Yesterday I blew my power amplifier. I’m quite sure what’s broken (some transistors) and that’s not the problem. The problem is what caused it to blow.
On the back of the amplifier the speaker outputs says 200w/4 ohms. Usually you can connect speakers with higher impedance at the cost of power output. The speakers I connected was at 8 ohms. The amplifier is rather old, about 20 years or so. It’s a Peavey CS400.
When the amp blew, it was fed with an overdriven signal. The amp was connected directly from a mixer console, and the VU meter at the mixer peaked. However the output from the amp, to the speakers wasn’t particulary high as I could tell. (There’s no VU meters on the amp)
I have been testing the amp earlier using other speakers at 8 ohms, and fed it directly from a CD. The amp rocked away with good volume, and no problems occured.
I have two questions for you:
Can connecting a higher load to the speaker outputs cause any damage or drawbacks other than the obvious drawback of less amplification?
Can a overdriven signal that is fed to the amplifier through the unbalanced inputs destroy the amplifier?
I hope someone can help me, becase I don’t want this to happen again once I have fixed the amp.
I doubt it. The higher impedance will reduce current through the power transistors, and current is what causes them to smoke.
The amp probably has two parallel speaker hookups per channel (is a CS-400 a stereo amp, or is it one half of a CS-800?) If you had both 8-ohm speakers hooked into the same channel at the same time, then each channel would have seen 4-ohms, which is at your limit.
FWIW, I’ve heard that tube amps are trickier in that the tubes and speakers have to have the tube and speaker impedances matched, as reflected through the output transformer (sorry for the tangent, but can anyone explain why higher impedances hurt the tubes/amp?)
You betcha! A square wave can be thought of as a sine wave that was too big for the electronics to handle. In response, the circuit clips off the top of the wave, and the amp is at full power for the entire cycle. The RMS output (more or less the average output) of the amp is much higher for a square wave than it is for a sine wave.
For graphics that explain this better (and link to sounds), go to the middle of this page.
Distorted input:
The strange thing that is I have not heard of this before, and there’s almost no mentioning of this anywhere on the net. I’ve been searching for hours and only come up with two pages pointing this out.
When I think of it, it all makes sense. The only think that feels a little strange is that the volume wasn’t very high when the amp blew. But maybe this doesn’t matter if the input is distorded?
So the thing is: Keep the output from the mixer low enough, so it will never distort. Right?
This is strange. Are there gain knobs on the amp? If not, and if the mixer was maxed out, then why wasn’tthe amp maxed out as well, at least to the level that the mixer could drive it?
FWIW, transistors do wear out, but it takes a long time on average. We’ve had them blow at work on tools that were idle, but don’t ask me why–I’m sure that current was going through the transistor on the tool, but I doubt that it was very much.
There are gain knobs on the amp, and they were set to approx
2/5 of maximum. The output was a little loud maybe, but not LOUD! At least that’s how I remember it.
There are also gain limiters on the amp (soft clipping). Could the amp blow if just the input stage distorted but the power stage was not maxed out?
I know I’m not being helpful, but I’ve always found the CS400 to be crap crap crappity crap. I sold mine for $150 and picked up a vintage 400W Vox head with a couple of old home made JBL/EV cabs for a hundred bucks off a restaurant that was cleaning out their basement.
