2. amps-preamps
  3. 126620-32

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?
phd
erik_squires
16,732 posts
 
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. .

Best,

Erik
phd OP
1,330 posts
 
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?
erik_squires
16,732 posts
 
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.

Best,

Erik
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