[sdiy] clickless muting

David G Dixon dixon at mail.ubc.ca
Mon Oct 29 17:13:10 CET 2012


> David G Dixon wrote:
> > However, for a high-quality audio signal processor, the 
> muted signal 
> > should be well and truly absent, and this requires true 
> disconnection 
> > of the input from the output; hence, a switch of some kind
> 
> Looks like we're designing another great product for 
> Intellijel!!  w00t :)

You might very well think so; I couldn't possibly comment.  :)

But actually, silent and effective muting is a problem of general concern,
which is why I brought the question to this list.

Based on all I've seen and heard so far, I'm veering heavily toward the
circuit in Self's book (Fig. 16.34 and 35).  It seems to represent the best
solution in terms of cost, adaptability, and performance.  I'm looking at
the dual SOT-23 JFET PMBFJ620 (which, in addition to appearing to be
virtually identical to two J112s, also would appear to be just about the
only widely available dual JFET out there.

Something interesting, though.  I made some (rough) measurements with my
oscilloscope and my 2164 VCA yesterday, and using the 2164 as a switch gives
at least 80dB of "off-ness" (it gives 100dB according to the datasheet, but
I couldn't tell beyond 80dB because this was below the noise floor of my
signal -- i.e., it disappeared completely within the noise band on the scope
at about 3mVpp).  This is actually not too bad compared to many of the
options discussed in Self's book.  Since the thing I'm shutting off is going
through a 2164 (of course!), this would in fact be the cheapest and easiest
option.  I'm thinking that perhaps just one JFET to shunt the current to
ground prior to the 2164, along with a large positive voltage to the 2164,
would be the best option.  The best part about this is that I could use a
single voltage source (presuming the 2164 is linearized) -- simply the
negative rail voltage with a suitable lag -- for both the linearized 2164
and the JFET shunt.  I'm pretty sure that this would turn off at least as
thoroughly as the two-JFET system.

I suspect, as long as you shunt the signal to ground prior, that the
performance of the series switch doesn't really matter too much.




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