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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

Showing 7 responses by rwwear

rwwear
2,847 posts
 
Tube preamps can have DC leakage that's why they sometimes use coupling capacitors. And since Krell amps have no capacitors in the signal path, they can shut down when fed a signal that has DC present. This also occures with McCormick amps. I once worked at a store that sold BAT and McCormick and everytime we hooked a BAT preamp to a McCormick amp, the amp would shut down. The jumper inside the amp inserts a capacitor in the signal path with Krell. I used to use a Counterpoint SA-3 with my Krell and never had a problem without using the jumper.
rwwear
2,847 posts
 
Well Newbee you got me on that one. But just because equipment is old doesn't mean it can't sound good. By the way, BAT has probably fixed the DC leakage problem that they had in their earlier preamps. I guess an SS preamp can leak DC but it is not as likely. You can easily measure for leakage with a voltmeter to the outputs of the preamp. I am pretty sure that Krell amps will only shut down when DC is present at the inputs. Once the problem is removed the amp will come back on without damage.
rwwear
2,847 posts
 
How can you say such a thing Newbee? If one can hear the difference between cables, one should surely be able to hear a capacitor in line with his equipment. After all, they make a difference in speakers. And if someone did make an amplifier that sounded like tubes, who would admit it that it did? I like tubes and use them but they don't always sound better.
rwwear
2,847 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.
rwwear
2,847 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.
rwwear
2,847 posts
 
Yes, the positive inside the jack and the negative outdide.
rwwear
2,847 posts
 
I have been reading that the VAC preamps use output transformers and really sound great. I would like to hear one with my Krell 600c but they are quite expensive if you want the top model.
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