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Krell amps and Tube Preamps - enemies?

Wow! I never realized how difficult it would be to add tubes with a KRELL amp. I'm no engineer, and, I don't pretend to be into the "technical" side of the audio world. So I'll try to explain.

It seems that tube Preamps create DC. I've been told that DC will dammage my Krell amp (FPB400cx), and possibly my speakers(Tyler Acoustics Linbrook Sig, System 1 piece). UNLESS, I open the amp up and do something to the "jumpers on the digital board" . This puts some protection into the signal path. The Bad thing is, I'm told, - it also colors or degrades the audible signal coming thru the speakers. So, if I want to use my SS set up, say for SACD or DVD-V/DVD-A, I would have to open the amp up again and reverse the proceedure. This would remove the "blockage" in the signal path. :-( This of course, would have to be re-done when I want to go back to the tube setup. WHAT A PAIN!

Needless to say, this has thrown the proverbial 'wet blanket' on my tube enthusiasm. Is this problem common to ALL SS amps? Or is it just a KRELL thing?

Any thoughts would be helpful.
(This is part of a thread i started on the speaker threads, but i thought i might have more luck here on the "amp' Threads)
Thanks
oldpet
rooze's avatar
rooze
645 posts
 
Oldpet, send an email to Krell through their website at www.krellonline.com and include a fax number with the email. They'll fax you a sketch of the internal layout of the amp with arrows pointing to the two jumpers that need to be removed. It's very easy to do. Again, I didn't hear any difference in the sound.

Rooze
oldpet OP
295 posts
 
Thanks Rooze, will do. I figure, the worst that can happen is, I'll have to change it back if i don't like it. Of course, I'll try it with my SS stuff, BEFORE I buy the tube pre. I was thinking of the ARC LS16.....or maybe a p-12A, H-CAT. If i go the H-CAT route then this all becomes a moot point since the p-12A is a SS design. I Think? hmmm.......
rwwear
2,883 posts
 
The reason Krell doesn't ship it with the jumper engaged is that they feel it will degrade the sound. And they are probably right. According to the Krell service manual "Full Power Balanced amplifiers use unobtrusive direct current(DC) protection circuitry that strips DC from the signal without corrupting sound reproduction. Full Power Balanced amplifiers feature direct coupled circuity from input to output. This topology eliminates all coupling capacitors from the audio signal path. Coupling capacitors block damaging DC but have sonic characteristics that corrupt sound reproduction." It also states that the high DC output of tube preamplifiers MAY exceed the DC protection of the FPB amplifiers. So in my opinion get a meter and measure the outputs of the preamp for DC. I have used various tube preamps with my FBP600c and never had a problem and have never had to insert the jumpers.
oldpet OP
295 posts
 
Rwwear - thanks for the technical info. :-) What type of meter? And what should the 'safe' readings be?
rwwear
2,883 posts
 
Just get a regular voltmeter and set it for DC. I don't know how much is too much but if there is no leakage then you should be safe.
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