[sdiy] Xfade *jackpot*

[Richie Burnett]

> Could someone please link to a schematic?

Something like one of these:

http://www.richieburnett.co.uk/temp/X-FADE_CCT.GIF

The top circuit tries to minimise the loading of the mixing amp on the 
x-fade pot to ensure -3dB attenuation for both channels at the mid-point of 
the linear pot.

The bottom circuit uses lower value resistors around the op-amp in the 
mixing circuit, and has _RAISED_ input resistances to compensate.  This 
preserves the -3dB attentuation at mid-point of the pot for constant power 
mixing.

Both are "constanst power" to within about +/- 0.3dB when fed from 
low-impedance sources.  Also, both designs possess gain in the mixing 
circuit to make up for the attenuation in the x-fade circuit so that overall 
input-to-output gain is close to unity at the ends of the pot's travel. 
(The bottom circuit has lower impedances around the op-amp, and also its 
input impedance varies less as the pot is swept.  Useful if these things are 
important to you.)

Disclaimer:  Add supply decoupling, DC blocking capacitors, op-amp feedback 
capacitor, and op-amp output resistor, etc to suit particular application!

-Richie,