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Old 25th April 2007, 12:37 PM   #1
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Default LM3875 - newbie gainclone project

I have a few questions about lm 3875 gainclone design shown below.HERE is the scheme
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1.Why are that 100nF caps needed, do I have to include them in circuit and what can happen if not?

2.How to properly test the amp output(DC voltage)?
*I have first measured the DC voltage without load-20mV, than I have connected 5.6Ohm/5W resistor and the DC voltage was raising or jumping quickly (I think it was more than 1V) and became very hot, so in moment I turned it off

NOTE:
Im using +/- 34V DC power supply, 2x1000uF caps per channel BUT I did not include that 100nF caps in circuit

Thank you very much
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Old 25th April 2007, 01:00 PM   #2
kscharf is offline kscharf  United States
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The small caps are to bypass any noise from the ps around the amplifier. Depending on the quality of your ps filter caps they may not be needed and you might not hear any difference. In the very worst case the amplifier could be slightly unstable without them, but if the amp is being operated with a gain above 11 this is unlikely except to very poor circuit layout.
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Old 25th April 2007, 01:23 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
the 100uF decoupling on board are probably mandatory.
The 1000uF smoothings caps would need to be very close to the board to replace the 100uF, possibly less than 50mm (flow and return) of wiring.
The heating when connecting the dummy load sounds like oscillation.
Houston, are you there?

BTW,
I have a big disagreement with the gainclone builders.
Inverted and omitting part of the DC blocking is asking for offset problems.
Passive attenuators add to these problems.
Omitting adequate smoothing cripples the continuous power ability and low bass response. But what do I know? It is only my opinion.
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Old 25th April 2007, 01:58 PM   #4
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How do I fix oscillation than?
btw; my caps are about 10cm from chip
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Old 25th April 2007, 04:14 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
fit the on board decoupling.
Fit the output terminal Zobel network and/or the output Thiele network.
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Old 25th April 2007, 05:39 PM   #6
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Ive found THIS about Zobel, but still not have idea what exactly could be(some stabilisator I guess)

btw; Can I go after original (National Corp.) design - not inverted?
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Old 25th April 2007, 06:32 PM   #7
Fenris is offline Fenris  United States
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Zobel is mereley the description of the topology. The purpose of a zobel in a crossover is to flatten the impedance of a driver in order for the crossover to see a constant impedance, making the crossover more predictable. The purpose of the zobel with a gainclone is to dampen high frequency (above audible range) response. It is merely a capacitor and resistor in series with each other, and placed across the output terminals of the amp.

And yes, you can go with a non-inverted design. Most builders actually recommend it.
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Old 25th April 2007, 07:45 PM   #8
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inverting or noninverting, this should be nothing to do with stability
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Old 26th April 2007, 12:36 PM   #9
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Wow, I have a little problem here;
When I make connection between signal and GND input, I get DC supply voltage at output (I had a luck that I didnt connect speakers)
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Old 26th April 2007, 04:15 PM   #10
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No ideas how to fix this?
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