Is this a problem or not?

[chipper]

I bought a punch 200.2 off of ebay for 40 bucks it appears to work fine no problems only loose power and speaker blocks but i re-flowed the solder so they are not loose anymore. But now i'm just letting it play on high pass mode on a pair of pioneer hpm 100 home stereo speakers listening to some misfits and it gets nice and toasty (meaning hot but not very, very hot) is that normal for it to run like that? I'm not listening to it very loudly either. I didn't even touch the biasing pots either because i don't want to ruin this nice amp. Can someone please let me know why it runs a little hot at normal listening levels? My speakers are 8 ohms a piece and 100 watts max so those should be cake to this amp, huh? Thanks guys!

[ppia600]

Older punch amps seem to feel like they're hot but I think its just the sink design getting the heat out. Running at 8 ohms is also going to keep the outputs at a higher voltage level. Add to the fact that the contrast between your a/c cool room and the amp's heat and it will seem like a big deal. Put a thermometer on it and let it warm up to see what it reads.

[Perry Babin]

Let it cool completely and then power it up and let it idle (no audio). If it gets hot, the bias is too high or it has other problems.

[chipper]

Originally Posted by Perry Babin

Let it cool completely and then power it up and let it idle (no audio). If it gets hot, the bias is too high or it has other problems.

I did let it cool off all the way, what happens if you turn the bias pot all the way down? I adjusted them all the way down and let it sit for 20 mins and no heat. I started to play music loudly on it and it barely got warm. Can you leave them all the way down or not? I know how deadly those controls can be to the amplifier so i don't wanna mess with those anymore

[Perry Babin]

With the bias fully off, the amp becomes a class B amp. It's likely that it will sound OK because the drive circuit can react quickly to eliminate any distortion. If it can't compensate fast enough, there may be a bit of distortion but it's unlikely to be audible in real world conditions. If it does distort, increase the bias for each channel just to the point where the amp starts to draw just a tiny bit more current. It should run cool with the bias set like that. The following shows the basic procedure.

http://www.bcae1.com/temp/ausettingbias.swf

[chipper]

Originally Posted by Perry Babin

With the bias fully off, the amp becomes a class B amp. It's likely that it will sound OK because the drive circuit can react quickly to eliminate any distortion. If it can't compensate fast enough, there may be a bit of distortion but it's unlikely to be audible in real world conditions. If it does distort, increase the bias for each channel just to the point where the amp starts to draw just a tiny bit more current. It should run cool with the bias set like that. The following shows the basic procedure.

http://www.bcae1.com/temp/ausettingbias.swf

Wow very interesting how that affects the amp. I didn't hear any distortion when i bridged on a woofer sounded very clean and loud! Also played a little cooler didn't get to hot while i pushed it.

[chipper]

Can i get a bit of info on what the biasing serves for? Does it make any sound difference?

[Perry Babin]

Transistors don't switch on until you get about 1/2v of drive on the base (referenced to the emitter). So that there isn't a 'dead' area in the audio signal, the transistors are driven with the 1/2v that's needed to get them ready to conduct the instant that the drive voltage changes. This prevents distortion. It's not always necessary to have them biased on because many times the drive circuit can compensate quickly enough to eliminate the distortion.

The attached image shows an amp that was intentionally misbiased to show the distortion. Most amps don't have enough range in the bias control to make the crossover/notch distortion this bad.