[sdiy] switching without popping

Richie Burnett rburnett at richieburnett.co.uk
Sun Apr 17 22:16:44 CEST 2016

Instead of using a switch for A/B comparisons, use a linear pot to 
cross-fade between the two sources (like the cross-fader in a DJ mixer.)

This won't pop because even with a good firm swipe the pot will still take 
20ms or more to travel from one end to the other.


-----Original Message----- 
From: Tim Ressel
Sent: Sunday, April 17, 2016 8:20 PM
To: Synth-Diy
Subject: [sdiy] switching without popping

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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