[sdiy] CMOS V.C.R. with 1V/Oct Response

[eq927 at freenet.carleton.ca]

Thanks Grant and Rene! I've used the circuit in both a HP filter and a 
phaser (using two randomly selected CD4049, where the processed CV from 
the first 4049 is also applied to the 2nd 4049 - it probably works 
better if matched 4049's are used, or a separate servo for each 4049 but 
sounds like a phaser as is.) Take a look at:

	http://www5b.biglobe.ne.jp/~houshu/synth/mike_irwin/

and there is also the schematic for a rezzy 24 dB/oct HP VCF and a photo 
of the board. It is meant for line-level signals. If you look at the 
voltage on the drain of the slave MOSFET's with a scope you can see that 
it swings symmetrically about ground (at least for small peak-peak). 
There is very little CV thump  with this circuit. I haven't tried using 
the "2R+C" linearizing approach yet but it probably would make an 
improvement in distortion. ARP replaced the "2R+C" linearizer with a 
TL081 follower in the compander used in the Quadra phaser. This is 
probably a good idea if you want to use sample-hold modulation, since it 
will avoid the CV slewing that would occur with a R and C connected 
between the gate and drain. I haven't tried this with the CD4069, but 
suspect it won't work as the 4069 has additional protection diodes.
Regards, Mike