Do all tube preamps leak DC?

 I want to pair an Audible Illusions L1 tube preamp with a Job 225 amp. Although both pieces have high gain, the gain can be reduced manually via two gain controls on the L1 preamp coupled with a master volume control. But here is my concern, the Job 225 amplifier has a Direct Coupled circuit that could potentially pass damaging DC leakage on to the speakers. So the question remains, do all tube preamps pass DC to one degree or another, specifically the Audible Illusions tube preamp?
Almost all preamps and amps have a tiny bit of DC offset. Maybe 5 mV or less for a pre, 60mV or less for an amp.

Tube preamps have a blocking capacitor on the output. That should not leak any DC, or TINY amounts. However..... when you first turn a preamp on it will take a few milliseconds for the cap to charge, and there will be a very loud thump.

Preamp makers deal with this usually by relays on the output to mute it for a few seconds. Otherwise, they may deal with it in the manual: "Please let the preamp warmup for 10 seconds before turning on your amps".

The way an AC coupled amplifier would deal with DC is usually by an input cap, but since you already have a cap in your preamp, it is redundant.

If you are worried, measure it. Put a 30K resistor on the outputs of your preamp and use a volt meter set to DC. See what you find after the relays click off. .


Thanks Erick, the Audible illusions L1 has four dip switches that can be turn off four caps on the output. Audible Illusions recommends for better sonics and after the preamp breaks in to keep them caps (dip swithes) in the off position. Would this further open the possibility for DC leakage?
Talk to them, I am not sure how their caps are arranged, and it's most likely that these caps are in addition to the main cap. There's no way I'm aware of to make a single-ended preamp without a blocking cap. :)

The voltages inside the pre are around 300V and would easily fry electronics expecting a maximum range of +- 2V.  This is part of what a DC blocking cap would prevent.

Like I said, if in doubt, measure and listen.


So I just found a picture of the guts. As expected, it has two very large caps and a relay at the output. :) I think you are all set, but caps do fail, film caps VERY VERY rarely, so by all means, spend $20 on a multimeter with 100mV resolution and you can see it yourself. :)

If you measure any DC offset at all (relative to connecting the meter’s pins together) then contact the amp maker and ask them if it’s within tolerance.  Remember to use a 30K to 100K resistor while measuring, as this will be more realistic than the voltage across an open circuit.


The AI preamp have a protection circuit that will not allow the unit to output if there is excessive DC at the output. I wouldn't worry about it. Just use the basic protocol. Amp on last, amp off first.
I have been running Audible Illusions preamps (Modulus3, Modulus3A, and Modulus3B) for years into 3 different power amps, 2 Classe' amps and an Odyssey and never ever had a problem.  My only gripe is the preamps have too much gain!!!
Thank you Erik & paul79 for your input, I'm feeling better with this combination already. Although 6 Moons considers the Job 225 has set very high standards & his first listening tests with a tube preamp was one of the cons as he was getting some crackling sound which suggested that there was some DC leakage and quickly switched to a ss preamp. He also thought  Job should consider DC servos in the future to correct this. This is why I was somewhat concerned.
Stereo5, yes I do see your concern with too much gain and although Audible Illusions did provide a way to reduce the overall gain on the L1 that provision was not afforded to the M3a (still is a spectacular sounding preamp in its own right though).
Next you can get into the world of cap upgrades...:) :) :)


Most great ones do not.
Erik, cap upgrades, maybe Mundorf.

Steroeo5, I'm willing to bet that even though the M3a has no gain adjustment, it is probably more detailed because the sound travels through less pots.
Actually, if you wanted to get a very nice bypass cap, here's one

You'd put it in parallel with your existing caps. Give 48 hours of play time.

I've not tried them in electronics but in speakers they are excellent. :)

Hi Paul,  you could have called me on the phone with these concerns,  however,  I see Eric_squires  has answered you as thorough as I would have,  cheers. 
Hello Keith, I'm going to call you anyway, we have some catching up to do, Paul.
Almost all preamps and amps have a tiny bit of DC offset. Maybe 5 mV or less for a pre, 60mV or less for an amp.
A preamp with 5 mV of DC at its output is rare. Our preamps have direct-coupled outputs and they have less DC Offset than that!

With **any** preamp, tube or solid state, it is advisable to turn on the preamp first and let it stabilize before turning on the amplifier. All preamps can exhibit turn-on thumps unless the manufacturer installed a mute relay.

Krell started the 'DC at the output' rumor with tube preamps; it was entirely false. Tube preamps usually have less DC at their outputs than solid state! This is mostly due to the output coupling capacitors used; tube preamps usually use a film cap at their output as stated earlier; solid state preamps often use electrolytic capacitors which are far more prone to leakage and failure.
It's been a very long time since I touched a tube preamp, but as I recall, the film caps do have a smidgen of DC leak. Maybe 50uV was closer to the truth, and yes, film caps will tend to have less DC than SS state pre's.

I've never seen an AC coupled solid state preamp in the high end, certainly not one with an electrolytic coupling cap. 

I _HAVE_ seen lots of bi-polar electrolytics as coupling caps in pro gear though, especially amps. 

But my experience is all pretty random, and from a small sample. I'm sure examples of all of these exist. 


atmasphere, thats a remarkable statement and it makes sense as well. If this is true then the reviewer over at 6 moons is full of it.
He might have got that from Krell. Its a rumor and nothing more.

It's been a very long time since I touched a tube preamp, but as I recall, the film caps do have a smidgen of DC leak. Maybe 50uV was closer to the truth, and yes, film caps will tend to have less DC than SS state pre's.
I work on tube preamps all the time. Right now I'm restoring a very nice condition Marantz 7c. Like most tube preamps, it employs a film coupling cap at the output. There is not even a 'smidge' of DC Offset. If there were, the problem of DC Offset would have been documented a long time ago, but it was not until Krell put that comment in their manuals that this rumor got started! Similar caps are often used in the output section of tube power amps, where small amounts of leakage can cause a lot of problems with the bias on the power tubes.

The cap at the output of any tube preamp has a resistor to ground at the output. Typically this is about 1 meg ohm, and is there to discharge the capacitor due to the effects of the voltage applied to the other side of the cap during warmup. It can be rather high in some cases (several hundred volts, although not in the case of the Marantz), and if there were no resistor to discharge the cap it would sit indefinitely at the B+ voltage or at whatever voltage the cathode follower at the output of the preamp runs. The resistor discharges this voltage (as does the input resistance of the amplifier) and as a result, the preamp after it has warmed up has no DC at its output at all. If you measured something, it was because of a measurement error or the preamp was not stabilized or the cap was actually bad (the only real case of this I have seen is in Fisher preamps, the ones that have those black coupling caps that otherwise look like Orange Drop capacitors).

Some preamps (tube or solid state) don't have regulated power supplies. If the AC line voltage were to vary slightly, such preamps might appear to put out a slight amount of DC but if one were to study the phenomena a bit closer it would be found to be a bit low frequency AC noise. To be clear here that's not a phenom of tubes, its a design issue.

Well then, I stand corrected! :) 

As I said, it's been a very long time, and quite possible I'm confusing the behavior of electrolytics. :) 

Good to have those with more and more current experience here to correct me. 


Hi Paul,  3 years ago Ralph Atmas-phere convinced me to open up my tube/solid state hybrid digital player that I run directly to my modified krell 700cx amplifier to prove that I had no DC leakage,  as a matter of fact, I have two coupling capacitors behind each tube,  krell once accused my player was the fault of on going problems with my amplifier,  it took Dan D’Agostino himself to tell me by speed dial on my phone to share with me what was actually wrong with my amplifier after modifications,  it was two heat sensors on the left side of the amplifier,  the amplifier has a total of six,  three on each side,  when I told krell how to fix the problem with the amplifier,  they wanted to know how I knew that,  of course I could not divulge who told me,  any way,  Ralph is a good friend of mine,  when it comes to this subject here of DC leakage,  I learned every thing I know on the subject from him,  I assure you Paul,  you can take what he has said as the gospel! 
Hi Keith, I believe you are correct about Ralph & would agree that he is very knowledgeable. It is nice to know that we have people like him to provide reliable info on most given subjects (questions) but you are also in the "A" Class, an asset to the audio community and a hell of a good guy to boot.
Paul,  have you taken the hood off the Audible illusions l1 preamplifier to visually check if you have coupling capacitors located next to the tubes?, and check the condition of the capacitors?, also,  I would ask Ralph here how to check if the specifications are up to par on the coupling capacitors even if they pass the visual inspection,  sometimes capacitors dry out,  or are just to old,  if all this passes,  I believe you have no worries of DC,  however,  if the capacitors fail any of the mentioned here,  I would replace before proceeding,  cheers. 
Keith yes, although this particular Audible Illusions L1 is factory refurbished, I removed the top cover after I received it to inspect the tubes. There are two large caps near the output and four smaller Panasonic caps next to those. According to the manual the four dip switches are there to turn off the four smaller Panasonic cap only leaving those two bigger caps remaining in the signal path. At this point I feel very confident based on what I have read here on this thread, that I don't need to be concerned about DC and would like to thank all those that participated!