M100 left channel weirdness

[fuzzysnuggleduck]

So today I notice my bass output was all wrong, dirty and just plain horrible.

I narrowed the issue down to the left channel on the M100.

To troubleshoot, I did the following:

1. Tried a single sub on each channel. Left channel output is terrible, right channel sounds normal.

2. Swapped RCAs to see if problem channel changes, it doesn't.

3. Bypassed TBAt, EQ and Xover by plugging the RCAs from the HU directly into the M100. Left channel still dirty and wrong while the right channel was still normal.

4. Tried using and not using the LP Xover on the amp, no difference in output.

So, I opened up the M100 to see if I could find any obvious signs of damage. I took the amp totally out of the covers so I could see both the front and the back side of the board. No visual damage whatsoever. I spend some 10 minutes looking at it very closely. Everything that I could see looks clean and normal.

I didn't check under the heat sink though because I'm not interested in getting that deep into it without knowing what I'm doing. I may not be capable enough to put that back together. From what I could see of the FETs, nothing looked burned, but like I said, I didn't take off the heat sink so I can't be sure of anything related to the FETs.

The only thing I did recently that I think maybe could have caused damage was attempt to RTA my system. I was pretty confident that I wasn't pushing the system too hard during the RTA session but who knows, this is the only thing I can think of that could have damaged the amp recently because I haven't been totally blowing the subs away or anything. Everything what setup correctly during the RTA session so it's not like I was feeding the sub amp high frequencies or anything out of the ordinary.

The weird thing is only one of the channels is messed up but I'm running the subs bridged off the amp so I would have figured pushing the amp too hard would have damaged both channels if anything but maybe not, I don't know enough. Maybe this was just waiting to happen, too. The amp is older...

Now, to describe the bad output.

The first thing I noticed is the volume is way too low. The second thing is the sound is really muffled and very scratchy. It's very dirty and shitty. It's still playing the music... but it's just gross, horrible output.

Cecil, Eric, anyone else... any ideas? Does my description of the output sound like anything you've seen or heard before?

Cecil, I may try and hit you up for your services if this is something I can't resolve on my own. I'll PM you in that event and we can go from there. I know you're a busy guy so if you don't have time, that's cool and all.

Thanks. I appreciate any and all information that can help me resolve this issue. I'm willing to put time and money into fixing this but only after I've moved to Vancouver as I'm too busy over the next week.

[1moreamp]

Well, yeah I have heard of this before. its a fairly normal over drive condition and you stretched something a bit further than it was supposed to go. Probably current limiting resistors have had a stretch put on them.

I have heard and seen some of the little 1/8 watt resistor giving up on high current loading and rising in value from there norm.
If it was a MS amp i would say start checking out all of the green and grey bodied resistors, as these are fuse resistors and they blow out just like fuses and Viola ! shitty sound. These are meant to blow before transistors go, so if any of these are blown you must replace them with exact parts only.

Unless your use to teching these things out I would say send to the tech of your choosing. The M series amp are very similar to the MS series amp. So I would say please check the right and left speaker output for DC offset voltage. Compare one to the other with no RCA's hooked up and no Speakers hooked up. just turn the amp on and read the DC voltage on each output set of terminals.
If there is no unusual DC offset then its most likely leaky diodes and resistors that have changed value. If on the other hand you have higher than normal DC output I would venture to say you might have a bad transistor or two.

Get back with me about what you find when you can..... C

[fuzzysnuggleduck]

First of all, thank you so much. This helps me understand the problem, which is something I intend on doing. This is my baby and I have to treat her right. I did notice unusually high temperatures on the heat sink maybe a day before this happened, but it didn't actually start sounding bad until after I did the RTA. Maybe the ultra low frequencies (I had it playing 1Hz-20KHz for a couple tests) at those settings was just too much.

Well, I should get over feeling like a dumbass over pretty quickly. I don't want to dwell on that, just learn from it.

I'll take those readings you asked about and see what's up. While I've always thought of myself as a rather poor solderer, I've also always thought of myself as somewhat tech-savvy. I'm willing to learn a little bit and I've been meaning to get a good DMM, so I might as well go out and get it. I have an analogue MM, but it's pretty much shit. I'll see what else I can do from there. I suspect it has to do with taking various readings over resistors and diodes. I can do that. Replacing them... perhaps. Not very confident at this point though. That might change after I get familiar with the problem and resolution.

I value your input greatly.

[fuzzysnuggleduck]

Got a decent DMM today, going to get readings a bit later in the day. I might even open up the amp again and look around for those 1/8 watt resistors and see what's up with them.