composite amplifiers

That it works under certain conditions is always a good start. Then the basic circuit should be correct.

The quiescent current per OPA2134 is around 10mA.
I only see one LM1875 (mono) so you only use one of the OPA2134 OP-AMPs I assume. Did you "tie" the other OP-AMP such that it does not oscillate?
Do you have an oscilloscope so you can see if there is any HF oscillation on the output of the LM1875 or on the output of the OPA2134?
What are your supply voltages for the LM1875 and the OPA2134?

My LM1875 does HF-oscillate when the peak output current approaches 3A. For initial testing I use only +/-18V. For my OPA2134 +/-15V.

The LM1875 has a tendency of HF-collapsing on one (the lower) half of the sine-wave. That will leave a resulting DC at the output. You cannot hear the HF self-oscillation as it is in the order of 1MHz (for mine at least).
 
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Hi, no, I havn't an oscilloscope, and if I had one, I wouldn't know how to use it. I am a total newbie and I'm glad, I know how to measure voltage, resistance & current with my supercheap multimeter.
I've never heard of tieing one half of an opamp, but thanks for the tipp. I'll google & try it.
The supply voltages are +/-15V & +/-26V.
 
You make a simple circuit for the unused OP-AMPs (eventually a simple voltage follower tied to ground) to prevent them from HF-oscillate and thereby pulling all the OP-AMPs down. Thus, unused OP-AMP outputs to the inverting inputs and the non-inverting inputs to ground.

With +/-26V supply of the LM1875, mine would HF-collapse at higher sound levels. Try for a start to use a 10 Ohm power resistor in series with the speaker. Then the LM1875 has much less tendency to HF-collapse.
With this, can you then make the OPA2134 work?

Is your other 8-pin IC a voltage regulator?
 
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.

If you ordered from an official TI distributor (Newark/Farnell, Digi-Key, Mouser, RS, etc.) you have real parts. The parts undergo a final test before they ship from TI, so unless you or the distributor damage the parts (which is unlikely) the parts you receive will work.
If you ordered from eBay, Amazon, or some flea market, all bets are off.

I bet the reason the circuit works with one opamp but not the other is that the circuit is unstable and rails with the OPA2134 opamp. Without running a simulation of the circuit (or taking some measurements) it is impossible to tell for sure what's going on.

Composite amplifiers are not good candidates for random opamp substitutions. To ensure stability, the composite loop relies on the characteristics of the specific opamp used.

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?

I'm assuming you mean mA not mAh. Current is measured in ampere (A). Battery capacity in ampere-hour (Ah).

You can find the quiescent current of the OPA2134 in its data sheet (which you can find on TI's website or through any internet search engine). 100 mA should be plenty.

Tom
 
Sorry for the late Answer, I must have missed the question. The other 8-pin is a RC4195 +/-15V Regulator. RC4195 pdf, RC4195 description, RC4195 datasheets, RC4195 view ::: ALLDATASHEET :::
Don't worry, I didn't pay for those, it's part of a legacy of a good friends father in law and noone but me could make a use of the electronic stuff the man left behind. Most is as it is said "new old stock", old parts never used.
 
Composite amplifier. OU LF357/LM1875
 

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My source for this circuit is almost 20 years old*, so a forum and web search didn't turn up anything about it. I'm soliciting comments, opinions, and possible mods using other op amps or more powerful chip amps. I'm attracted to its elegant simplicity. And the numbers ain't bad either:

10 watts RMS into 8 ohms ____________ >0.003% THD @ 1kHz

* Charles Kitchin, Scott Wurcer, and Jeff Smith. Composite Audio Power Amplifiers. Electronics Now, Nov 1992: 38-44
check out this thread:
Modern Version of Scott Wurzer's turbocharged Power Amplifier with TI's OPA1611 ?