Advice on Summing & Center Channel Circuit

[K1ngR0y]

Folks,

I have been struggling with this project since last April. And I received good advice and suggestions back then.
But it got to the point where I felt that I no longer believed that the LM386 was the right IC for this project. (It was pointed out to me that it was ancient.)
So I've redesigned things a bit and will be using two MC33171N ICs.

The schematic was redone but was largely unchanged. That is, I substituted the new ICs and added, changed or removed connections accordingly.
So, for example, there's no longer the bias cap from the LM386 pin 7. And there's no longer a pin 1/8 gain adjustment.
But as the overall design is from an LM386-based schematic that I found on the net (and failed to store the link) certain components are still in place.
For example, the product info on the 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?
Also, there are a few holdovers as I mentioned. What purposes are served by the 220μF and 470nF caps on the output stage? For that matter I never understood the purpose of the two 470nF caps leading into the "Summing" 2-gang balance pot.
So I welcome your answers and suggestions.

Thank you,
Roy

 

[KlausST]

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?

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?

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

 

[FvM]

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.

 

[K1ngR0y]

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.

Got it, thank you

--- Updated Yesterday at 1:20 AM ---

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

understood . . . thank you