[sdiy] switching without popping

[Tom Wiltshire]

I don't know whether it works on the DG449, but with the old 4053, it was possible to put an RC filter on the control signal and turn the switching action into a noiseless crossfade.
This won't work if the IC has level-shifters or schmitt triggers on the control signals, evidently.

Tom

On 17 Apr 2016, at 20:20, 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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