Some of you folks have seen this schematic which I’ve attached a few times.
I believe I’ve corrected the mistakes and hope to start building.
But aside from what errors you might uncover I have a question about grounding as pertains to the three ICs (MC33171N [spec sheet also attached]).
But first the purpose of the device:
Does this seem right?
I believe I’ve corrected the mistakes and hope to start building.
But aside from what errors you might uncover I have a question about grounding as pertains to the three ICs (MC33171N [spec sheet also attached]).
But first the purpose of the device:
- Produce a center channel (mono) output
- Produce an output consisting of what is unique to each of the channels of the stereo signal
and - Produce an output consisting of what is common to each of the channels of the stereo signal
Does this seem right?
They are op amps and as long as the inverting input mirrors the outpu pin, it will be happy.
Don't you think the ics will struggle with a 200R load; R12 & R14.
I would choose a better, production ic; TL071 etc.
Don't you think the ics will struggle with a 200R load; R12 & R14.
I would choose a better, production ic; TL071 etc.
I chose 200 ohms because from what I read (unless I'm cuckoo) the stated output impedance of the MC33171N is 200 ohm.
As unity gain is satisfactory for IC3 I simply matched them. What do you suggest?
As for the TL07X group what advantage is there? (I have the 33171s and am really not looking to continue to buy stuff.)
As unity gain is satisfactory for IC3 I simply matched them. What do you suggest?
As for the TL07X group what advantage is there? (I have the 33171s and am really not looking to continue to buy stuff.)
Never load an op amp with less resistance than stated as minimum in the data sheet.
This is typically 2k, but some are as low as 600R.
This is typically 2k, but some are as low as 600R.
Where did you get 200 ohms from as an output impedance?
The output impedance of any opamp like this configured with feedback as here is miniscule, tiny tiny fractions of an ohm.
The circuit doesn't quite make sense tbh. The LPF at the top looks to be just a load to ground when switched into circuit which won't do anything. IC3 doesn't seem to be configured correctly with the inverting input going directly to the output of another opamp. The 200 ohm resistors appear as a load to both opamps IC2 and IC3
Any common opamps should work including what you have (and even 741's) but not with those 200 ohm values which would limit maximum output swings to just 3 or 4 volts rather than the full -/+ 12 volts you could expect with correct values.
The output impedance of any opamp like this configured with feedback as here is miniscule, tiny tiny fractions of an ohm.
The circuit doesn't quite make sense tbh. The LPF at the top looks to be just a load to ground when switched into circuit which won't do anything. IC3 doesn't seem to be configured correctly with the inverting input going directly to the output of another opamp. The 200 ohm resistors appear as a load to both opamps IC2 and IC3
Any common opamps should work including what you have (and even 741's) but not with those 200 ohm values which would limit maximum output swings to just 3 or 4 volts rather than the full -/+ 12 volts you could expect with correct values.
"The associated high frequency low output impedance (200 � typ @ 1.0 MHz) allows capacitive drive capability from 0 pF to
400 pF without oscillation in the noninverting unity gain configuration."
This was the only reference to output impedance in the IC's spec sheet. Have I misunderstood?
What R12 and R14 values would you recommend?
400 pF without oscillation in the noninverting unity gain configuration."
This was the only reference to output impedance in the IC's spec sheet. Have I misunderstood?
What R12 and R14 values would you recommend?
I'm with you 🙂 You've not misunderstood as such but it needs qualifying.
The opamps in your circuit (any audio circuit with any opamp suitable for audio) behave as 'perfect' voltage sources. That means if the signal output from the opamp is lets say 2 volts peak to peak and you load the opamp with a 1k resistor to ground then the output voltage does not change at all. It remains at 2 volts because the output impedance is almost zero (that tiny fraction of an ohm I mentioned).
If the output impedance were really 200 ohm then adding our 1k load would see the voltage across the load fall to around 1.6 volts peak to peak (just ohms law). That doesn't happen in this circuit or any other normal audio circuit using opamps but this only applies at DC and relatively low frequencies, and in the scheme of things all audio is very low frequency.
The very low output impedance we see in the audio circuit is achieved because of the action of the negative feedback applied around the opamp by your other components. The opamp itself will have a much 'higher' output impedance when run without any feedback (which we never do in audio). The figure in the data sheet partly relates to this and is saying that the opamp remains stable driving capacitive loads because of this. It gets a bit more complex because capacitive loading affects the phase margins provoking instability.
Because of this rise in output impedance at high frequency the 400pF cap loading will not provoke instability or oscillation.
The opamps in your circuit (any audio circuit with any opamp suitable for audio) behave as 'perfect' voltage sources. That means if the signal output from the opamp is lets say 2 volts peak to peak and you load the opamp with a 1k resistor to ground then the output voltage does not change at all. It remains at 2 volts because the output impedance is almost zero (that tiny fraction of an ohm I mentioned).
If the output impedance were really 200 ohm then adding our 1k load would see the voltage across the load fall to around 1.6 volts peak to peak (just ohms law). That doesn't happen in this circuit or any other normal audio circuit using opamps but this only applies at DC and relatively low frequencies, and in the scheme of things all audio is very low frequency.
The very low output impedance we see in the audio circuit is achieved because of the action of the negative feedback applied around the opamp by your other components. The opamp itself will have a much 'higher' output impedance when run without any feedback (which we never do in audio). The figure in the data sheet partly relates to this and is saying that the opamp remains stable driving capacitive loads because of this. It gets a bit more complex because capacitive loading affects the phase margins provoking instability.
Because of this rise in output impedance at high frequency the 400pF cap loading will not provoke instability or oscillation.
First, I will change the 200 ohm resistors to 2.2K as another poster suggested. Would this do the trick?
Second, let me address "the inverting input going directly to the output of another opamp."
Is there a difference between what you wrote and "the output(s) of IC1 and IC2 going into IC3?" Really, I'm not being sarcastic; it's just that how you phrased it is the opposite of the circuit flow and I may not understand a subtle difference.
Regarding the LPF, when SW3 is down the SW2 output is passed through and the LPF is completely isolated from the circuit.
When SW3 is up the SW2 output enters the LPF and emerges from it hopefully boosting the ~70Hz neighborhood.
Or have I miswired it? (Had I been able I'd have rotated the RC circuit 90 degrees.)
When you write "The 200 ohm resistors appear as a load to both opamps IC2 and IC3" if you mean R12 how else could I wire it?
My grand nephew has gotten into music in a big way. He's teaching himself guitar and learning songs as he does.
I figured that if he could listen to a song without vocals it'd be easier for him to pick it up.
Also, as there is a balance (for the IC2 input) he could vary the output content.
Believe me, if I thought that a karaoke machine would do the trick i'd have saved myself a lot of grief.
Apropos something Mooly brought up, the purpose of the LPF is to boost the bass (~70Hz ) since bass is often common to both channels.
This one:
It is a data sheet value giving clues to how the amp will behave when capacitively loaded.
For R12 yes. C4 can then be reduced in value by a similar factor.
There is a big difference. Output of IC1 going to the + or non inverting input is fine. Output of IC2 going direct to the - or inverting input is not fine. IC3 will try and bring he difference in voltage between the two inputs to zero. That is done via the 200 ohm feedback resistor but as you have the - input connected to what is a 'zero ohm impedance point' which is IC2 output the circuit will not work in any form.
The circuit itself is hard to accurately analyse visually without very detailed study but these errors are fairly basic. Just take IC 3 in isolation and look at how it behaves when you ground the - input (which is effectively what IC 2 does). When signal is applied it all gets a lot more complex but the basic error remains.
This is what happens for real...
This shows that area of your circuit being supplied with two dissimilar input signals, a 1kHz sine and a 1.2kHz sine with a 45 degree phase shift. So two valid input signals each 1 volt peak in amplitude.
Now we look at the two opamp outputs. Op amp IC3 output is so distorted because the opamp is running at maximum gain due to the inverting input being tied to IC 2 output.
Remove the input signal from IC3 and we get this. The opamp is running flat out for gain and clipping. Even the clipped output level is low because it is struggling to drive the 200 ohm feedback load.
If we reduce the opamp loading and more importantly configure IC3 to a workable configuration we get this which are correctly mixed signals. Wheter all your circuit will achieve what you want in terms of removing vocals etc etc is another totally different question.
Mooly,
You recommend changing C4 from 220 to 4.7. Do you also suggest that I eliminate C5?
-RF
Typical values I see for that chip (NS8002) are as low as 0.1uF. If that cap is already present on a pre built board then you don't need C5 and neither would you need C4 and R12 on your diagram.
Does this method actually work to remove only the vocals from a stereo recording?
Sorry, I don't mean to be negative.
I'm just having a hard time understanding how those three signals can be used to effectively remove only the vocal content.
I've attached the sheet for the NS8002A module. Unfortunately these sheets are written by guys who spent three days learning English.
I have found contradictory statements in a few of them. In this one it's unclear to me whether or not an external decoupling cap is needed because of two contraditory statements.
The 200uF value is a holdover from a year ago when I was using an LM386. But the MC33171N is not designed to drive a speaker so I can see that value being off. I know that schematic symbols vary. To my American eyes the cap you've circled is not electrolytic. Am I correct?
Were I to keep the decoupler could I substitute the 220uF+ with a non-electrolytic 1uF ceramic? Or is a decoupling cap necessarily electrolytic? To be honest I'm tired of buying stuff. I have a few 4.7uF+ and a 10uF, as well as a few 47uF+. I also have a ton of 1uF ceramics.
I have found contradictory statements in a few of them. In this one it's unclear to me whether or not an external decoupling cap is needed because of two contraditory statements.
The 200uF value is a holdover from a year ago when I was using an LM386. But the MC33171N is not designed to drive a speaker so I can see that value being off. I know that schematic symbols vary. To my American eyes the cap you've circled is not electrolytic. Am I correct?
Were I to keep the decoupler could I substitute the 220uF+ with a non-electrolytic 1uF ceramic? Or is a decoupling cap necessarily electrolytic? To be honest I'm tired of buying stuff. I have a few 4.7uF+ and a 10uF, as well as a few 47uF+. I also have a ton of 1uF ceramics.
IC1 (hopefully) removes the vocals.
IC2 turns both channels into mono.
Then, according to my brilliant plan, feeding a L+R signal and a no-vocals signal into a third IC (IC3) I hope to get only the vocals.
I'm mainly looking for the ouput of IC1. But then I decided to get ambitious.