tube pre amp and Krell FPB 600


Krell in its reference manual says : Tube pre amplifiers , by design, are capacitively coupled. For this reason, the benefit of a direct coupled amplifier can not be realized when used with a tube preamplifier. Additionally , many tube preamplifiers output a great deal of DC. This DC may exceed the servo of a FPB amplifier. Excessive DC level in a signal can damage amplifier , speakers , or both.
Can I use Air tight ATC 2 pre amp with Krell FPB 600 ?
Please advise.
Thanks
fpooyandeh
Well! My experience is different.

What happened is that when I connected my BAT Vk-3iX preamplifier to Krell FPB 300 without knowing about the coupling capacitors engagement, the combo performed wonderfully. However, one day there was a loud popping sound from both speakers and right channel went mute. I reset everything but now Krell FPB 300’s right channel had intermittent operation, it will work sometime but not the other times. I sent Vk-3iX to BAT and FPB 300 to Krell. BAT gave the ok signal that Vk-3iX was performing as per specifications but Krell told me that right channel of FPB 300 had intermittent short in the driver board and reason seems to be from high DC at input of FPB 300. Krell changed the board and a controlling processor but engaged the coupling capacitors in FPB 300. BAT Vk-3iX is from new line of BAT but I measured max of 11.3 mV DC at it output. BAT says up to 14 mV DC is normal for it. However, Krell says that it is not average level of DC that hurts direct coupled amplifiers as DC servo in amplifiers can and do counter it but it is the extremely fast spurts of DC as high as 100-200 mV DC from tube preamplifiers that does the damage. I measured max spurt of 56.67 mV DC at VK-3iX output. However, BAT says if max function in digital multi-meter is used then it can measure low frequency AC magnitude which can depend on line supply quality…………………………………………………I can continue writing about this for one more page.

I sold FPB 300 to experiment with Parasound JC 1s but I hated JC 1s sound. Whatever the Stereophile writes in favor of JC 1s, they are not even close to Krell’s FPB series. Now I have been using BAT with FPB 400cx with coupling capacitor engaged in Krell for six month and so far everything is fine.

But this is not an isolated problem with Krell only. All amplifiers which does not have coupling capacitors as protection against DC and only rely on servo to counter DC face the similar risk. I once read somewhere about a Parasound JC 1 that got fried when the owner turned off the preamplifier before turning off the amplifier. Some preamplifier (even solid state) spurts DC when they are cycled on/off and this alone can fry an amplifier which doesn’t have coupling capacitors as protection when its servo failed to counter high level of DC. JC 1 doesn’t have coupling capacitors thus in above case it got fried.
It is a well documented fact that the BAT VK-3 was a DC leaking fool. It shut down McCormack amps all the time at a store I was once associated with.
HI Gr8deal3,
can you email be the decoupling procedure for the Krell FPB 200?

Thanks in advance!
I realized that this is an older thread but I am concerned. I've purchased a vk-3ix (have not received it yet) used. However reading Haroon's comment on how it fried his JC 1 because it doesn't have coupling caps concerns me because I have a A21, which is basically the baby brother of the JC. What I don't understand is this from the Parasounds Owner manual:

"Parasound power amplifiers incorporate ingenious and fast-acting DC servo circuits, which completely eliminate the need for coupling and blocking capacitors. The JC 1 is direct-coupled from its input jack to its speaker terminals.This advanced circuitry never needs adjustment or maintenance. It operates outside the audio signal path to keep the DC offset at the output of the JC 1 at a constant 0.00 Vdc.The results are startling clarity, freedom from listening fatigue, and formidable bass response."

In relation to the comment should I not use the BAT?
Have anyone have any experience upgrading the Krell input coupling caps ? Can you share your experience ?