakers), I don't see that there is a problem.

Regards,
-- Al

Lightspeed attenuator with a battery pack... I can't figure out why the noise isn't there when I have the lightspeed turned on...

"The noise only comes from one channel, and only when my passive pre amp is turned off and my amp is still on."

How do you turn a passive preamp off?

I can only think to suggest looking inside your 200 to see if maybe there are any hairline fractures anywhere with the circuit boards possibly. Or a cracked solder joint. Or it could well be a part is out of spec, but it may be a resistor or diode. A very cursory way to check caps, other than looking at them for any signs of swelling or leakage, is to feel their temp with your fingertip after warm up and a little playing time - if in pairs, does one cap feel any warmer than its partner? Disclaimer: not responsible for anyone shocking themselves into next week...or further...!