Thread: Amp distorting on 1 channel (large pics)

If they are mosfets, they are not npn and pnp but n-channel and p-channel. npn and pnp only applies to bipolar transistors.

Unfortunately, i'm clueless when it comes to things like mosfets and transistors, otherwise I'd probably be able to be more detailed... It is a MOSFET amp tho, It says so on the front.

Would it be worth swapping some of the irfz44n's around, and see if different channels are affected? (theres 8 of them in the amp)

Also, heres a link to a pretty detailed datasheet I found, forgot to post the link first time around: http://www.loutre.org/schemas_chaine...ts/IRFZ44N.pdf
The chips are made by International Rectifier, and by looking at the date thing in the datasheet, they were made in 1997.

Maybe this might shed some light on something? I can link up a scope to the dodgy channel and post some test tone results if needed?

Originally Posted by MikeHunt79

I can link up a scope to the dodgy channel and post some test tone results if needed?

That would actually be very useful. Depending what class of amp it is, it might be one of your output drivers gone or the crossover trimmer got some dust in, which would result in only one side of the signal coming out (top or bottom half) or distortion at the crossover point. All just speculation atm.

I'd be surprised if it was the power supply unless each amp has it's own supply (which is possible on high quality amps), but then the symptoms would be different (loss of a channel for example).

Rob

Both good news and bad news.

I've literally put the just amp back together, and re-installed it in the car. I first tried it out with my minidisc player before I started to dig out my laptop to make the test tones.

First of all, I plugged a single RCA into the bad channel, and it still sounded the same (crap). I then tried plugging the same RCA plug into each of the other channels, and they still sounded good. While doing this, the bad channel let out a crackling sound for around 2 seconds even tho there was no RCA plugged in.

I then tried plugging the RCA back into bad channel, and it now sounded good! I tried my utmost to get it sounding bad again, but I couldn't do it. :/

I've had the problem intermittently for the last week or so, but I only took the amp out when it started sounding bad most of the time.

If it starts sounding bad again, I'll get my laptop and scope so I can get some results out of it.

you mentioned that you thought running it at a 2ohm load might have caused the prob...
what amp is it?
what is it rated at (rms at 4ohms and *2ohms* into 4 and 2 channels)
stable down to how many ohms?
most amps can totally handle that load, so i recommend double-checking your WIRING, as that is much more commonly screwed up than an amp at 2ohms, esp if the other 3 channels work fine.

Update: Took my car out for a long spin earler, cranked the amp right up, and it sounded fine.... no crackling or distortion at all.... i'm pleased but puzzled. I Just hope it stays good, even tho I didn't change any components.

Originally Posted by stkdidy ntshidy

you mentioned that you thought running it at a 2ohm load might have caused the prob...

I'm still not 100% sure about it, but i'm now running it at 4ohms again, just to be sure. Theres a wiring diagram on the bottom of the amp, and in every diagram (bridged, etc.), it shows 4ohm speakers being used.

Originally Posted by stkdidy ntshidy

what amp is it?

I dont know who makes it... I won it at a police auction about 5 years ago, and have had great sound from it until about a week ago. There were a few on e-bay a couple of weeks ago, but there not there any more.

Originally Posted by stkdidy ntshidy

what is it rated at (rms at 4ohms and *2ohms* into 4 and 2 channels)
stable down to how many ohms?

It claims to put out "150wx4" on the front of the amp, but I think this equates to 75w rms per channel at 4 ohms. I've tried quite a few different speakers, and found it has had more than enough power (unless your're using subs).

Originally Posted by stkdidy ntshidy

most amps can totally handle that load, so i recommend double-checking your WIRING, as that is much more commonly screwed up than an amp at 2ohms, esp if the other 3 channels work fine.

I double and triple checked the wiring, and it's all good.

Maestro: I am gonna replace the IRFZ44N's, i've already ordered them, it can't hurt to replace them.

Also, when putting the amp back togesther, I noticed the heatsink screw was very close to the leftmost IRFZ44N, so i moved the IRFZ44N a bit to the left... I think the screw could have been shorting the IRFZ44N to the heatsink/case of the amp.

The amp started to sound bad again, but this time I was ready. I busted out the laptop, and run some test tones thru it... I'm afraid the pictures didn`t come out too well... lcd screens are a pain because they are so reflective, so i had to experiment with the cameras flash a bit (and use photoshop for sharpening). Heres the results:

50Hz sine being put thru a good channel:


Same thing being put thru the bad channel:



I know the pictures are pretty bad, but the the bottom half of the bad channel is fine... It's just the top half that is being clipped.

By now the transistors had arrived... So I took the amp in for soldering...

All I did was replace the very bottom left hand transistor (the IRFZ44N), bolt it all back together, and went to put it back in the car, but when I connected the power, the wire i was connecting at the time (+12v) gave out and spark, and I heard a loud BANG! noise come from the amp. It sounded like a component dying (the IRFZ44N maybe?), but I carried on anyways to see if any of the amps channels would now work.

And they did! The bad channel sounded even worse, however, it sounded so distorted that you could barely tell what sort of music you were listening to. :-/ By this time, it was dark, and I was tired, so I gave up for the night...

I'm gonna have another go at this tomorrow, but any help/advice/information would be appreciated, I really want to get this sorted.