Explain this circuit to me - Output Protection?

barefoot · May 21, 2005

Can someone explain the highlighted section of this circuit to me? It appears to be some sort of soft-start, anti-thump or output-short-protection arrangement, but I can't really figure it out.

Thanks,
Thomas

CJ · May 21, 2005

That's a good guess. As the plus rail comes up, it pulls in the relay, and thus, un-shorts those two emitters. That's as far as my brain will go.

When I see stuff like that, I rip it out of the circuit, since I don't know what it does, and eliminating one more potential failure source in a power amp makes my heart soar like an eagle. Knowledge replaces fear, no knowledge rips out excess baggage.

barefoot · May 21, 2005

Yeah, this really has me scratching my head.

It's highly unlikely that any relay is going to be activated with just a diode drop across it, right? So, the only way I see it switching is if the zener side is more positive than V+. But how can that happen?

Kev · May 21, 2005

DC offset ... failure scenario
not a delay turn on / soft start
but I don't think it looks like the typical mods for the Phase Linear 70's stuff.
why is it on the other side of the diode ... halfwave ?
power rail monitoring ?
:roll:
relay is closed when there is a level of power rail but if the output DC rises to ??? it opens ....

why am I having trouble with this ... is it the way it is drawn ??

arrgghhh ... :sad:
stupid Kev ... have another coffee.

barefoot · May 21, 2005

Well, at least those shorted emitters gave me an idea. What if I put a pair of RC time constants across the bias references in my buffer?

Like this (in red):

This would turn the biasing to the subsequent stages on slowly as the supply powers up, avoiding any potential thumps if I happen to use this as a power amp. Look like it will work? Common thing to do?

I just finished my 3rd coffee for the day. :wink: