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Need help with my new buffered gainclone!
http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=180
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Author:  bsgd [ 12 Oct 2008, 13:17 ]
Post subject:  Re: Need help with my new buffered gainclone!

RiGuy wrote:
Thats insane and really cool. So, you put it on the output. I wonder if you could use a filter at output instead of the snap on. Not that the snap on is a bad idea to have there. Just wonder if a filter would do the same thing.
So how did you come up with this idea? Just desperation?
Is the output line just straight through the ferrite or did you wind it?
Anyway, really interesting and makes me want to try it on mine just to see if there is any difference. I have cleared up all problems on mine except a humm from my preamp and some HF from the power supply. I think its the fans on the supply but would be neat to try ferrites as I bet they don't mess with the sound as much as caps.

Well, I guess you could call that desperation, yes. I knew my wiring was perfect and still somehow the chips were oscillating. The only thing I hadn't tried so far to fix that was some ferrite chokes, so I thought it was worth a shot. Indeed it worked.
Also the output line is just straight, my wire was too small to wind it around the ferrite. But maybe I'll wind it in the final version.

BUT... there are some bad news. I'm getting a quite dirty top end from the amp. I don't know why really, and I'm 100% sure its not the ferrites (I removed all of them and disconnected both chips, so I had only one working, and still the top end was quite dirty).
What could cause this? All my parts are premium, except for those 68uF caps (cheap no brand). Right now I'm using 26V DC at each 3875.

I would strongly recommend some ferrites in your amp Ri. I never really believed they did help at all, but now I really do.

Author:  bsgd [ 12 Oct 2008, 13:19 ]
Post subject:  Re: Need help with my new buffered gainclone!

gmilitano wrote:
Hi Bernardo, I wonder if the problem is your speakers / speaker cable. Are the cables high capacitance? Perhaps the speakers are a very difficult load to drive. Perhaps you can try the amp with another pair of speakers.

I'll try it with another pair. Actually the cables are just cheap ones. But still, the chips without the ferrites and with a dummy 8R load still heat up.
What do you think could be causing the dirty to end?

Author:  bsgd [ 12 Oct 2008, 13:21 ]
Post subject:  Re: Need help with my new buffered gainclone!

johnmcquay wrote:
I can't help but think you have a problem with earth loops. :|

Hi John, thanks for your help. I checked and re-checked everything but there are no ground loops. I'm using star ground and all chips have a separate ground wire running to the star bus. I really dont know where is this oscillation coming from. Right now Im even using coax cable everywhere, even for the speaker connection!

Author:  johnmcquay [ 12 Oct 2008, 13:46 ]
Post subject:  Re: Need help with my new buffered gainclone!

You're welcome. It was a shot in the dark for me, I've never built a gainclone amplifier so had to just take a guess.

Kind regards, John.

Author:  Gio [ 12 Oct 2008, 22:34 ]
Post subject:  Re: Need help with my new buffered gainclone!

bsgd wrote:
I'll try it with another pair. Actually the cables are just cheap ones. But still, the chips without the ferrites and with a dummy 8R load still heat up.
What do you think could be causing the dirty to end?

Right, so that rules out the cables and speakers. I'm stumped and out of ideas. Sorry.

Author:  johnmcquay [ 13 Oct 2008, 03:00 ]
Post subject:  Re: Need help with my new buffered gainclone!

Have you tried putting a 1n Cap between the supply rail and ground?

Kind regards, John.

Author:  johnmcquay [ 13 Oct 2008, 14:48 ]
Post subject:  Re: Need help with my new buffered gainclone!

Would it be possible to see a photo of the inside of the build? I work with someone who's built more amps than I've had hot dinners. It's a possibility he may spot something that has been overlooked. I know we all hate a smartass (me especially) but a fresh pair of eyes may be all it takes.

Kindest regards, John.

Author:  bsgd [ 14 Oct 2008, 12:14 ]
Post subject:  Re: Need help with my new buffered gainclone!

johnmcquay wrote:
Would it be possible to see a photo of the inside of the build? I work with someone who's built more amps than I've had hot dinners. It's a possibility he may spot something that has been overlooked. I know we all hate a smartass (me especially) but a fresh pair of eyes may be all it takes.

Here's the pic of the amp built based an BR100 circuit;
download/file.php?id=202&mode=view

Right now the innards is really a mess (cause Im experimenting with everything I can) but I can post a pic anyway. But, aside from small changes, its just the same.

Author:  bsgd [ 14 Oct 2008, 12:15 ]
Post subject:  Re: Need help with my new buffered gainclone!

johnmcquay wrote:
Have you tried putting a 1n Cap between the supply rail and ground?

I tried that with 100n and 220n. Didnt help.

Author:  RiGuy [ 28 Oct 2008, 15:44 ]
Post subject:  Re: Need help with my new buffered gainclone!

Ok. Another suggestion.
Actually you are not following the circuit as shown in 1192. You are using monster caps on your chips. These are supposed to be about 4.7uf and polypropylene. Thats it. The power you need is going to come from your supply. Trust me you can get bass without those monsters on the chip.
Now we may want to ask Gio on this, but the schematic says otherwise. Mike Feurbacher has used this chip with NO caps and claims decent, not fantastic, bass.
Check the datasheet on page 6. It says use of large leaky electrolytics will effect the offset of the ouput voltage.

There seems to be a lot of talk about how hard it is to bridge this chip. Some people are using a DRV134 to help with the problems of differences in input voltage. I need to look into this more and discuss it with Gio.
I am going to post this in the thread so he sees it and comments.
I will work on your boards when I get this figured out because right now I think individual amp boards may not do the trick since these issues and the lack of the DRV134 or some similar circuit may be causing your problems.
Not to worry though :) We will get it done.
Ri

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