Analog Bit Crusher - Duty Cycle Control?

[thehallofshields]

I've got the Duty Cycle Comparator going.

My Potentiometer is useful in a swing from about 1V-4V. Too close to the Vb voltage and it cuts out. At about 1V it goes totally clean with no carrier signal bleed-through. Sounds identical to True Bypass on my breadboard.

I used V+->147k->100k->22k->V-

This really does act as a perfect Depth control. Useful at lower frequencies that would otherwise crush the notes to bad they all sound the same.

[thehallofshields]

The use of the Comparator makes a big difference in the dynamics of the circuit.

Standard mode it is very compressed and soft sounding with a little bit of treble loss. A little too compressed.
On my breadboard I have zero Clk Signal bleed-through when not playing.

Through the Comparator (on minimum Duty Cycle) it still sounds louder, more treble, and very uncompressed. Maybe a little too boomy.
A little bit of Clk Signal bleed-through on my breadboard. It might be the extra wires or Breadboard capacitance. I'm not really sure.

[thehallofshields]

Maybe someone could give me an idea of why the overall Volume level and clarity jumps way up when the Clk signal is coming from the comparator?

I'd like to find a sound in-between the two extremes.

[thehallofshields]

Okay. I switched things around a bit and am now having the Comparator compare Vb and V- and it's working pretty well. I also added a voltage divider at the comparator output because the 1M Resistor wasn't enough to lower the Amplitude of the Pulse. Circuit is much more tame.

BUT! Can someone riddle me why any of my MPF102's won't work at all in this circuit? J201 working flawlessly.

[blackieNYC]

Hey Slacker - thanks for sharing.  What happens to the gate of the fet? 

[anotherjim]

FET Vgs value is probably critical - it can only have close to 4.5v in the circuit because that's where the amp bias point is. If the signal level thru the amps is small enough, you can probably raise the bias towards around 6v and that might make it less critical for the FET's.

[thehallofshields]

FET Vgs value is probably critical - it can only have close to 4.5v in the circuit because that's where the amp bias point is. If the signal level thru the amps is small enough, you can probably raise the bias towards around 6v and that might make it less critical for the FET's.

I looked up the cutoff Voltage of each:

MPF102 Vgs: -25V

J201 Vgs: -40V

I don't understand these figures but... the problem is the MPF102 isn't able to turn off? That would seem to be what is happening.

[thehallofshields]

I'm being a stickler about this because I have almost 100 MPF102 and only 1 J201 on hand.

[slacker]

You need to look at the vgs(off) figure. This is how far below the source the gate has to be before the fet turns off. In the design the source sits around 4.5 volts and the lowest you can get the gate is to ground which is -4.5 volts relative to the source.
On the data sheet I looked at Vgs(off) for the mpf102 is -8volts maximum with no minimum specified. So some might work but no guarantee.  For j201 it's -1.5 volts max.

[thehallofshields]

Wow. Thanks for all of the help.

I threw a Diode in series with the MPF102 and now it wants to start Sampling.

I'm not sure why the Diode helps but I do see it in a lot of Sample and Hold Circuits out there.

[blackieNYC]

I think a frequency independent Duty-Cycle control is a must for this circuit, even if it is going to require another opamp or more.

You can do this by taking the output of X4 into the + input of an opamp, connect the 1Meg to the fet to the output of this opamp instead of where it is now. Then connect a pot, 1 outside lug to 9 volts, the other outside lug to ground and the wiper to the - input of the opamp.
This makes a comparator where the threshold is set by the pot, the output of X4 is a sawtooth wave, when the wave goes above the threshold the output goes high, when it's below the threshold it's low. This gives you a variable duty cycle that doesn't interact with the speed of the oscillator. You'll probably find you want some limiting resistors between the outer lugs of the pot and the supply to set the limits once you see if it does what you want.
My question wasn't clear - Slacker, what happens to the gate connection when you move the 1meg from it to the op amp? Not following you.

[slacker]

Sorry my explanation was badly worded, I meant the 1M now goes from the gate to the output of the new opamp instead of to the output of X3.

[thehallofshields]

This circuit is coming along great. The MPF102 actually needs an LED sized Forward Voltage at the Gate to Sample. It just so happens a Blue LED was the first thing I tried. Lucky.

I wanted to add more, but playing with my controls, I don't think Frequency Modulation of Pulse-Width Modulation would sound good with this circuit.

Sweeping the Frequency Pot just sounds like clean signal with Varying Clock Feedthrough. Copilotfx Antenna added an Exp Jack for a Rate Control and it just sounds like white noise when used. Sweeping the Depth Pot kind of kills the Harmonic Presence.