[sdiy] clickless muting

[David G Dixon]

Tom,

> Yeah, 16.34 hits the best of both worlds.  The J111 is only 
> sixteen cents from Mouser in unit quantities (Fairchild) and 
> the J112 is nineteen cents.  That's pretty cheap and is 
> probably a lot better than a relay.  It consumes a whole lot 
> less control current, too.  The control gets a bit more 
> sticky with the J111 but can be managed with difficulty.  
> Yeah, the J112 is probably the better choice for ease of 
> control due to relaxed turn-on requirements.

Yes, I'd stick with the J112 -- I don't really want to have a -23V rail!

> (Update:) Now that part is as cute as a bugs ear.  A little 
> quick on the turn-on/off times but could probably be managed 
> by ramping the LED current.  Since the "load current" is 
> minuscule, slewing through the linear region more slowly 
> would not be an issue.  I like!  The on resistance is awesome 
> as well ... a lot better than a 4391 or J112/J112.  At a buck 
> apiece it is a bit spendy but is offset by a simpler control 
> arrangement.  Mouser has a bunch on order for Xmas delivery 
> 2012 but none in stock at the moment.  Avnet says none on US 
> soil and none in Asia or Europe.  Element14 says no cigar as 
> well.  I guess bodging up a prototype would have to wait awhile.

It would be a simple matter to ramp up the brightness of the LED, but would
that have a significant effect on the turn-on time of the MOSFET?  I.e.,
even if the ramping occurred over, say, 100ms, the transistor may still
switch in a fraction of that time, somewhere in the middle of the ramp, and
this obviously wouldn't have any benefit.

> I used a similar device on an isolation circuit a few years 
> ago.  I have no recollection of the part number I used but I 
> think the on resistance was higher.  I'll dig up that part 
> number at work tomorrow and report back if worthy of 
> consideration.  I think it was also available in duals and 
> quads.  That would be sweet.  (The on resistance is higher, 
> IIRC, because the isolation was 4KV.  I needed that isolation 
> to make UL happy.)

I'd really like to know what that part was!  I'd be very interested in the
dual.  I'm surprised they don't have complementary ones that can use the
same control signal to both disengage and shunt the signal (maybe they
do...?).  This is a very common requirement in audio circuits.