Boss ce2 c22 cap query

[paul525715]

I'm looking into creating a CE2 style chorus pedal with a switchable intensity mod. I think it's cap C22 on the original Boss schematic. The original value is 47pf and I want to switch in/out another 47pf using a logic operated switch (e.g. a CD4066 chip). Does anyone know how the "on resistance" of such a chip (about 120r) will affect the sound of the intensity mod, or will it work exactly the same as if I were using a physical switch?

Many thanks,

Paul

[antonis]

47pF exhibits a capacitive reactance of 34M at 100Hz, 3M4 at 1000Hz & 340K at 10kHz..
So, I presume 120R resistance wouldn't do any harm..

P.S.
Welcome..!! 

[Rob Strand]

If you look at the datasheet you will see Cis and Cos at about 8pF max.  Not all datasheets show it.  The typical capacitance is not spec'd but it might be about 4pF.

The way I interpret that spec is there's capacitance from the switched connections to Vss (or Vdd) on *each* side of the switch.   With a total of 8pF (upto 16 pF) connected to ground and only a 47pF cap for timing I'd expect a significant effect.  So the way to reduce the problem by half is to switch the side of the cap on pin 5 of IC4. The switch capacitance on one side of the switch is driven directly by IC4's output and is largely removed from the picture.  On the other side what you have is the 4pF to ground in series with your switched cap (Cadded) then that connects to pin 7 of IC4.   When the switch is off that 4pF in series with Cadded still connected top pin 7.  This will affect the circuit a bit but you might be able to compensate by changing the value of the 47pF.

The way to set it up is to set the depth to minimum and measure clock frequency with 47pF but without the CMOS switch present at all.  Then add the switch and tweak the capacitor to get the same frequency.

The simple answer is to try it but without carefully measuring before and after it's easy to miss a problem.

Another scheme would be to switch the 150k on the +5V side using either a series or parallel resistor.  Without doing a lot of analysis it's hard to know if it would work out better than switching the cap.

[paul525715]

Thanks both for your help!

Cheers,

Paul