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)
You can always check your pre-amp for DC leak. I use an tubed pre with my Krell and read the same thing you did about the internal tweak..I didn't bother with it and have no problems at all.

Krell Ksa-250
Krell Kav-500

There was a thread about BAT pre-amp's and Krells, you should do a search for this one.

Call up Krell and ask them.
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.
Does it damage the amp if this occurs?
I bought a "pre-owned" Krell FPB300 that had been used with a tube amp without doing any internal alterations.

It had to go back to the factory for repairs. Luckily, it was close enough to being in warrantee that Krell didn't charge me.

So to be safe the jumpers SHOULD BE MOVED. Right? How does this effect the sound of the amp? Anybody know of a SS preamp that sounds like a tube preamp?
If there were such a beast as a SS amp which really sounded like tubes then the manufacturer would get a corner on the SS market (IMHO)! Some solid state amps that I have heard that come close that won't cost an arm and a leg are made by Muse and Plinius... I'm sure there are others. Personally, if I were going to have a SS amp in my system I'd find an older Threshold amp - the SA series. With your speakers the SA3 or SA2's would be great and they are not a problem with tubes - I used them for years w/o problems. I think they may well be the best amps Nelson Pass ever designed. By the way, I doubt that you will hear the insertion of the capacitor in the Krell unless you have everything else in your system optomized, with either a tube or SS pre-amp. Oh, an extra "by the way" DC offset from a CDP shouldn't be a problem - it should stop at your pre-amp, and even if it didn't the volume control would substantially reduce the signal if it did pass.
It seems as if BAT preamps are prone to this and if you wonder whether it damages amps, it might in the long run.

Check another thread of the same nature...
I use a BAT preamp and also First Sound preamp with my Krell 450 mono's without any problems.
You should call the manufacture of your preamp and ask them if your preamp leaks dc current. I called BAT and First sound and they told me that their preamp has no dc current leakage so it was safe to use.
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.
Wvick - I think dammage DOES occur. I'm not sure what exactly happens, and I don't want to 'experiment' with my Krell.

Newbee - Thanks for the second BTW :-) I might just try a Tubed CDP. I wonder if the (Modded Exemplar, APL) 3910's counts as a tubed cdp?

Nickt - Great Idea. I haven't bought one yet. (good thing I suppose)
But I was considering an ARC "LS" or "SP" 16 (not sure of the difference) or a Cary SLP 2002. I'll contact them to see what they say. Thanks
Rwwear, What would you expect from a newbee who still listens to a 20 year old pre-amp and 10 year old amps? :-)

My point was only that while you might be able to hear the capacitor it might not really make any substantive difference sonically speaking, different perhaps but not better or worse, and if it allows an upstream improvement it would be worthwhile.

Solid state preamp can leak DC also. It is not unique to tube.
Will the manufacturers say if their products "leak" DC? It doesn't seem to be something a company would want to admit.
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.
I've used a BAT KV-51SE, Calypso, and SF Line3 with a Krell KSA-200s with good results. (Also earlier a AR LS2B witched worked but the combo with THiel 3.6s drilled a hole in your ears.)
I used an ARC LS 15 with a Krell FPB200 for a few months. Initially I didn't remove the jumpers in the amp, and I couldn't leave the amp switched on for any length of time, it would get hotter and hotter (without signal) then eventually shut itself down. I then swtched the jumpers and everything worked fine. It did not degrade the sound in any way that I could detect and it took less than 5 minutes to remove the jumpers and replace the lid.
I wouldn't let this put you off using FPB's with tubes....the FPB needs all the help it can get in the warmth and tonal balance departments.....go for it, it's a good amp and the LS range made a good match in my system.

Rooze - Where did you get the instructions on how to do the operation? Why does Krell NOT ship the amps with the protection?
If I had a SS main that wouldn't accept a Tube pre, I would get rid of it.

Like Rooze, I used a Krell 200 FPB with a tubed line (AE-3)level pre-amp. I called Krell and they faxed me the instructions and general jumper location.. I am not a technical person and it took all of five minute get it done. The only issue I had was volume distortion. When played really (Megadeth) loud it appeared to get muddy, less defined but I figured it was the tubes not the Krell. Beware tubes can be habit forming.
Oldpet, send an email to Krell through their website at 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.

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.......
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 - thanks for the technical info. :-) What type of meter? And what should the 'safe' readings be?
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 - would i just hold the meter contacts on the 'outputs' of the preamp?
Yes, the positive inside the jack and the negative outdide.
Rwwear - thanks for your help. I just tried a tubed pre and found that the bass was simply insufficient. I'm returning it asap.

Does anyone know of a tubed pre amp that - 1. Doesn't leak DC. and - 2. has GOOD bass response?

Oldpet, First Sound Presence Deluxe II has GREAT bass response. I don't
have any idea whether or not it leaks DC, but you can contact Emmanuel
Go and ask him. He's very open and eager to discuss his preamp.
Tvad - thanks the First Sound Presence Deluxe II is a bit mor then i wanted to spend. I didn't want to go above 1500. maybe I'm kidding myself.

Call me crazy, Paul, but mating at $10,500 amp with a $1500 preamp seems to be doing your system a disservice, don't you think? If you're a little patient, you might find a used First Sound Presence Deluxe II for around $2200 - $2500, and for the extra $700 - $1000 you'd be buying a world class preamp instead of a pretender. Read the review at
There is a used Audioprism Mantissa (black) for sale on's only single ended though.

I have used one with my Krell...great sound quality that takes nothing away from the Krells.

Tvad/Sogood51 - Tvad - Good point. I need to be patient. I also need to find out about the DC situation.

Dave - Thanks of the tip. Do you happen to know if they leak DC? If you used one with a Krell they probably don't. Single ended is not an issue; I've had custom cables made. ;-)

I've not had any problems. The only pre-amps I have read about giving Krells problems were BAT pre-amps.

This could have been an isolated problem even at that I would think? I have used my Mantissa with my Ksa-250 and Kav-500 both at one time or another without problems.

This ongoing subject of tubes vs solid state is so stupid as to be almost laughable.It is the component matching(impedences,capacitance etc.)within our set-ups that determine how effective our stuff is in reproducing a musical signal.That goes along with low distortion and low coloration,as well as non-resonances.I've heard spectacularly performing products(in actual set-ups)that were solid state as well as those that were tube based.If the system was matched well the end result was MUSIC.This is 2005,there is a wide chasm of products to choose from,some of which are great and some not so good.That's it!Regardless of "The grass is always greener on the other side"marketing of either the tube OR ss camp.
Well, sirspeedy. Why don't you give all of us stupid people an exhaustive list of SS and tube gear, which will match well with each other. So then we won't have to exchange ideas with one another any more.
Correct me if I'm wrong, but the original question didn't have anything to do with SS vs. tube. It did have everything to do with tube preamps that have good bass and don't leak DC.

So, what's the relevance here of a debate about SS vs. tube gear?

Another instance of not reading the question.
Tvad - Thank you! :-)

Tvad - Just a follow up to your First Sound recommendation. Emmanuel designs them so there is NO DC LEAKAGE! YAY!!!
I suppose the search/ wait now begins. Thanks again for the recommendation.

Paul, post a "Wanted" ad for a First Sound. You never know what might come up. My only are aware that the First Sound does not have a remote control, right? This is an obstacle for some people, but I don't find it to be a big deal.
Tvad - if it sounds the way it is described in the review i read - I'll live with out a remote. :-) Especially if i can grab one in the price range you mentioned. :-) The "wanted" is running.
I asked Audio Research if I can use a Refr 2 mk2 pre to my Krell 450monos. They answered:
Teh ref2 mk2 has 4+MFD output capacitors, ie it is NOT direct coupling. We use it with our solid state power amps and have experinced no DC problems. They have direct coupling like the Krell amps"
So I can assume that it OK to use the Ref 2 without connect Coupling Capacitor in my Krells?
I just finished building and "burning-in" a Raleigh Audio line stage kit that was featured in a series of articles in AudioeXpress magazine. The circuit was in part inspired by an article on parallel feed circuit topology by Lynn Olson that appeared in Vacuum Tube Valley. The relevance to this thread is this: it is transformer coupled on the input and output with Lundahl amorphous core transformers. If you haven't heard these trannies, don't let any pregedices you may have for transformers close your mind to this option. These amorphous core transformers have "magic" in them. And more to the point, no DC in output. Also, all your components will be galvanically isolated from each other. Since I have inserted this component into my system I have noticed a real lowering of the noise floor. This is one of the most open, realistic and revealing preamps I have ever heard. If you are looking for "natural presense" and "holographic" imaging check this preamp out.
I don't know anything about the amorphous core transformers, but if they work that's the only thing that matters. I'm curious as to where you got the pre amp, and the approx. price range. Thanks

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.
It's funny that you have an FPB 600c. In my continuing research on Tube preamps, Tvad, on this thread mentioned the First Sound Presence Deluxe II. So, I contacted Emmanuel Go. He (Emmanuel) LOVES his Presence Deluxe II design with Krell amps. He had personal experience with the 600c and his design and absoluteley RAVED about the combination! So, his designs are safe to use with Krells. I'm looking for one, but I'm trying to wait to see If I can get one used before I would spring for a new one. Here's hoping. :-)
Oldpet - log on to: or check out the K&K Audio sponsored forum over on the AudioAsylum. There was also a series of articles published in AudioXpress by Dave Davenport under the title "A Line Stage Odessy". A very complete kit is available for $1799 which is what I purchased and built. The kit has 6 inputs (one XLR, 5 RCAs with one of the RCAs a fixed volume one for HT cinema use), remote volume and mute and two outputs, XLR and RCA. The only change that I made was to substitute Cardas caps for the supplied Kimber parafeed blocking caps.
i've been using a cat preamp wiyh my krell fpb 250 mc for the last three years and haven't had any problems. sounds wonderful
John/Mike - thanks for the info John. It looks very interesting. Did you need to do any soldering?

Mike - Is the CAT a tube pre?

Take a look at the SAS 10A. Steve Sammet has designed it in such a way that DC is not present in the signal path