[sdiy] switching without popping

mark verbos mverbos at earthlink.net
Mon Apr 18 23:54:42 CEST 2016


I use DG409 switches for PWM in my Multi-envelope. I actually PWM using the enable pins on two chips, but that’s another story. I’m not switching audio, but my oscillator is at 30kHz. I have the outputs through a sallen-key lopass at 10kHz cutoff or something. Without a lopass filter you will surely get aliasing, but if you clock it high enough you might be able to get away with very chintzy ones (like just a small cap in the feedback loop of the following opamp). They make very reliable crossfaders, BTW.


> On Apr 17, 2016, at 3:20 PM, Tim Ressel <timr at circuitabbey.com> wrote:
> 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 expensive.
> 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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