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Old 6th July 2018, 08:06 PM   #51
mondogenerator is offline mondogenerator  England
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Hi.

I've long been planning to try a composite amplifier using LM675, which I believe is close to LM1875 but stable at low gains.

Got to love the empirical work. That probably how much of my builds go.

Even a rough sketch would give us an idea of the general layout.
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Old 6th July 2018, 08:11 PM   #52
DPH is offline DPH  United States
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Quote:
Originally Posted by mondogenerator View Post
I've long been planning to try a composite amplifier using LM675, which I believe is close to LM1875 but stable at low gains.
DS quotes a minimum gain of +10, so it's the same as the 1875. Frankly, looking at the respective datasheets, are they exactly the same part?

Edit: [Resolved] LM675: vs LM1875; what's the difference? - Audio Amplifiers Forum - Audio Amplifiers - TI E2E Community (yup!)

Edit edit: IIRC, the 3886 is a faster part, similarly compensated into a G=10, and should be okay with the lower voltages the same as the 1875/675. Speed on the output opamp of a composite is a big_deal.
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Old 10th July 2018, 05:20 AM   #53
danielwritesbac is offline danielwritesbac  United States
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The LM675 is lower bias. So, I'm wondering if a composite build can redact the higher crossover distortion? Or, if running slightly cooler isn't worth decreased audio quality? You'll get one case or the other.
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Old 10th July 2018, 12:29 PM   #54
mondogenerator is offline mondogenerator  England
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Quote:
Originally Posted by danielwritesbac View Post
The LM675 is lower bias. So, I'm wondering if a composite build can redact the higher crossover distortion? Or, if running slightly cooler isn't worth decreased audio quality? You'll get one case or the other.
Good questions....

The only composite amp I have built so far is a current boosted op amp using a push pull follower BJT stage, diode biased, with tip41/42.
I could never bias it heavily enough into AB with the limited current available from the bench supply so settled with the best result I could obtain empirically (low bias, low beta OP, limited base current from op amp).
That meant low watts, but at least on a scope the output looks clean until 4 or 5 watts.
That's my head amp lol
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Old 11th July 2018, 01:58 PM   #55
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by danielwritesbac View Post
There's probably a lot of people with LM1875 & OPA2134 available.
Do you have a schematic to share?
A sketch/drawing is fine.
Also, what power voltage did you use?
Thanks!
Finally managed to make a schematics. 98% certain on the component values. Not in my labs.
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Old 11th July 2018, 10:40 PM   #56
Clueless but avid is offline Clueless but avid  Germany
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Many thanks! I'll try it soon.
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Old 14th July 2018, 05:49 AM   #57
asuslover is offline asuslover  Romania
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mother /or father of the lm1875 composite amplifiers
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Old 14th July 2018, 05:20 PM   #58
tomchr is offline tomchr  Canada
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The original article is from the November 1992 issue of Radio Electronics/Electronics Now. Pages 38-44. Along with a few textbooks and app notes, that article was a source of inspiration in my development of the Modulus-86. You can find the full issue of RE/EN here: https://ia601701.us.archive.org/15/i...ember_1992.pdf

Tom
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Old 14th July 2018, 05:34 PM   #59
asuslover is offline asuslover  Romania
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You are right Tom, I tried to attach the whole pdf file but it was to large.
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Old 16th August 2018, 05:09 AM   #60
Clueless but avid is offline Clueless but avid  Germany
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I didn't believe I could make it work, but it does . Many thanks @ FauxFrench!

But, beneath all the light, there is some shadow, too. I ordered four OPA2134, but it seems these are not working, or maybe they're fake. With the OPA2134 I get immediately DC (I think its DC, when the speaker cone jumps outward, or what is it? I've got not so much experience yet, so maybe I'm wrong?) on the output when I switch it on.
When I replace the OPA2134 with a NE5532 it seems to work really fine.
Or, maybe my voltage regulator is to "weak" as it can only deliver 100mAh. I don't know what the power consumption of a 2134 is. Could it be more than 100mAh?
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