Audio Research tube problem with Aragon 4004 MKII

I have used an SP-6b with an Aragon 2004 MK11 without problems for more than three years... NO HUM. I also have an SP-9 MK 3 (with GNS ref mods) also with NO PROBLEMS now for two years. I have coupled these with Precision Fidelity M-8 amps also with out a problem.

I use a Sonic Frontiers Tube Pre, and used others in past with my Aragon 4004MKII and never have had a problem so I am suspect of Johns claim you have to add DC blocking cap.

I had a similar problem as Kenscollick did.

If you are set on using a tube pre with the 4004, you'll need to add a DC blocking cap for both channels. I know' direct is supposed to be better, but in this case, your options are limited. Add a film cap to the 4004 inputs and enjoy your tube pre.

Most solid state amps can accept certain amounts of DC to be passed from the preamp to the power amp via existing filters. It is when too much DC is passed that overloads the filtration capabilities of the power amp that these problems occur.

Tube amps are reportedly more likely to pass differing amounts of DC than solid state amps due to the tube fluctuations.

Krell amplifier manuals (at least mine) states that when using tubed preamps that one should activate their input coupling capacitors by removing the jumpers to protect against this more volatile DC from tube preamps. In speaking directly with Krell on this very topic, I was informed that I should find out if a tubed preamp has output coupling capacitors. If the tubed preamp has these, the DC voltage leakage should be well within the "Krell" power amps filtering capabilities. If the tubed preamp does not have the output coupling capacitors, then the input coupling capacitors should be activated (witin the Krell amps by removing the jumpers around the capacitors) for the proper protection, but at risk of impacting the sonic performance of the amplifier.

Since I have a tubed preamp on its way to me (Calypso) which according to the manufacturer has output coupling capacitors, I will not pull my jumpers from my Krell amp. But I feel compelled to send my preamp in after a couple of years to have the caps replaced (yeah, wouldn't that be great if we could keep any component that long?).

Years ago I tried to mate my SP6c with a Aragon 4004 after shutting down the amp a few times a conversation with Tony Fredricci [sp] he told me that the pre was passing DC then I contacted AR sent it back they changed caps and tubes at no small cost. when I got it back still had the problem. Called AR and was told that the tubed pre will always pass DC. that was it for me no more tubes and I could not be happier. Good luck trying to build your own "hybrid"