Help, amp problem

[dswiston]

Hey guys, I need some help. I may have a disaster on my hands. After testing the thermal system of a P101 (rod elliot mosfet based amp) for more than 2 hrs and the heatsinks never exceeding 100F, I decided to finally get around to setting the bias as I never formally did. I put some 100Ohm 1/4W carbon (all I had in the way of 100Ohm) resistors in series with the supply lines, turned the amp on, and took some readings. I got around 1/2V across the resistor which wasn't what I expected, and then the resistor (or at least I think it was) smoked a bit. Hastily I turned the amp off, figuring that the 1/4W resistor just wasn't going to cut it being so small and all. I hooked the supply line back up w/out the resistor, hooked some speakers back up, and turned the amp on. I got some oscillation type noise instead of nothing. I hooked up an input and repeated. I got mostly oscillation type noises and in the background a very faint signal that was lacking all bass. Upon further inspection, the negative supply line is supplying 3amps or so to the board but the positive supply line is supplying none, with no signal hooked up. The scope on the output shows no whacky signals from the outputs. I tested the mosfets in the circuit and none are shorted but they do warm up quickly and the power resistors in the circuit next to them get toasty too.

Any ideas what on earth could be happening? I changed nothing except the series resistors and then back again.


Thanks so much guys.

[dswiston]

I believe I have figured it out but I am very confused.

It appears as though the parallel R/C combo that connects the signal ground to the power ground was cooked. The 1W resistor appears to be an open. I put in a new resistor of the correct resistance with some alligator clips and all is well again, well except one of the woofers in the speaker I was using it is toast as in voice coil = roasted

The question is, what caused this? I will absolutely need to stop this from happening again and potentially ruining some of my much more precious speakers than my bench testing speakers. Maybe a DC protection circuit is in order?

[dswiston]

The odd thing is, I didn't see any noticeable DC output from the amp module when I looked at the output on the scope. How much DC does it take to ruin a speaker, electrically or mechanically?

[anatech]

Hi dswiston,
It takes very little DC to destroy a loudspeaker. 10 VDC will do it easily as the voice coil gets pushed out of the gap. Normally the heat gets radiated to the magnet structure and dissipated from there. A little gets cooled if using a vented pole piece. The part of the voice coil outside the gap will be burned much worse. Normally, the glue will fail, the former may warp as well.
So, it looks like you lost your ground reference for the circuit. Make sure the ground return goes to the supply common. There should never be any current flowing through the reference ground connection (for the input stage and input jacks).
I feel it is irresponsible to design or sell an amplifier without DC offset protection. It is a fact of life that devices will fail, that means that some amps will fail with a DC offset across the output terminals.
Try 1 ohm resistors or less next time for bias measurement. Do not hook up speakers until after you measure the amp under test for DC offset.
-Chris

[hughmon]

"well except one of the woofers in the speaker I was using it is toast as in voice coil = roasted"

Sounds like we know where that -3 amps was going. A disrupted ground can do all sorts of strange things. Just taking a guess from what you've said, your scope was probably referenced to a point at a different potential then the speaker's ground, so you wouldn't have realized there was a DC offset flowing current through it.

[dswiston]

Good to know, I wasn't aware that just 10v DC could do it, never thought about the voice coil moving beyond the magnet like that.

Any ideas why the resistor between signal and power ground went bad?

When speaking of the ground return, are you making reference to the power ground? The power ground is connected directly to the supply common ground. I'm thinking what may have happened is while I was playing around with the wires one of the psu caps discharged somehow by some random event, not sure how.


Any ideas for DC protection outside the amp and inside the speakers/crossover, or a VERY small DC protection circuit? With 10V DC causing problems a simple fuse will not help. I don't have much room left in the amp and did not plan ahead as well as I should have. I do however have 12v DC avaliable in the amp and would not be opposed to using a wallwart to supply a circuit outside the amp with 12v DC.

Thanks so much guys.

[hughmon]

"Any ideas why the resistor between signal and power ground went bad?"

It's tough to say without actually being there and looking things over. Sometimes it can be as simple as a hidden component defect, or mechanical stress. There maybe some electrical issues during testing also.

As for protecting speakers from offsets, just for bench testing like you were doing it can be as simple as a coupling capacitor between the amp and speaker. For enjoyment listening in the easy chair with the "real" speakers, an active protection scheme is a better way to go. Since you have little room in the chassis, building one in an outboard chassis might be the better way to go. Placing a relay in the wires between your amp and speakers will, in all likelyhood, effect the sound to some extent. With that possibility in mind, it's probably better to have the protection circuit cut-off the AC power to the amp, as opposed to just disconncting the speakers from the amp. You may want to look around on the web for some design ideas.

[dswiston]

Well cutting off the AC leaves the caps to discharge, and the caps have enough juice to keep the amp running at decent levels for maybe 10 seconds so that may still not cut it, but I'll search online. I read some mosfet based scheme last night but it didn't make too much sense to me as I was too tired at the time.

BTW, both amp modules (left and right) had the same 10Ohm resistor go to resistor heaven, which is interesting.

[anatech]

Well dswiston,
Many kits and circuits exist for that purpose. Should be easy to find. Some of these are fairly small and 12V is a popular voltage.

As a note, a relay contact will affect the sound less than a fuse element. I wouldn't cut the AC with the relay, but you could shut down the current sources (if you have). That will effectively shut down the amp. If you are worried about damage to the amp, don't. Done is done. There is enough charged energy in the power supply caps normally to finish blowing stuff up.
-Chris

[anatech]

Hi dswiston,
Seems I posted while you were at it. Do you have a schematic of your amp to show us?
-Chris