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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
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.

shadorne's avatar
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shadorne
7,467 posts
 
If the CD player is indeed overdriving the preamp what would be the remedy short of getting a new CD player?

You simply need to attenuate the signal a little - a longer interconnect (12 foot or so) might do it as unfortunately your CD player output is probably fixed.

You could easily test to see if this is indeed the problem by placing the old SS preamp (which you claim works fine) between the CD player and your new tube preamp ....then vary the volume on your SS pre to see if you can reproduce the problem.
jylee OP
306 posts
 
Ok, I have spent last 30 minutes crouching down next to the equipment rack swapping cables. The preamp has been exonerated. The problem is between my Unidisk and Rotel class D amp that I'm using as a backup.

When I said previously I didn't have this problem, that was with Levinson 335 amp. Levinson broke down a few weeks ago, around the same time I swapped the preamp. Rotel is doing the main duty for the time being.

The problem wasn't there with Unidisk and Levinson combo. But this scratchy sound is evident with Unidisk and Rotel, with or without CJ preamp.

The problem is still puzzling to me. I can't find any explanation why it happens only with Unidisk Rotel combo.
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