tube pre amp and Krell FPB 600

When I talked to Krell about using the VTL 7.5 with my FPB650M monoblocks they said no problem at all. I did for awhile and the sound was great, and no problems. I used two other tubed preamps with these amps. Again no problems. Right now I am waiting for a custom made tubed preamp(Singlepower Audio) to go with the Krells. I have never changed over the Krells to "adapt" them for tube preamps. If your thinking about the 7.5 with your Krell, IMO they go very good together.

I compared two preamps over a local audiophile home who owned a FBT 300 amp. One was the tubed Modwright SWL 9.0 (w/ upgrades)and the other was a Belles 28a reference preamp solid state. There were no issues running the Modwright with his amp, but I did have to have caps installed in my Pass labs X-150 amp which is also servo I believe. All Pass amps have an input cap in them now to be safely used with tubed preamps. I don't think you'd have a problem. For what it is worth the Belles sounded better in his system. More depth, dynamics, soundstage and control over the Modwright, and you would not have to switch in any caps (if that is the case) in your amp to use it. Which would result in a cleaner sound.

I drive my FPB300c with an Aesthetix Calypso, which is a capacitively coupled pre-amp. I've had no problems at all and am very happy with the results.

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.