Dc voltage sent to my speakers

Thanks, @atmasphere  , I guess I am going a few extra miles then.  I will worry no longer.  Thanks again.

My experience with tube preamps is very limited, but the one or two that I played with had a tiny bit of DC on the output.  In the range of 5mV or less.

@erik_squires Some preamps like the Dynaco PAS3 make a small amount of DC like that but its not on account of the output coupling cap. The circuit employs feedback to the cathode of the input stage so there is a small amount of DC on the output as a result. As you already know though, its harmless.

this is pretty much what I do now-a-days, but how much time should it take the preamp to stabilize?

@immatthewj Normally about 30 seconds; your preamp is an exception. But if you're talking about a tube power amplifier its not a concern since the tubes can't be damaged that easily. 

So if the preamp is turned on after the amp is already on, the amp is in danger! That is why the common protocol is the preamp of any system (tube or solid state) is always turned on first and allowed to stabilize prior to powering up the amp! It really doesn’t matter if the preamp is tube or solid state in this regard.

@atmasphere , this is pretty much what I do now-a-days, but how much time should it take the preamp to stabilize?

As far as understanding electrical theory, much beyond which wire goes to where is beyond me, but here is what I have:

the separate power supply of my SLP-05 has two meters on it, and one is supposed to read "210 vdc nominal" but usually reads ibetween 230 to 240, and the other is supposed to read "60 mA dc nominal" and usually reads between 55 to 60. After turning the preamp section ’on’, it takes a couple of minutes for the meters to come up to within the paramters I just listed and then a couple of more minutes for a red light on the front panel to illuminate which theoretically means it is ready to play.

(My amp has two switches on the front panel, one power switch I am assuming provides power to the four large ower caps; according to the manual "left and right bias voltage and high voltage" and the second switch is what gets the tubes glowing. After the preamp meters and red light say it, the preamp, is ready to go, I normally turn the first switch of the amp on, but then I usually wait about 20 to 30 minutes before turning the second switch on & lighting up the tubes.)

Is my amp at risk the minute I hit that first switch, and would you think that I should wait longer on that? Thanks.

@atmasphere  My experience with tube preamps is very limited, but the one or two that I played with had a tiny bit of DC on the output.  In the range of 5mV or less.

If a preamp is outputting a little DC, is it always because of faulty output coupling caps?  Should those caps always be expected to block all DC?  I had one tube preamp that would trip the protection circuit of one of my amplifiers due to DC leakage, and I'm wondering if replacing the output caps would have fixed the issue.

@erik_squires @ketchup Unless the coupling caps are really old, its unlikely there's any DC at all at the output.

However, when you start up a tube preamp, there is a voltage rise on the input side of the coupling cap. A corresponding rise will occur at the output which follows an exponential curve. Usually a resistance is supplied at the output of the preamp to discharge the part as the tube warms up.

So if the preamp is turned on after the amp is already on, the amp is in danger! That is why the common protocol is the preamp of any system (tube or solid state) is always turned on first and allowed to stabilize prior to powering up the amp! It really doesn't matter if the preamp is tube or solid state in this regard.

I for one would be really interested in hearing the results of actual analysis of the preamp and amp. There are lots of great theories here, it would be really be good to hear what actually happened.

I don’t think the take away is to go buy a VARIAC. First, I have never heard of a preamp frying a set of speakers. But I am not versed in practical electrical engineering. But, even if this is a thing, then the take away is to buy from a reseller that verifies it functions properly (like Echo… and a host of others) or to have the device checked over by a audio shop. 

Thanks a ton my Audiogon family . I can't scream this loud enough I SHOULD HAVE USE A VARIAC . Because the tube pre amp was in storage for years.

Should also note that SS amps also may suffer from small imbalances, usually under 0.1 V though.

@ketchup - Even with the best caps there can be some leackage, but usually in the microvolts ( < 1 millivolt ).

Yes, it's very common for the output caps to leak DC and need to be replaced.  Not saying everyone's caps will fail, but that this is a very common reason techs see tube preamps. 

If a preamp is outputting a little DC, is it always because of faulty output coupling caps?  Should those caps always be expected to block all DC?  I had one tube preamp that would trip the protection circuit of one of my amplifiers due to DC leakage, and I'm wondering if replacing the output caps would have fixed the issue.

Some amps are heavily current limited and under low impedance/high output conditions will clamp their output so that the output resembles straight DC or clipping (or a square wave, for that matter). Because this keeps the amps VA output within safe marginsn, nthis is an amp protection scheme, not a speaker protection one. 

Some amps are heavily current limited and under low impedance/high output conditions will clamp their output so that the output resembles straight DC or clipping (or a square wave, for that matter). Because this keeps the amps VA output within safe marginsn, nthis is an amp protection scheme, not a speaker protection one. 

My DIY amp went south a while ago. Not sure if it was the emitter resistor that shorted and caused the output transistor to fail, or vice versa.

Thankfully, I had installed protection boards on the outputs, so speakers were spared.

Had they not been there, the speaker would have been toast.

It’s a 30w class an amp, so there was only around 30vdc, but would have blown the speaker for sure. Rail voltages can go waaaaay high in high power amps, so should something fail, it could get dramatic.

Sorry this happened, a costly episode no doubt.

Should you buy a used amp again, one that doesn’t have DC protection, or some other kind of fuse on the output/rail, then any tech should be able to install DC protection for not very much money.

If you want fancy amplifier output protection you need to stick to Class D. 

Not really joking but looking at the Hypex and ICEpower spec sheets they have a lot of fault detection and shut down circuitry built in that most linear amps never even think of.  Maybe the need to target pro sound helps them build paranoia into each amp?

Um, not the one's I've seen especially "high end" amplifiers that pride themselves on not having any.

@erik_squires 

Sounds cringeworthy to me- yikes!

Lesson learned always have pre owned equipment checked .

Oh, forgot one, often BJT amps will also have some sort of thermal runaway protection.  Otherwise, no I haven't seen a lot of SS amplifier schematics with "turn off on DC" on the outputs.

But, I'm not a tech, just a hobbyist who stopped pouring over schematics a long time ago.

@mulveling  , forgive me for my limited ability to understand electrical theory, but based on what you typed, assuming the applicable cap failed inside a tubed preamp, the output transformers of the tubed amp (assuming a tube amp was behind the pre) would still protect the speakers from DC voltage?

Most solid state amps have a protection circuit to prevent damage to the speaker in case of output section failure

Um, not the one's I've seen especially "high end" amplifiers that pride themselves on not having any.

It varies a lot by make and model IMHO.  I've seen plenty of schematics with just fuses and an anti-oscillation circuit on the outside.

Most solid state amps have a protection circuit to prevent damage to the speaker in case of output section failure (which is how they typically put out DC that can damage speakers). So this is a bit unusual!

Can you share what amp and preamp you were using?

@mulveling is correct in all counts.

The amplifier transistors swing the output voltage between 0 and one of the two power rails (plus and minus).  Anything from bad input, to a shorted transistor along the way could cause the output voltage to stick to the rail.

Fortunately you can use a voltage meter to check this.  Set it to DC and measure the amp outputs with no speaker attached.  First with the preamp off.  If you have more than a few millivolts it's probably the amp.

I am very sorry to hear that! A power amp failure is likely in your case. But it could also be your preamp.

Tube amps are generally not DC coupled, and they have output transformers which would block DC from your speakers and protect them. Such amps are very unlikely to be the culprit. So you’d likely either have a failed SS amp, or a DC coupled SS amp that is working properly but simply amplifying a failure from upstream (preamp).

Tube preamps often rely on output capacitors that block DC. These caps can fail over time, and leak DC. So if you had (say) an older tube preamp into a big DC coupled Krell amp, that could be bad news - the leaked DC from the preamp would be dutifully amplified by the Krell, and yes this could very easily fry your speakers.

Audiophile tube preamps have used lots of exotic film caps over the years, and not all of them have the best track record for reliability & longevity. I think Jensen oil caps were a problem at one point? Lower end preamps might even use electrolytic output caps (bad for sound, poor longevity), and those can definitely go bad.