2. amps-preamps
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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
11 responses Add your response
shadorne's avatar
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shadorne
7,477 posts
 
I expect The Linndisk is overdriving the new preamp and this produces distortion - especially on a modern "loud" CD but less so on a CD made in the late 80's...

The PS1 may simply have lower output and therefore it does not overdrive your new pre.

Microphonics are a bigger problem with power amplification rather than the smaller tubes in preamps - of course it is always a drawback of tubes and it is best to place tubes away from a corner and preferably in another room if you listen loud.
digsmithd
913 posts
 
You tap the suspect tube and if the filaments buzz or make noise youve found your culprit.
mofimadness's avatar
mofimadness
4,982 posts
 
"Microphonics are a bigger problem with power amplification rather than the smaller tubes in preamps"

Really? I have owned hundreds of tube amps and preamps and phono stages and have NEVER had a microphonic tube in an amplifier. I've had plenty in preamps and phono stages.

Huh, you learn something everyday...
entrope
482 posts
 
An annoying buzz, hum, squeal or whine much like a spouse.
jea48
4,425 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
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shadorne
7,477 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.

shadorne's avatar
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shadorne
7,477 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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