[sdiy] clickless muting

Neil Johnson neil.johnson97 at ntlworld.com
Mon Oct 29 17:14:33 CET 2012

Hi Tom,

>> Look up Teledyne relays, and mercury-wetted relay technology.  Used
>> extensively in RF and ATE.  As always, quality costs.
> Fair enough. Budget is always an issue though, isn't it. And we're only on audio here, not RF, so I'd seriously doubt it's worth it.

I thought we were designing...

David Dixon wrote:
> a high-quality audio signal processor


David Dixon wrote:
> the muted signal should be well and truly absent

Since there's no such thing as "well and truly absent" we try to do
the best we can within our financial and physical budget.  Once you
get to better than 100dB attenuation you really have to think about
the physical things like PCB layout in order to meet the separation.
More so at higher frequencies, say 10kHz and above.  If you want much
more than 100dB attenuation then the THAT Corp SIL-packaged VCAs do
better physically than the 2164 by virtue of the input and output pins
being much further apart.  On the 2164 they're only 5mm apart, even
less when allowing for track widths.

> True enough, and in my view this makes an electronic "switch" that can be slowly faded from one to the other over several milliseconds much the preferred solution. Even if your switch contacts have no bounce at all and you've achieved mechanical nirvana ("toggle on, toggle off, ohmmm"?) if you switch the signal at a peak you'll get a good click - hardly ideal.

Agreed.  That's one of main reasons that electronic morse keys are
low-pass filtered, to reduce the level of harmonics created by
switching the carrier.

> I can see that you'd want to stay away from OTAs, and perhaps away from multiplexers too for some applications, but there are definitely plenty of devices that are good enough for audio. There are even plenty of video crosspoint switches out there, so they at least must have some reasonable bandwidth.

Audio, yes.  Video, yes.  But not every problem can be solved with a
chip.  Sometimes relays are the right solution.  In the particular
story I was referring to, found here:


was this thoughtful vignette:

"In 1998, I had a contract at Hewlett Packard designing diagnostic
equipment for automobile service. I was having a heck of a time trying
to make a solid-state variable attenuator front end. I was using
multiplexers and variable gain amplifier and all kinds of other
approaches. The hardest thing was the whole front end had to withstand
high-voltage faults. On top of that, the customer wanted 1dB accuracy
over 10 MHz. I had spent weeks on the problem. Finally I figured I had
to consult an analog great. So I called Jim up at Linear Technology. I
was a little sheepish, since this time I was going to be asking for
free consulting. Jim patiently listened to my problem and my extensive
description of the system. He asked a few questions. He asked what I
had tried and listened patiently as I described all my false starts.
He paused for a few seconds. Then he asked, "Have you ever used a
Tektronix oscilloscope?" I said sure, there was one a few feet away in
my office. Jim then asked, "What happens when you twist the vertical
attenuator knobs in that 'scope?" I instantly saw where he was going.
I smiled and said, "I hear relays clicking as I change attenuation
levels." "Yup," Jim said, "Expensive little high-bandwidth relays."
Jim then politely explained that if Tektronix could not make a
broadband solid-state attenuator, maybe I should not be trying. I
understood the implications and changed the design to remove the need
for variable attenuation in the front end.
-Paul Rako, analog engineer and EDN technical editor"

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Homepage: http://www.njohnson.co.uk

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