symmetrical 400 buzz ...ooops ...help!

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Mattyo5

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Joined 2002
#1
Ok. Very quickly...what happened was in effect, I connected the speaker out + of both my anthony holton symmetrical 400's monoblocks. (don't ask...it was a mistake) Music still comes out of both monoblocks, but one has a buzz now. Worse, instead that amp also has -200 mV dc offset rather than the +3mV that it should have. ALSO, when I shut the amp down, the DC offset climbs from -200mV to -1.2V!

http://www.aussieamplifiers.com/sym-sch.htm

As I said, the other monoblock is fine, still silent, and only 3mv dc offset and no turn on thump or off thump. Any ideas? Where should I start looking for trouble? Does it sound like a driver transistor is going? Thanks!!

-Matthew K. Olson
 
#4
One of the grounds was connected to the speaker....thats the amp that is buzzing I think. The other amp...which is ok...only was connected at the speaker + out. Yes...speaker + out was connected to speaker + out. yikes!

-Matthew K. Olson
 
#5
No music was playing. Imagine, one speaker was connected... - and + and as soon as I connected the other + ....POP...icky sounds...and I immediatedly disconnected it...this is embarrassing. Anyhway, we connected everything correctly...and the amps still played music...even cranked....but w/ a nasty buzz. Again, the buzz I think is coming from the amp that had been grounded to the speaker. Later!

-Matthew K. Olson
 
#8
Matthew,

The DC offset seems to increase up to a good fraction of a volt when I turn the amp off (normally). R9 could be damaged, I smoked mine once when it accidentally conducted speaker current. Besides that I am not sure what could have happened, but it can't hurt to check the bias on each output device. Once one of my output devices got damaged and its Vgs went down to 2.6 volts [!], I'm glad I caught that before it took out anything else.

It was nice meeting you, by the way! (It may be years before my amplifiers are anywhere as nice as yours.)
 
#9
Well, my DC offset is still negative after removing a bad R9, but at least its at -3mV...and never climbs when I shut the amp off....so thats good 🙂 Any idea on why the dc offset is still negative? I haven't checked Anthony Holton's dc nodes yet....I'll work on that in a few minutes. Just wanted to post my results.

Kilentra...don't worry...you'll get there 🙂 I have spent MANY hours reading and many many huge $$$ on learning how to do amps. Plus I have found a lot of good cheap sources. Some day I'll post all my sources for all the cheap stuff I've found. :-D Back to playing w/ a toasted gainclone (which was part of the sym 400 toasting too...don't ask) Later!

dc offset negative? comments?

-Matthew K. Olson
 
#15
Well...beats crap outa me. Ok...here's the real scoup on what happened. I made a gainclone custom for a guy ....i've never met him until this last weekend when I finally met him and we listened to my sym 400's. During setup...since he doesn't believe in banana's....he had his gainclone connected to his speakers via solid core which was just screwed to the terminals of the gainclone. Well...so I figured...I'd just keep the gainclone off and plug the sym's into the terminals ...because I had bananas and it'd make it easy. Dumb me. The gainclone is an inverting amp, and the + terminals on the gainclone are really tied to ground. THEY ARE BOTH TIED TO GROUND. So when I went to plug in both amps into the gainclone...pop...pow...icky. I'm lucky I didn't destroy his speakers. Ok, long story short...I developed a buzz in my mono's....we went...huh...well its probably the cables or something...who cares. We cranked it for about half an hour...everything was fine except for the buzz. Man did we have it cranked and the amps didn't even heat up. Ok...so then I was like, uh, lets plug in the gainclone and see how it sounds compared to the mono's. Well, haha. uh...we plug it in...turn it on...and snif...snifff......something smells. I open the gainclone case and poof...a puff of smoke comes out :-D DOH!!!!!:smash: :devilr: :dead: :devilr:

So now I've finished fixing both...and my wife prefer's the mono's :-D Later ya'll 🙂

-Matthew K. Olson
 
#16
Well that is an interesting story, I will keep that in mind when working with inverting circuits. 🙂

While you're here, I might as well ask, how did you mount/connect the output devices? I used Holton's suggested method using aluminum angle-brackets and the temp drop is just too much, I want to re-do it with the devices mounted directly to heatsink somehow (except his PCB isn't designed to allow under-board direct mounting a la Pass DIY). I have these huge heatsinks, 0.28C/W per channel and the transistors still heat up a good bit (45C idle to 60[?]C) from music over 90db. This would merit fixing before I decide to really crank it anymore. I didn't get a close enough look at yours on Saturday.
 
#17
Ok, this is really simple. Have a look at holton's other amps. If you notice he has his outputs mounted via an aluminum bar effectively bolting the heatsinks right to the main heatsink without individually bolting each output right to the heatsink.

http://www.aussieamplifiers.com/avamp.htm

I thought it was a good idea...I bolt down w/ 1/8" aluminum bar and brass #8 screws...works well...nonferrous etc. Time to get working on dinner and listening :-D Laters!

-Matthew K. Olson
 
#18
Kilentra said:
Well that is an interesting story, I will keep that in mind when working with inverting circuits. 🙂

While you're here, I might as well ask, how did you mount/connect the output devices? I used Holton's suggested method using aluminum angle-brackets and the temp drop is just too much, I want to re-do it with the devices mounted directly to heatsink somehow (except his PCB isn't designed to allow under-board direct mounting a la Pass DIY). I have these huge heatsinks, 0.28C/W per channel and the transistors still heat up a good bit (45C idle to 60[?]C) from music over 90db. This would merit fixing before I decide to really crank it anymore. I didn't get a close enough look at yours on Saturday.
Click to expand...

I mounted my devices directly to the main heatsink, look

here

Cheers

Andrea
 
#19
Dang andy you stole my idea on the caps! haha! looks great 🙂 Hope you are enjoying those caps...I know I am. Pics of guts will be up soon. Actually, I think Kilentra got some pics of my amps this weekend. We'll see if he posts them. Later!

-Matthew K. Olson
 
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