switching at zero cross/noiseless switching

[The Old Crow]

On Wed, 10 Nov 1999, Buck Buchanan wrote:

> I'm wondering if there's anyone out there who can point me towards a
> resource on switching audio at the zero crossing.  Or for that matter,
> other techniques for noiseless switching at speed.  I'm open to using
> specialized ICs.  Because of high speed, parts like the SSM2402 and
> other "slow on/off" techniques won't work.

  National Semiconductor explains how they did this in a 14-channel
graphic EQ front-end chip they made in the mid-80s.  THe appnote is
doubtless on their website.  The chip itself was just a bunch of
numeric-controlled resistors, the actual bp filters were external.

> Am I correct in thinking that even IF the switch itself adds no pop,
> there will still be a pop since an instantanious switch at some
> arbitrary point in the wave will cause a sharp transistion (fast rise
> time) which itself will be the pop?  And furthermore, if I did manage to
> make it switch at the zero crossing, wouldn't this be eliminated?

  There will be a step response, yes.  The age-old example is SCR lamp
dimmers that reek of RFI until they started using zero-crossing triac/SCR
drivers.

> So far I've tried running the audio through a 4052 (2x 4ch mux) running
> +/-12VDC supplies.  I tried this part first since it will switch

  Ack.  That is too much voltage for a 4xxx chip.  Better to use +/-5,
+/-8 at most.  I've seen 4066s spontaneously combust at +/-12.

> My thought so far was to try and use some kind of clocked latch to
> switch the 4052 only during zero crossings of the audio.  The clock
> would of course be based on the zero crossing of the audio.  Yes, this
> falls appart when switching speed is greater than the audio frequency
> but for most apps this might work???

  Make a zero-crossing detector on the output of an opamp with fairly high
gain (50) such that anything -100mV<x<100mV of the amp output is
considered "zero".  Then use this zero-sync signal to enable the clock
(and gate, whatever) on your switch latch.  For AC power control, a
zero-crossing detector can be made from an LM393, some diodes and a few
passive parts.  For KHz response things will need tweaked around, of
course.

  --Crow

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