[sdiy] RE: cross-fades (was ...)

[Tom Wiltshire]

On 16 Jan 2009, at 18:45, David G. Dixon wrote:

>>> Can't you just feed the two signals into either ends of a pot and
>>> grab the result from the wiper?
>
>> This only works if the two source impedances are zero.  Otherwise
>> you'll never completely select one or the other source at the ends
>> of pot travel.
>
> ...so this means the signals should be sent through followers  
> first, right?
> So, effective (manual) cross-fader = 1 TL072 + 1 Pot...?
>

Another way to get a crossfade function with a single gang pot and a  
TL072 is to feed input A to an inverting mixer and one input of a  
differential amp. Input B goes to the other input of the differential  
amp. Output of the diff amp goes to the pot, with the pot wiper taken  
to another input of the mixer.

This makes it sound complicated. What you're doing is adding a  
variable amount of (B-A) to A. When you don't add any you get just A.  
When you add half, you get A/2 + B/2. When you add it all, the A's  
cancel and you just get B.

I've used this circuit successfully for wet/dry mixers on stompbox  
designs.

T.