[sdiy] switching without popping

Tim Ressel timr at circuitabbey.com
Sun Apr 17 21:20:47 CEST 2016

Good day everyone,

Basic question: If I wanted to make an A-B switch and not have it pop, 
what is the best way to do it?

I tried to do an A-B with a DG449 but the popping was unacceptable. 
Popping occurs when the incoming signals are at different voltages. Then 
you switch between them you get an instantaneous transition from one 
voltage to the other, resulting in a pop.

I have mulled this over quite a bit. The obvious answer is to cross fade 
instead of switch. So instead of a DG449 you use a pair of VCAs. This is 
where I run into trouble. VCAs and their associated control circuits are 
huge, and my products are small. I thought of using vactrols but you 
need 2 to get 80+ dB attenuation, and again they are big as well as 

I have tried to use a FET as a cheap VCA but can never get it to work. 
Lots of distortion.

Query: Can one use a DG449 switch and a high frequency PWM, say 100KHz 
or higher, to do a cross fade?  Or would that result in aliasing?

There must be a good solution. Ideas?

--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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