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Old 19th July 2004, 08:34 AM   #1
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Default another failed gainclone

ok, i'm getting close to giving up on this stuff in general...

i made a demo amp about 9 months ago, lm1875. it worked ok, but had issues with noise.

upon returning home from school the issue became major and the amp was unuseable!

later i made a tiny pcb for a 4780. it worked, just not at the same time i used the 1875 amp...

so now i've made a new 4780 amp. there's a bizzare wizzing sound in the tweeters and a quiet hum in the midwoofers. its unlistenable due to the wizzing sound.

and i'm just thinking wtf. i hope its not RF, becuase it should have 0 RF issues. i used a seperate ground plane for the signal processor and the amp. i bypassed everything just as stated in the datasheets. i added the RC network to remove RF. i added a zobel network to reduce RF. and still it sounds terrible!

so unless anyone has any quick suggestions, i guess i'm gonna scrap that amp. i guess the input buffer was too complex. i've found that things don't ever work.
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Old 19th July 2004, 08:45 AM   #2
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It is probably RF oscilations - look at output with good scope. Can you show pic of PCB ?
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Old 19th July 2004, 09:38 AM   #3
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Possibly you are trying to do too much at once. Make an amp, keep it simple, then add to it. That way you can troubleshoot in a logical order.
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Old 19th July 2004, 09:46 AM   #4
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no, the program i made is in dos. i'd agree its ossilations, but for another month i have no oscope. after 20 minutes of light music (midwoofers only...) the heatsink is too hot to touch. the heatsink is 11" x 3" x 2" with about 20 1/4" fins and there is an aluminum clamp holding the ICs to the heatsink.

it just makes me mad becuase i followed all the guidelines and suggestions and it works worse then some amp i threw together 9 months ago just to see if it would work -- no DC or RF protection.

its annoying because after all the parts i've bought, nothing works and i could have bought a real amp...
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Old 19th July 2004, 10:27 AM   #5
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Are the chips isolated from the heatsink? I think there's a simple error in the circuit thats allowing current to heat the sinks - can you post a picture? Try a gainclone with a pcb - easy and works everytime for me.
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Old 19th July 2004, 10:33 AM   #6
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It is clearly oscilations thats playing around here.

A small cap between the + and - input of the opamp might help.

See http://www.ben.cz/download/121098/lm3886.pdf Test Circuit #2 it is cap Cc¨.

You can also try with a small cap over the feedback resistor, that has help me alot with some troblesome opa627.
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Old 19th July 2004, 11:02 AM   #7
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You can have hf signal on the input - have you there any low pass filter ?
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Old 19th July 2004, 05:10 PM   #8
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the chips aren't isolated from the heatsink, do you think this is the problem? i may be able to fix that.
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Old 19th July 2004, 06:04 PM   #9
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like i said, the thing is made for RF rejection!

i have 2 lowpass filters (not in feedback loop)
the first cut out things above 0.5Mhz and the seconds 0.1Mhz. both are basic RC networks. further, there is the zobel (RC) network on the output, and all ICs are bypassed using 10uF tantalum + 100nF ceramic for the lm387n signal processing opamps, and using 1000uF + 10uF + 100nF for the lm4780's. the 100nf is about 1-2mm away from the leads of the IC.

at this point i can't add anymore components.

isolating it from the heatsink did not help at all.
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Old 19th July 2004, 07:03 PM   #10
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If its heating that much then its oscilating. You just need to find whats making it unstable. Sounds like your've tried everything so its just trial and error until you sort it out.

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