Samson RS110a

[drewl]

Sorry, no schematic, posted a request in that forum, no responses.

So, powered speaker was getting HOT
While testing output blew.
Basic power amp, two 150watt transistors one npn, one pnp.
Each has a TO220 sized driver of it's polarity and each side has two smaller current driver transistors, everything else surface mount.
I replaced the blown drivers and anything else suspect and amp works, drives a load, signal looks great, but it's drawing too much current
Bypass light bulb limiter and POOF! There go the output transistors
It's got to be biased too hot.
Looked for what looks like the bias string, found a 330 ohm resistor, shorted it with tweezers and bulb barely lights.
Lowered it to ten ohms, amp works, signal looks great, drives a load and bulb barely lights

It's pulling about 200 milliamps at idle, that sound right?

Only thing left to do is plug it in without the limiter
Or should I try and get idle current lower?

I'm afraid to try it without the limiter

Thanks

[Jon Snell]

200mA is far too much for a class B amplifier., more like 15 - 20 mA is what you need.
With a bulb limiter, the voltage is reduced and the readings will be incorrect.

[Helmholtz]

It's pulling about 200 milliamps at idle, that sound right?

Is that AC or DC current?
If DC, I'm with Jon Snell.

[g1]

200mA where? I thought you were talking about from the AC line, Jon and Helmholtz seem to think you mean idle current per output transistor.
Please specify.

[Helmholtz]

200mA where? I thought you were talking about from the AC line,

Yeah, that's what I meant with AC current. Would include the PT exciting current.

[drewl]

Sorry, 200ma AC from the ac input to the unit.
Even under load and putting out a few watts.

[drewl]

Why didn't I measure the dc bias?
Sometimes I'm stupid

Well I did, about 22mv across the .22 emitter resistors which I think is a little high

[drewl]

Got the bias down to 15mv per side, at idle with or without a load, limiter just barely lights, you have to turn off the overhead light to see the bulb.

[drewl]

Just to wrap this up-
Bias kept rising even though I lowered it.
Traced the bias circuit out and a 1.5k resistor was either bad or too high, even though it measured fine.
Lowered it to 1k and amp is stable, bias stable, puts out full volume with no distortion.