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noisy tube preamp

I'm fairly new to tubes but have recently acquired a tube preamp. I'm really liking the air and the soundstage it provides, something the solid state stuff haven't done as well but the one issue I have is the elevated noise floor. It's really only audible when no signal from the source is being played, like when the system is simply on or when the song stops. Also there is an occasional quiet, short lived buzz from the speakers that pretty much goes away after the preamp warms up and lastly, after the unit has been on for several hours, like 24+ it will start to emit a high frequency pitch that also is only audible with no music playing but it is louder than the inherent noise floor of the pre. My question is, is this something I can remedy by rolling tubes or is it something that has to do with the pre itself like the design or a failing part? Btw, it's a jolida fusion preamp.
lukaszwk
newbee
4,767 posts
 
Could be as simple as needing some very low noise tubes. Could also be too much gain for your amp/speakers and the 'noise' issues could be solved by using a fixed in-line attenuator. But that turn on 'buzz' and high frequency noise after 24 hours, I don't know. But then I would never leave a tube unit on for 24 hours......The turn on buzz is probably nothing more than the unit stabilizing, something many pre-amps avoid by having a mute circuit which engages on turn on (for up to several minutes).

Tackling the tubes first is a good place to start. Sooner or later you will have to change them out, so even if this is not 'the' problem having the spare set is a good idea anyway. 
hifiman5's avatar
hifiman5
1,832 posts
 
The high frequency whine that you mentioned rings true to me.  I had the same experience and found the problem to be with the tubes, not the preamp.  Not sure what tubes you are using but I have had great luck with Cryoset.  They cryogenically treat all of their tubes which, in my experience, has resulted in very quiet tubes that seem to last forever.  I buy new production Gold Lions and they have been great.

Good luck!
atmasphere's avatar
atmasphere
11,841 posts
 
after the unit has been on for several hours, like 24+ it will start to emit a high frequency pitch that also is only audible with no music playing but it is louder than the inherent noise floor of the pre.
That is a form of something called 'microphonics' and is a property of all active devices. Tubes have it more than solid state, and its definitely something that you can get rid of by changing a tube!
lukaszwk OP
63 posts
 
The tubes are stock (as far as I know, I'm a second owner) 12au7 and 12ax7 and this being an inexpensive preamp I'm thinking they are not of any significant quality and should be replaced shortly after purchase which is fine by me. Also as far as voltage gain from the pre, I'm pretty sure that is also the issue, I have never dared to play it louder than maybe 11 o'clock position on the volume potentiometer because it's starting to clip a 300 watt amp so the in line attenuator sounds like might be a good idea although I'm pretty wary about putting another thing in the signal path. With having next to zip knowledge in electronics, is there a way to change this value internally? Jolida states that the max output voltage is 30v and voltage gain is 20db (although they have these two values reversed on their website)
newbee
4,767 posts
 
Rothwell -  stock 10db in-line attenuators should work just fine. No real downside unless you have ultra high end stuff and you are anal! :-) You can select other values if you want. Lot cheaper than trying to mod your pre-amp internally.  BTW, if you are changing tube brands be aware that you will want to insure that their 'tonal' balance is one that you will want in your system. Do lots of research on this before you leap and beware of salesmen's advise and advise from folks who don't know much more about tubes than you do, they abound! :-)
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