A way to match two LM3875 for a parallel circuit? - diyAudio
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Old 1st October 2003, 07:00 PM   #1
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Default A way to match two LM3875 for a parallel circuit?

Background:
Well I have inverted paralleled lm3875 chip amp here, built after “Application Note 1192 Overture Series”.
All resistors are matched within 0.01% to really balance between the chips. After the first built I measured the temperature on the chips after running the amp pretty hard for an hour and it differed 5 degrees Celsius between them. I replaced one of the chip’s did the same thing again and the difference was almost the opposite 7C. I had a few chips to test and eventually I found two that that had the same temperature after running them hard. After building the second channel with the same hazel and played it for a while. I love it and it sounds great and has tighter bas than my unparallel GC.

Conclusion:
Their is small differences between different LM3875 (I might be wrong, maybe something else affect the temp difference but I haven’t thought about it)

My question:
Is it any fast and easy way to match or measure the chip’s other than doing it the brute force way?

/Magnus
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Old 1st October 2003, 07:37 PM   #2
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you could try to test them with a small DC signal (DC couple the inputs before), say 0.5V (small enough to prevent the output to clip), and look at the output voltage
repeat the operation with all chips, and adjust the gain (you'll have to use pots for input and/or feedback resistors) so that all the chips output the same voltage with the small DC input

This is a rather complicated way to do this.


You could also simply put a series resistor between each IC's output and the "common" output, say 1R. Exactly like with paralleled voltage regs
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Old 1st October 2003, 08:33 PM   #3
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Magnus, lost swede in big US... think about this: What happens if you take two lead batteries with different charge status and connect them in parallel? Right, huge current from one of the battery to the other. You have the same problem with two outputs connected together. You must insert a small resistor if you don't want a current between the amps. Those IC's aren't any precision devices. Check the AN9-1192, sensible values!

You know, you can't trim this because you will have temperature drift. The only way to avoid those output resistors is to use DC-servo or doing some selection but how long term stable will this be?

Answer: Connect only one IC at the time and measure the output offset. Then take two IC's with the same offset.
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Old 1st October 2003, 08:34 PM   #4
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After viewing the "Application Note 1192 Overture Series" from the National semiconductor website, I see that the IC's are operated in a "bridge output" mode.

One explanation for a higher temperature rise on one IC might be that the audio being fed to the amplifiers isn't symmetrical.
Almost all audio is asymmetrical in nature. The difference between positive and negative voltages in the waveform can be greater than 10dB. Under such circumstances, one IC might be delivering more power than the other IC.

As Bricolo advised, if the IC's have a slightly different voltage offset at their outputs, you might try adding a small series resistance (.27 ohms/5 watts) between the output of each IC and the loudspeaker. This will help balance the load between the two IC's.

I hope I have been of some help to you.
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Old 1st October 2003, 09:48 PM   #5
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Well I know they are not any precision devices, thats why I had to do it brute force, it really works nice now. I had small series resistor between each IC's output and the "common" output from the begining. I have probed the temp for two weeks now, and they stay within one degree all the time.

Just asked to get to know if it was any easier way to find the one's that worked together. I might test with a little dc signal next time.

Maybe a really stupid question, but what happens if I cross the feedback paths, could that balance them, is it even possible to do? Building one more, to play around with, I'm to happy with this one to play around with it anymore.

/Magnus
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Old 1st October 2003, 10:00 PM   #6
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Well it looks to me as if it is parallel for sure, did you really look at the one on page 7?

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Old 1st October 2003, 10:01 PM   #7
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The offset problems comes ONLY from the input stage alone. Your idea seems bold but I don't think it's so good after all.
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Old 1st October 2003, 10:03 PM   #8
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Oh no! Do not cross the feedback paths.
That will give you positive feedback and an oscillator that can deliver lots of power.

I'll look at the application notes again. You might be able to add a little bias voltage to the inputs of IC's through a balancing potentiometer. That might null out the dc offset.
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Old 1st October 2003, 10:09 PM   #9
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I'm bold, fat and ugly, so any good reasons till that I shouldn't try it?

Efter ett par flaskor OP vågar man allt!

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Old 1st October 2003, 10:17 PM   #10
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Another thought.
You might also rid yourself of the d.c. offset problem by adding a really large capacitor between the output of one of the IC's and the loudspeaker.
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