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new design for chorus thing?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author new design for chorus thing?
guest
ok, not sure what to call this, but it seems like it could be used as a chorus effect, or maybe a harmonizer.



the circuit is composed of two portions. the top is a topology ive been looking for an excuse to use for a while now. it basically is a voltage in, current out, absolute value circuit. the nice thing about it being current out, is that it has a really wide dynamic range. with the capacitor at its input, its a differentiator, so the output current is a linear function of the input frequency. for a triangle wave, this is an ideal relationship, with a constant current out for a fixed input frequency.

so the top portion would make a very nice, and extremely fast responding F/V converter. typically solutions for F/V count zero crossings, which has a really long lag time at low frequencies. this responds instantaneously. the output current could be put into a log converter to give a 1V/octave output.

the bottom portion of the circuit is just current steering into 2 VCOs. these VCOs can be set to be at the same frequency, or spread by equal amounts from the center frequency by the input CV. this ratio from the center frequency is fixed regardless of input signal. you could also do an offset rather than ratiometric current differencing, so you always get 100Hz difference, for example. or, you could use 4 transistors and 4 VCOs and get a bunch of voices that spread out or come together.

the ratio of the input frequency to the output frequency is set by the amplitude of the input signal. so a VCA at the input would allow you move everything up by an octave, for example. other waveforms besides for triangle can be used, and they track, but they make funky output waveforms.

ultimately, the bottom half of the circuit could just be connected to an exponential converter to make a VCO that natively does this, but i think its nice to be able to essentially sync up to something else. it makes the output wavefroms non-ideal, but this adds a fair bit of personality to it.
DMR
This is interesting, if I understand correctly the outputs are v/oct signals to be sent to stand alone VCOs?

It might also be used to create variations on a modulation signal?
jorg
The top portion will output a series of very short positive spikes, time duration set by the current limit of the op amp and series resistance of the transistors. It won't give a constant current. You could extend the width of the spikes by adding a resistor in the feedback loop. This resembles the LM2907 F-to-V converter. The LM2907 also has a 2nd capacitor to average the current pulses. It still won't be ripple-free or instantaneous though.
hox3d
If I understand correctly you simply need to mix the two VCOs to get a chorus effect?

Well actually, it could also be a CV processor for chorus-like effect (considering you would have two spare VCOs).

Or also, a VCO-controlled swarm oscillator, if current is fed into 4 oscillators. Like, supersaws.

Are my assumptions correct?
guest
hox3d wrote:
If I understand correctly you simply need to mix the two VCOs to get a chorus effect?

Well actually, it could also be a CV processor for chorus-like effect (considering you would have two spare VCOs).

Or also, a VCO-controlled swarm oscillator, if current is fed into 4 oscillators. Like, supersaws.

Are my assumptions correct?


yeah, thats all correct. the swarm oscillator i think would sound pretty good, as it already sounds pretty good with just 2.
guest
jorg wrote:
The top portion will output a series of very short positive spikes, time duration set by the current limit of the op amp and series resistance of the transistors. It won't give a constant current. You could extend the width of the spikes by adding a resistor in the feedback loop. This resembles the LM2907 F-to-V converter. The LM2907 also has a 2nd capacitor to average the current pulses. It still won't be ripple-free or instantaneous though.


if i were to feed it a square wave, it would produce spikes, but with a triangle wave, its a constant current output. the equation of one side of a triangle wave is V =k*t, and the first half is a differentiator, so dV/dt = k. so for a sine input its a sine output, and a sawtooth input produces an almost constant current if you shunt the edge somewhere else.
guest
DMR wrote:
This is interesting, if I understand correctly the outputs are v/oct signals to be sent to stand alone VCOs?

It might also be used to create variations on a modulation signal?


the outputs are oscillators, but the top portion could be reconfigured to output a V/oct signal.
jorg
If you have a perfect triangle wave, you probably are using a VCO, and you could just add offsets to its voltage and drive additional VCOs. I'm thinking more of the general pitch to CV task with a less well-behaved input signal.
Ayab
I really like the description of the application of this circuit as a quick responding chorus/harmonizer. I imagine something like a dual PLL but faster.

Not sure I have quite got my head around some of the concept such as the amplitude of the input governing the ratio of the input freq to the out freq. Should certainly be some experimental possibilities also.

It is also an interesting circuit. I am not familiar with the ven diagram type symbols - current source amplifiers? What IC would this be?

Best wishes for 2019 to all
jorg
The "Venn diagram" parts are Operational Transconductance Amplifiers: current-controlled current sources such as a CA3080.
Ayab
Hi Jorg

Thank you for explaining this to me.

I think I will try put this circuit together. Am away from my diy stuff for the next three weeks but when get back will breadboard it up.

And thanks to Guest for presenting the interesting idea.
guest
great, let me know how it goes. i made the 2.5V reference with a zener, but a voltage divider with an opamp follower would work as well. and the schmitt triggers there are HC14, but that makes the amplitude only 1V. if you use opamps, you can set the amplitude as you like.
EATyourGUITAR
Ayab wrote:
I am not familiar with the ven diagram type symbols - current source amplifiers? What IC would this be?


jorg wrote:
The "Venn diagram" parts are Operational Transconductance Amplifiers: current-controlled current sources such as a CA3080.


I will agree with this definition but I would add that theory and practice are sometimes more complicated than a simple definition. there are many ways to use an OTA that make it possible to have a voltage controlled current source. there are also many OTA VCA designs that are in total a voltage controlled voltage amplifier. we basically convert voltage to current, do the multiplication in the OTA and the convert back to voltage.

here are some drawings that show how voltage and current are directly proportional and perfectly linear based on Ohms law.





this is basically saying that if your dummy load is constant then you can trust that a change in voltage will always correspond to a change in current. and a change in current will always correspond to a change in voltage. the volt meter has an input impedance almost infinity Ohms in theory so if you follow kirchhoff's current law we can say that 0 current flows into or out of the volt meter. then we can say that the current measured coming through a battery terminal on either end, is exactly equal to the current flowing through the resistor or through either of the terminals of the resistor.

sorry to thread jack you carry on!

(hides)
Ayab
Hey guest - yes & am looking forward to seeing how the input output ratio works.

Must be the info what Eat is trying to force into me thick skull hmmm.....
looking forward to attempting testing in practice. Still little idea of electronics theory - must read up!
cornutt
. editing screwup, disregard
cornutt
guest wrote:


if i were to feed it a square wave, it would produce spikes, but with a triangle wave, its a constant current output. the equation of one side of a triangle wave is V =k*t, and the first half is a differentiator, so dV/dt = k. so for a sine input its a sine output, and a sawtooth input produces an almost constant current if you shunt the edge somewhere else.



Hmm. You could convert almost anything that was input to it (assuming no non-harmonic overtones) to a triangle wave by severely slew limiting it, in a linear fashion. The problem with that is that the output level will be extremely low, and it will still take at least a half cycle to produce a useable value.
guest
unfortunately its amplitude dependent, and a fixed slew rate will change the amplitude of varying frequency inputs. as far as a F2V circuit goes, it really needs to be right at the output of a VCO to really work.
Picard
@guest
A classic opamp differentiator would be a high pass used at a frequency well below its corner frequency. So in your case which feedback resistivity of the BE juction do you assume hence what corner frequency and how do you care about its current dependency?

BTW nice circuit, thank you for bringing it into discussion!
guest
in this case, there is no corner frequency. a standard opamp highpass filter will have a second resistor in series with the capacitor, which sets the corner frequency. the feedback resistor sets the gain. in this case, the input capacitor has virtual ground on one side, and the input signal on the other, so the current through it is: I=(1/C)*dV/dt. this current just gets passed by the transistors, so their impedance doesnt matter too much.
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