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Sorry... :/ I started a new one because I'm not just talking about this filter, but ANY Sallen-Key (or even RC, now that I think about it) filter I've built. They all seem to follow a similar path.
I can get close. I got the first filter from here: http://www.doepfer.de/a1011_tec.htm . The second one you need to sign up to get, unless there's one someone leaked somewhere: http://korg.com/monotronschematics . Look at the monotron schematic (it comes with several)
That is entirely possible. Newbie in electronics that I am, I figured I could just take the Sallen-Key cutoff control setup and use that since that seems to work in theory. Never having much training in electronics, I can really only go off of what I have. Even the Art of Electronics (which I have as an e-book) show similar designs and theory, yet different outputs than what I get.
I can get close. I got the first filter from here: http://www.doepfer.de/a1011_tec.htm . The second one you need to sign up to get, unless there's one someone leaked somewhere: http://korg.com/monotronschematics . Look at the monotron schematic (it comes with several)
That is entirely possible. Newbie in electronics that I am, I figured I could just take the Sallen-Key cutoff control setup and use that since that seems to work in theory. Never having much training in electronics, I can really only go off of what I have. Even the Art of Electronics (which I have as an e-book) show similar designs and theory, yet different outputs than what I get.
Earlier in this thread you posted a circuit that you said was not working. I simulated it and it did exacly what I expected. Perhaps your expectations are wrong.
Why don't do try a fixed frequency filter and tell us exactly:
1. What is the intendeded cutoff frequency.
2. What is the intended slope.
3. What is the input signal frequency and waveform.
4. What you expect in the output.
5. What is your test setup.
6. Oh, and of course the schematic with component values.
Bob
Why don't do try a fixed frequency filter and tell us exactly:
1. What is the intendeded cutoff frequency.
2. What is the intended slope.
3. What is the input signal frequency and waveform.
4. What you expect in the output.
5. What is your test setup.
6. Oh, and of course the schematic with component values.
Bob
Possibly. I think low pass filter, my 5+ years in synthesis comes into play. I expect a reduction in cutoff frequency to filter higher harmonics until any waveform achieves a sine wave.
I can do that!
f=100Hz (actually 102)
slope=not sure how to word this, so 12dB/2-pole
input f=41.2,square wave
Expectation (recorded with KORG MicroKORG with approximate same criteria):
Test setup: 555 oscillator powered by 9V>filter>Monster guitar cable>mixer>computer>Virtual Analyser 2011.
Reality:
Schematic:
Schematic values acquired from http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm
I have problems with Virtual Analyser's Spectral Analyser, so things are only close. Oh, and tell me if you need the spectral in linear mode.
I can do that!
f=100Hz (actually 102)
slope=not sure how to word this, so 12dB/2-pole
input f=41.2,square wave
Expectation (recorded with KORG MicroKORG with approximate same criteria):
Test setup: 555 oscillator powered by 9V>filter>Monster guitar cable>mixer>computer>Virtual Analyser 2011.
Reality:
Schematic:
Schematic values acquired from http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm
I have problems with Virtual Analyser's Spectral Analyser, so things are only close. Oh, and tell me if you need the spectral in linear mode.
I agree with you that this looks wrong. For one thing the spectrum analyzer is showing a waveform at about 80Hz rather than 40Hz. And the harmonics are not falling off as they ought to at 12db / octave.
First of all, what op amp are you using, and with what supply voltage(s).
Are you using dual supplies for your opamp? With this circuit you either must do that or use a virtual ground that is 1/2 the single supply.
How are you coupling the square wave into the opamp? If through a capacitor and you have a single supply, you are going to be cutting off the negative half of the waveform.
I suspect that the op amp is clipping and turning what you should get out back to something close to a square wave.
Bob
First of all, what op amp are you using, and with what supply voltage(s).
Are you using dual supplies for your opamp? With this circuit you either must do that or use a virtual ground that is 1/2 the single supply.
How are you coupling the square wave into the opamp? If through a capacitor and you have a single supply, you are going to be cutting off the negative half of the waveform.
I suspect that the op amp is clipping and turning what you should get out back to something close to a square wave.
Bob
I am using an LM741CN, but unpowered, as shown in the schematic.
Like, dual power supplies? How would I even connect that?
I just have pin 3 of the 555 connected to the input resistor on a breadboard. No special coupling or anything. You can imagine Vin as Pin3.
So, should I put a passive attenuator (aka a semi-large resistor) between the 555 output and the filter? Or, even better, I can run a pure square wave out of my microkorg and into this. That has a very low output voltage of I think 2.5v, max.
Like, dual power supplies? How would I even connect that?
I just have pin 3 of the 555 connected to the input resistor on a breadboard. No special coupling or anything. You can imagine Vin as Pin3.
So, should I put a passive attenuator (aka a semi-large resistor) between the 555 output and the filter? Or, even better, I can run a pure square wave out of my microkorg and into this. That has a very low output voltage of I think 2.5v, max.
Unpowered? No wonder you are having problems! The schematic leaves off the power connections, but you must have them.
A 741 will require a dual power supply, i.e. one that is positive wrt gound and one that is negative. Use two 9V batteries connected in series with the middle being connected to ground, the other + terminal to the V+ pin of the 741 and the other negative terminal to the V- pin of the 741. Then use a capacitor of about 1uF between the 555 and the input of the filter. I am sure your results will improve!
Bob
A 741 will require a dual power supply, i.e. one that is positive wrt gound and one that is negative. Use two 9V batteries connected in series with the middle being connected to ground, the other + terminal to the V+ pin of the 741 and the other negative terminal to the V- pin of the 741. Then use a capacitor of about 1uF between the 555 and the input of the filter. I am sure your results will improve!
Bob
Goes to show what I know! May I ask why it needs 18v, not 9v?
So I set things up as you said, looking for smoke (my first time doing a dual power supply like that), and got some interesting results.
So, playing around with things, I decided to remove the -9v connection (I have opamps to play with in case one burns out)
Then I connected the -9v wire to ground
I'm no expert (obviously), but it looks like the -9v supply creates a high pass filter and the ground connection makes it seem *more* like a lowpass. What would cause this?
So I set things up as you said, looking for smoke (my first time doing a dual power supply like that), and got some interesting results.
So, playing around with things, I decided to remove the -9v connection (I have opamps to play with in case one burns out)
Then I connected the -9v wire to ground
I'm no expert (obviously), but it looks like the -9v supply creates a high pass filter and the ground connection makes it seem *more* like a lowpass. What would cause this?
Last edited:
davenn
Moderator
its + and - 9V
you will find for many Op-amp circuits this is the way they need to operate ( split supply)
Dave
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