Not really a DIY question. Popping when switching inputs.

[mattjk]

I have a Pass Labs x2.5 pre, and an old tube phone preamp.

When I have the phono preamp powered up, I get a popping noise when I switch inputs on the x2.5. The phono preamp seems to be working fine... but the tube preamp is definately causing it. What would cause the popping noise that can be heard through the speakers?

The tube phono preamp is a Lafayette unit. I checked everything inside and it seems to be OK. Bass is very weak however. Could this be a bad powersupply cap?

Thanks,
Matt

[mattjk]

I just measured the output RCA's

23volts out of the right channel, and 16volts out of the left channel.

That's bad, right?

[Nelson Pass]

Either you need new caps (fancy schmancy film types) on the
output of the phono stage, or some bleeder resistors to
ground after the caps (maybe 47K) or both.

[GRollins]

Nelson is correct, popping and DC at the outputs in this case almost certainly indicates that the DC blocking caps at the ouput are leaking (meaning that they're passing DC, though not a dead short). Start by replacing those and do it soon--this is not something that will get better. I also agree with Nelson that film caps are a good choice for the new caps, both in terms of sound quality and long life.
And yet...and yet...
Your observation about weak bass bothers me. There may be other problems. If by Lafayette you mean the erstwhile competitor to Radio Shack, then that's an old unit, indeed. You might consider going through and replacing every electrolytic in the unit. Electrolytics don't last forever; they tend to dry out and die after twenty or thirty years. Yes, sometimes they last longer, but for fifty cents or a dollar apiece, it should not be all that expensive to replace them all. Your main problem will be the almost-inevitable multi-section aluminum can cap in the power supply. It's not that you can't find caps to replace the individual values, it's that you won't find a can like that--you'll have to find room to put individual caps somewhere, either in the chassis or in an external box. Sometimes that's a pain. Be patient and creative. It can be done. If at any time the cap you're replacing is small enough in value that it can be replaced with a film cap, do so. Yes, film caps are more expensive, but they are better in every way. Just be sure to observe the voltage ratings along the way.
Gotta go, I think I just heard reindeer on the roof...

Grey

[mattjk]

Thank you for the helpful replies!!

[Duo]

Interestingly, there are sources for those cans, and they can be re-formed. However, I find that there's nothing like a new cap to go in a power supply.

I have a box filled with those old caps, many NOS. Usually they still work if they haven't been used, but often need a bit of an upbringing to make sure they're good to go.