John Curl's Blowtorch preamplifier

[Wavebourn]

Exactly Scott! Computers, car radios, etc... All brought some great results... Even soft furniture!
A lot of things we could not dream about are cheap and affordable.

[Bob Cordell]

Originally posted by john curl
Well said Bob. I do believe that some sort of amp DC protection is useful and necessary IF the servo goes outside its limit, but that also means that something is wrong, either with the amp itself or the DC supplied by a poorly designed preamp.

Thanks, John. I agree with your point.

I usually monitor the output of the servo integrator and if the voltage gets beyond a certain point, I open the speaker relay or otherwise engage protection. Depending on the design parameters, however, this can sometimes interfere with full-power testing at frequencies below 20 Hz.

Cheers,
Bob

[janneman]

Originally posted by Bob Cordell



Thanks, John. I agree with your point.

I usually monitor the output of the servo integrator and if the voltage gets beyond a certain point, I open the speaker relay or otherwise engage protection. [snip]Cheers,
Bob

Bob,

Wouldn't it be wise to have an offset protection mechanism looking at the amplifier output to the speaker? One could possibly have a scenario where the servo still looks as if it is in it's design range while the amp has too high offset from another fault condition. In other words, detect excess offset where it matters, at the speaker jack.
Then this sort of protection would automagically also protect against servo failures and amp problems that drive the servo out of it's design range (including input offsets), so no separate servo error detection scheme would be necessary.

Jan Didden

[x-pro]

Originally posted by janneman
detect excess offset where it matters, at the speaker jack.

Jan,

That is exactly what the servo does, and if this offset present for long enough the servo output goes out of range and trips the protection. That what my circuit does and Bob is apparently using a similar approach. Obviously, the servo may fail but so may the protection circuit itself and after all there is such thing as a direct lightning strike .

Alex

[zinsula]

Originally posted by janneman
.....automagically......

nice one....


I like the idea of using the servo for protection. OTOH if the servo has a very low LF roll off as suggested earlier (Pavel mentioned 0.1Hz) it would be too slow to really protect in a failure case.

Tino

[Juergen Knoop]

what would be the typical integration time of a common DC-detector?
I don't know this, never designed one myself.
regards

[janneman]

Originally posted by x-pro


Jan,

That is exactly what the servo does, and if this offset present for long enough the servo output goes out of range and trips the protection. That what my circuit does and Bob is apparently using a similar approach. Obviously, the servo may fail but so may the protection circuit itself and after all there is such thing as a direct lightning strike .

Alex

Well, yes, a chain is only as strong as the weakest link. That's why I like as few links as possible in my chain. Like one

Jan Didden

[john curl]

Parasound senses at the output with a relay to open the output. The real headache is the relay, itself. A crowbar circuit across the power supply might even be more ideal.

[john curl]

I normally use 1 meg ohm and 1uf for a time constant of 1. It is easy, cheap, reasonably quiet, and we can just barely get mylar caps to fit close to the IC.

[Juergen Knoop]

thank you John, I have no idea how long speakers can withstand larger DC voltages.
One second is just a short moment. I imagine that a decent speaker should survive that.
Guess that wouldn't steal my sleep.
regards