Using 4066 to add CV control to Filters?

[loss1234]

So I was looking at the MXR circuit that uses 4066 and 4069's to make an envelope filter.

It is such a great way to avoid using an expensive CA3080! so i was wondering if there are other circuits around that use switches to change resistance.
and what is an easy way to vary the duty cycle of the clock feeding the switch?

and are there problems with bleed?

anyway...any help highly appreciated

thanks

[mdh]

R.G. has a schematic of a tremolo and a phaser based on this method (PWM, or phase width modulation) in his ASMOP (a simple matter of programming) article. I played with the phaser, and tried to design an analog replacement for the digital LFO, because I was getting tired of matching FETs for Phase 90 type circuits.  Honestly, I didn't have much success, but I think I went about things kind of backwards -- I started with the LFO and clock, and then tried to put in the audio path.  I wound up with a gate whose frequency was controlled by the LFO, and opened and closed with a nice pop.  I think those problems were just my errors, though, and you could well succeed at it.  I don't think that clock bleed is likely to be a problem, as long as you keep it out of the audio range.  The clock in the MXR EF runs at about 35kHz, and doesn't cause problems.

One other thing worth mentioning, though, is that 3080s aren't the only OTAs out there.  I just picked up a rail of 10 NJM13600s in my last Mouser order, and now I have a Ross Phaser on the breadboard.  It sounds amazing, and even with breadboard spaghetti syndrome, it's pretty darn quiet.  Those 10 NJM13600s cost me $6.30, and they're dual units.  The last time I tried to buy a CA3080, it was going to be $3 or more, and it turned out that the shop couldn't even get it (the NTE sub!) from their distributor.  The little bit of experience I've had with these chips has pretty much sucked all the fuel from my PWM fire, and the only reasons I can see for going back to those experiments are the need for an arbitrary, programmable (microcontroller-based) LFO, or just plain old curiosity.

[loss1234]

thanks a lot for the info

so really then i just need a way to vary the duty cycle of the clock right?

btw...is there a schematic for this lm13600 phaser?
thanks

[mdh]

Well, I seem to recall reading that the way the MXR EF does it is to actually change the DC bias of the clock output, which effectively changes the duty cycle, since the clock isn't a perfect square wave.  But that could be completely wrong.  Varying the duty cycle should do the job, though, yes.  Also, I think if you search for "PWM phaser," you might come across a similar project that was done by someone else on this forum.  I'm not sure if he ever published the schematic, though.

The schematic (and a pcb layout) for the Ross Phaser are on Tonepad.  It sounds great, and Francisco even designed an add-on board so that you can insert more phase stages, 4 at a time.

[notchboy]

Another PWM phaser:

http://www.uni-bonn.de/~uzs159/phaser.html