Advice on Summing & Center Channel Circuit

K1ngR0y said:

MC33171N shows me a freq. vs. impedance graph which suggests to me that the IC's output, unlike the LM386, is not 8 ohms.
Can anyone tell me what the likely impedance would be based on my circuit?

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FvM said:

Two additional thoughts about circuit aspects, KlausST has already characterized most basic issues.

- 470 nF capacitor is clearly a schematic fault also with LM386, it forms a low-pass with worst case 15 Hz cut-off frequency, shouldn't be more than about 330 pF for full audio bandwidth.
- LM386 circuit is actually no summer rather than a substractor. The difference matters because it cancels mono signals respectively the center sound of stereo signals.

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KlausST said:

Hi,

you may call it schematic ... but it does not follow any standards thus it´s hard to read, needs additional documents to be opened and cross checked ...
In the end I gave up to spend more time. Missing part names, missing standard symbols, missing pin descriptions...
A schematic does not need to be colorful to be "easy to read".

Also your text is confusing. I´m not sure: is it your design or did you copy a circuit (If copied --> contact the original author)

It seems you just removed the LM386 (it is no OPAMP - instead it is an audio power amplifier) and replaced it with a true OPAMP .. without reading any datasheet and adjusting the schematics accordingly. This won´t work. Even we professionals need to read and keep on datasheet informations.
--> OPAMPs need negative feedback.

It should be easy to do an internet search for "audio suming OPAMP circuit" ... then from all the hits, please choose the "professional" ones (best from semiconductor manufacturers) .. not the most colorful ones from dubious random internet sources.

Yes, datasheets. If you want us to read them --> provide links to the documents.

Also a lot of text ... with 90% of the informations I don´t know what to do.
On the other hand I miss important informations like: what is the circuit supposed to do ... and how?


The LM386 output impedance is not 8 Ohms either. It is designed for load impedances of 8 Ohms.
A "typical output impedance" of a feedbacked OPAMP may be down below 1 Ohms ... as long as you don´t overload the OPAMP (read typical output current spec.) Still this does not mean it can DRIVE that low load impedances.

And ... a typical OPAMP is no power amplifier! .. it is not designed to directly drive a speaker or a random headphone.

My recommendation: forget about the shown schematic ... there are so many mistakes ... I don´t think it´s useful to go on with it.
--> Please do research for professional informations and professional circuits.
Like standard circuits: search for "OPAMP standard circuits tutorial"..
(There you should find out that EVERY audio signal OPAMP circuit has feedback from output to inverting input)


If you want someone to design the circuit for you ... consider to pay for it.


Klaus

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