[sdiy] switching without popping

rsdio at audiobanshee.com rsdio at audiobanshee.com
Mon Apr 18 00:54:17 CEST 2016

Some home stereo components had depletion mode PMOS FET or JFET parts to mute the outputs at power-on and power-off. Because these FETs conduct with 0V gate signals, they connected the signal to ground to mute it, probably with some resistance added. You could use a pair of depletion mode PMOS parts with opposite gate signals to mute one or the other input to a simple active mixer. With the appropriate RC filter on the gates, this would be a smooth crossfade.

I think the key is that you don't need the precision of a VCA. All you really need is a mute with a soft engage / disengage. If the fade isn't perfectly accurate, that's no great loss.


On Apr 17, 2016, at 1:30 PM, "jarno.verhoeven at ziggo.nl" <jarno.verhoeven at ziggo.nl> wrote:
> You could try your switching scheme, but add a fet or vactrol mute. This would mute the signal during the time to reinstate dc balance.
> If you can't tack this on the original gate, you might need to add some cleverness (slew or gate delay).
> ------ Origineel bericht------
> Van: Tim Ressel
> Datum: zo, 17 apr. 2016 21:27
> 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 chea
>  p 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 ResselCircuit

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