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
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
Not really, is a MC33171N datasheet available?
The 220μF is doing the dc-decoupling, the 470nF + 10Ω is the zobel network.
As these caps act together with the two 100kΩ input pots, the two 250kΩ pots in tandem at the summing opamp and the two 10kΩ, the 10μF and the 50kΩ pot on the center opamp, as a somesort of low pass filter.
In the meanwhile, I'm scratching my head as why this arrangement is chosen. What is the intended difference between 'summing' and 'center'?
The 4kΩ-450Ω divider at the output is also very uncommen: - 20dB attenuation, rendering the zobels useless.
Hi,
Thank you for your replies.
I've uploaded the spec sheet for the IC.
The 4kΩ-450Ω divider is gone. It was there because I had originally been using LM386s which produce an 8 ohm output. The purpose of the complex on the lower left was to provide line output. But since the 33171 does not – and perhaps with the spec sheet you can tell me you think its output impedance is - that circuitry is gone. Instead I'll be feeding the output of both sections into a proper (little) 3W amp. (It's a PAM840 which requires 5V so I'll have to add an LM7805.)
The "center" is just that: a center channel feed. It is now less complicated and does not need a chip.
What I labeled "summing" is a differential amp, at least based on my understanding of that. The purpose is this: I want to suppress what is common to both left and right channels. So, given that voice is typically on both left and right channels, I can get an output without the voice. This is handy for someone (my grandnephew) who is learning guitar and wants to pick up songs from recordings. (I guess I called it “summing” inasmuch as I’m summing the right with the inverted left to get that result.)
Once I've redone the schematic I'll post it. I hope you'll offer some comments.
Thank you again,
Roy
Thank you for your replies.
I've uploaded the spec sheet for the IC.
The 4kΩ-450Ω divider is gone. It was there because I had originally been using LM386s which produce an 8 ohm output. The purpose of the complex on the lower left was to provide line output. But since the 33171 does not – and perhaps with the spec sheet you can tell me you think its output impedance is - that circuitry is gone. Instead I'll be feeding the output of both sections into a proper (little) 3W amp. (It's a PAM840 which requires 5V so I'll have to add an LM7805.)
The "center" is just that: a center channel feed. It is now less complicated and does not need a chip.
What I labeled "summing" is a differential amp, at least based on my understanding of that. The purpose is this: I want to suppress what is common to both left and right channels. So, given that voice is typically on both left and right channels, I can get an output without the voice. This is handy for someone (my grandnephew) who is learning guitar and wants to pick up songs from recordings. (I guess I called it “summing” inasmuch as I’m summing the right with the inverted left to get that result.)
Once I've redone the schematic I'll post it. I hope you'll offer some comments.
Thank you again,
Roy
Ahh... that kind of summing...
What you want to happen is that all the common parts of the L/R signal are suppressed, leaving the (stereo) difference intact.
But most of the 'L/R difference' signal is mainly in the upper frequencies. The input caps of 470nF acts as an low pass -> high cut filter, so pretty much nothing remains...
And apart from that, the intuitive 'substraction trick' by using the (+) and (-) inputs of an opamp has prooven not to be the best idea, alas.
Remove those 470nF's and see what happens, it might be usefull enough for the intended purpose. The IC is ok.
What you want to happen is that all the common parts of the L/R signal are suppressed, leaving the (stereo) difference intact.
But most of the 'L/R difference' signal is mainly in the upper frequencies. The input caps of 470nF acts as an low pass -> high cut filter, so pretty much nothing remains...
And apart from that, the intuitive 'substraction trick' by using the (+) and (-) inputs of an opamp has prooven not to be the best idea, alas.
Remove those 470nF's and see what happens, it might be usefull enough for the intended purpose. The IC is ok.
Thank you, again.
It was suggested on another board that those 450nF caps should be no more than 330 for full audio bandwidth. Would you agree?
Also, if you had a chance to look at the MC33171 spec what would you say is its output impedance?
It was suggested on another board that those 450nF caps should be no more than 330 for full audio bandwidth. Would you agree?
Also, if you had a chance to look at the MC33171 spec what would you say is its output impedance?
If the input pot is at 50%, the series resistance is 50kΩ. With the 470nF cap, it yields a 7Hz low pass filter. There will be no audio at all.
As with #6, opamps are not supposed to work like that.
You better switch to instrumentation opamps, like the INA series by TI. Read the datasheets and the Application Notes.
The amplification is set with one resistor, search for 'analog subtraction circuit'.
Depends on the amount of feedback, which is not present. Somewhere between 10 and 1000Ω ?
