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I need help: drawing a wave multiplier circuit w/ rectifiers
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page Previous  1, 2 [all]
Author I need help: drawing a wave multiplier circuit w/ rectifiers
Blackened Justice
Can't I basically divide the mixing part in 2 sections, so that I mix the rectified signal with the -5 in an op-amp and mix the rectified centered signal with the CV 2 in a second op-amp? So that the tap would just be a parallel branch after the first op-amp.
neandrewthal
Blackened Justice wrote:
Can't I basically divide the mixing part in 2 sections, so that I mix the rectified signal with the -5 in an op-amp and mix the rectified centered signal with the CV 2 in a second op-amp? So that the tap would just be a parallel branch after the first op-amp.


I was almost going to suggest that, but then I realized that the LFO signal you insert into the second op-amp would only be half wave rectified.
Blackened Justice
No, I mean actually adding another opamp dedicated to the mixing of the CV in, before the parallel branch to the diode.
daverj
Sure you can. At that point why not just build several single stage units , each with 2 inputs. Then normalize the output of the first unit into one of the inputs of the second unit (and second to third, etc...).

That way they can be used individually, or be cascaded in a chain if you don't use one of the inputs.
Blackened Justice
Blackened Justice wrote:
What about a 4 stage, or whatever, but with individual inputs for each stage, where each stage's output would be normalled to the next stage's input? It would allow for completely modular patching and still have them cascaded by default.


I suggested exactly what you did a few posts ago ;D /\
|

Anyway, I'm gonna breadboard it when I get the -5v regulators, and then I'll write back to comment on it.

On a more pratical concern, 2 ins and 1 out per stage leaves us at 3 jacks per stage. I'll probably put a polarizer on the CV ins and a regular attenuator on the Signal ins. So that leaves us at 2 pots per stage.

I was trying to design a polarizer myself, and came to a relatively simple design: put your input into a 1:1 inverting amplifier configuration, and put the output of that opamp through an attenuator. Then put the original signal through another attenuator. Then connect the output of both attenuators together. Of course, this would depend on the existence of dual pots, a single knob to control 2 resistors. Does this exist?
My other solution was to route the inverted and the original signal into both of the potentiometers "inputs" and then take the output from the pointer. Would this work? I know that even with the potentiometer at the extremes, there would still be a bit of the other signal present, but couldn't this be overriden by using a very large value pot?

I ask because I looked at the schematics for the Fonik attenuverting mixer, and it seems overly complicated.
daverj
Blackened Justice wrote:
My other solution was to route the inverted and the original signal into both of the potentiometers "inputs" and then take the output from the pointer. Would this work? I know that even with the potentiometer at the extremes, there would still be a bit of the other signal present, but couldn't this be overriden by using a very large value pot?


That will work fine as long as the signals are both buffered. You don't want to connect both the pot and the input to the inverting amp directly to the input jack. That could cause variations in the gain or oscillations. Run the jack to both a non-inverting amp and to an inverting amp and then those two amps to the two ends of the pot.

There's no worry about bleed since the signal at one end of the pot is exactly the inverse of the other end so the bleed is canceled out. You would only get bleed if using this method to mix two unrelated signals.
Blackened Justice
But won't I get slightly less than full voltage swing?

Why can't I connect the pot to the input jack? What do you mean variations in the gain?
daverj
Many modules have series resistors on their outputs. These range from zero (no resistor) to maybe 1K. If you connect one end of the pot to the jack and the input of the inverting amp, and the other end of the pot to the output of the inverting amp then the pot itself becomes part of a feedback circuit with the amplifier.

Tthe gain of that circuit is unknown since the series resistor on the output of whatever module gets plugged in is unknown.

With a buffer amp between the jack and the pot the output resistor of the module plugged in becomes negligible. And the inverted signal can't feedback backwards through that buffer so the pot is not part of a feedback circuit.

As for your other question, if the pot allows a small amount of the opposite signal to mix in when the pot is at an extreme, yes it slightly reduces the levels. But that is a constant and is easily compensated for by using a slightly smaller resistor on the input of the amp that the center wiper of the pot is connected to.

Or, better yet, it can be compensated by increasing the gain of the inverting and non-inverting amps. In fact, you could increase the gain of both to maybe 2x gain anyways. That gives the circuit more range, so it can go from about 2x positive to 2x negative gain.

You might also want to toss in a non-polar cap and a switch to connect it on at least one of the units, so you can AC couple a signal into that unit (maybe do it on the first unit in the chain).
Blackened Justice
Thanks for your description, it makes more sense now. Well, but I guess I'll use Fonik's design, even if I don't quite understand it, because it uses a single opamp per channel.

An AC/DC coupling switch seems cool, even if any possible offset the AC input signal has could be corrected by the internal offset summers.
daverj
Quote:
An AC/DC coupling switch seems cool, even if any possible offset the AC input signal has could be corrected by the internal offset summers.


An offset pot will remove DC, but won't remove low frequencies (sub sonic), which a cap will. In fact, it could be a 3 position toggle switch and the 3rd position could be a different frequency high pass filter by simply using a smaller cap.
Blackened Justice
Just an update: I've breadboarded the circuit (a single stage, using 3 opamps), and it works great! I put a guitar through it, and it sounded pretty much like an octave fuzz. With simpler waveforms, it added some pretty tasty and crunchy harmonics. It sounds really cool being animated with a low frequency CV, and is great for drones when using audio-rate CVs, with a harmonic relationship between the In and CV frequencies. I'll try another stage, and then I'll design a PCB for it. I'm using TL074s, so the design repeats itself after 4 stages (4*3 opamps=3 TL074s).
Blackened Justice
Okay, I'm at the process of designing a PCB, and I have to make some decisionsto make:
Should I use polarizers or just regular attenuators? Polarizers would add a pair of opamps and some resistors per stage, and I don't know if it's justified.
daverj
There's no point at all to having a polarizer on the first stage. You'll get exactly the same result with positive or negative signals going in.

On further stages it might be interesting in some cases (and make no difference in others) since it would add or subtract from the previous stage. It would have the most effect when the signals are similar or related.
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