View Full Version : Lazarus tube preamp noise ....


hotgas
12-08-2002, 12:12 PM
Hi all,

After listening for a while on my new tube system I decided to complete the whole tube line by using the Lazarus pre-amp's phono as input to my aux of the Legend preamp. Knowing in advance from the seller that this preamp had some sort of "hiss" I still bought it to experiment. Whoa ... the hiss is higher than I expected kind of ... you know ... like the hiss from a cassette tape but even higher :( . Here are what I've tried:

1) Leave the preamp on for 3 days in a row . The hiss noise is less dominant but is still there.

2) Change the 12AX7 tubes from Tesla to military 5751 ... Still noisy .. even more .

The next thing I plan to do is to replace the 12AU7's pair but doubt if this' gonna work. My friend recommends to replace all the caps but doing this is a major thing and the last resort if I have to. Any ideas to do next?

Thatch_Ear
12-09-2002, 12:21 AM
Look under the chassis and where you have electroliticics that are not power supply filter caps and if they do not have a poly cap in parrallel put one that is around 25% of the uF of the electro.
Also any resister that is in a circuit to the tube can be changed for high quality cracked carbon which are much quieter. Ask Twysted for his recommendations as he has done this a few times and knows brands. His brother is also a wiz at this stuff.
My recomendations are stock, or everyone should do them. If nothing else you will have the best sounding hiss available.
You also might try some of the tube shields that have a corragated inner sleeve and protect against microphonics. It might not help but sure won't hurt and keeps the tubes cool.
It is suprising that the 5751s were noisier than new 12AX7s.
It is always hard to make guesses without a schematic, but polys parrallelled with electros and good carbon on the cathode to ground, good cracked carbon on the grids to ground should make some difference.
You might ask if the gain is so high that it causes this hiss. 5751s tend to be lower gain overall than 12AX7s. It could be a voltage thing which would not allow you to use 5751s to beguin with.
If this does not make sense to you or somebody else has an idea, chime in. I can't guess any more.

hotgas
12-09-2002, 11:29 AM
Thatch,

Here attached a nude pic of the Lazarus. I'm not familiar with electronic terms so I put a pic in for clarification.

Are the round and gray caps electroliticics ? My friend recommends me to replace all these with higher quality caps. It sure looks tight in there but if I have to make it work by putting parralell ones then I will remove the cover to get more space.

On resistors ... there are tons of them , are you recommending replacing them all ? How cracked resistors look like?

Thanks,