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What does microphonic tube sound like?

I have this strange problem since I replaced my old SS preamp to a tubed preamp. Some of my CDs when played through Linn Unidisk sound scratchy. There is this annoying scratchy echoy sound throughout the CD and I can't listen to them. But the same CD, played through my PS1, sounds fine. I had both Linn and PS1 with the old SS preamp, and I didn't have this problem with that particular CD in either player.

I think the problem is with either the CD (bad press?) or the new tubed preamp. The current hypothesis is that the more extended sound from Linn is triggering preamp to misbehave and produce the scratchy sound, because the sound from PS1 is softer in extreme frequency ends. I don't know if what I'm hearing is microphonics, but if it is shouldn't it happen to all the recordings?
jylee
jea48
4,788 posts
 
This guy does a good job of explaining it.

My experience with a bad microphonic 6922 tube was as he described with a little ringing sound thrown into the mix.

Tap on top of your preamp lightly with two extended fingers. If you can hear your tapping through a speaker/s you have an excessive microphonic tube/s.
shadorne's avatar
shadorne
7,491 posts
 
Mofimadness,

Good point. I stand corrected. The higher gain stages will make pre and pre-preamp microphonics more audible. However the bigger tubes are actually more susceptible to it.

Most people like the microphonic sound. In David Gilmour's studio they have put all their tubes in a completely separate room in order to avoid this issue.

However, I still think the OP might be overdriving the preamp, as the PS1 plays ok.
samhar
1,048 posts
 
Tap the tubes non microphonic tubes won't make any sound through the speakers just a dull thud. Microphonic tubes have a hollow sound to the strike and will produce sound through the speakers, once you hear it you'll know !!
jylee OP
306 posts
 
Ok, I did a knuckle test on the preamp, and the tubes seem to be fine.

Shadorne, overdriven preamp is a definite possibility. Although the CDs with distortion problem are not the hottest CD that I own by any mean. It happens with a Diana Krall, and before I think one of the violin concerto. If the CD player is indeed overdriving the preamp what would be the remedy short of getting a new CD player?
vett93
205 posts
 
I am not sure about the hypothesis that your CDP overdrives your tube preamp. Your tube preamp should have a volume control that can attenuate the signal from the CDP.

My guess is your SS power amp overdrives your tube preamp. Your tube preamp may have a high output impedance and/or your SS power amp may have a low input impedance.

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