[sdiy] clickless muting

Tom Farrand mbedtom at gmail.com
Sun Oct 28 23:17:22 CET 2012


How "muted" do you need?  As you know, the 2164 can attenuate as much
as 100 dB but any noise on the control port is reflected in the
output.  A JFET like a 2N4391 has about 30 Ohms of on resistance.
Drain stuck on the junction of two series-wired (high value) resistors
in the audio path will mute when the gate is appropriately tickled but
I don't think that would be better than the 2164.  The old-school
method has a JFET in series with the audio signal as the first mute
element and the aforementioned paralleled JFET after the series-wired
JFET as the second.  (Sort of a one-two punch, so to speak.)  A Mackie
mixer just uses a section of a SPDT switch to interrupt the signal
path and ground the input to the destination.  That is the mechanical
equivalent of the one-two punch JFET solution.  Vactrol prices are in
outer space these days so I wouldn't likely try them.  Maybe a tiny
PCB mounted relay would be a good idea.  Keeps the "control" as
remoted as you'd like but keeps the audio localized to the PCB where
it belongs.

Another consideration is how "clean" the mute circuit must be when not
muting.  Your basic transmission gate does a decent job buts adds some
distortion.  I haven't look at a Rane mixer ... maybe they do
something different.  It is hard to beat the simplicity of mechanical
switching when "off" needs to really "OFF".  Sometimes surplus houses
have tiny PCB-mount relays designed for low-level RF switching, for
cheap.  They'd probably have better bleed-through specs when "off",
than run of the mill Taiwanese relays.  Switch the coils from 5V so as
nmot dirty up the analog rails.

Tom Farrand

On Sun, Oct 28, 2012 at 3:44 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
> Hey Team,
> Does anybody here have any insight into good ways to achieve "clickless"
> muting of audio signals?  My first inclination was to send a large (and
> slightly LPFed) positive voltage to a 2164 VCA (i.e., to use it as a
> switch), but I don't think that the 2164 VCA shuts off hard enough for true
> muting.
> Of course, the easiest way would be simply to put a toggle switch in the
> audio path.  However, I'm a bit concerned about feeding an audio signal
> through a panel switch.  (Am I right to be concerned about that, or is this
> a safe thing to do?)
> As usual, any and all help most appreciated!
> Cheers,
> Dave Dixon
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