[sdiy] Boss PC-2 CT pot replacement

[Steve Lenham]

On 09/09/2011 12:08, Ben Barwise wrote:

> My problem is that I have been trying to find an alternative to the CT
> potentiometer for sweep, the best solution I came up with was to use a
> switch here is my progress ... bottom part is were I am at
>
> http://img15.imageshack.us/img15/3672/pc2scem.jpg
>
> but the problem is that when the sweep is off by putting the pot CCW
> the pitch changes when you switch because of this offset on the
> envelope.
> I was wondering if anyone could give me tips on how I could null this
> offset (Is it not possible??)

Just off the top of my head:

- use an extra opamp to generate an inverted copy of the envelope.
- feed the inverted and non-inverted signals to the two ends of a 
standard (non-centre-tapped) 1K pot.
- connect the wiper of this pot to the upper 22K resistor feeding the 
inverting input of the opamp.
- connect the lower 22K resistor to the half-rail voltage.

By doing this you should get +ve sweep at one end of the pot, nothing in 
the middle and -ve sweep at the other end, with no need for a switch. 
There will be no interaction with the pitch control since the impedances 
around the non-inverting input of the opamp stay constant.

HTH - good luck!

Steve L.
Benden Sound Technology
www.bendensound.co.uk