analog switches and thd

[David Halliday (Volt Computer)]

Another option would be to use a small relay across the input of each
active loudspeaker and momentarily mute it when a switch is pushed.  Use
a 555 timer ( or just a transistor and R/C time constant ) to make the
muting a bit longer than the worst switching transient.

Also, part of the problem might stem from Capacitive coupling of the
stages - this is normally a *Very*Good*Thing* but static charges can
accumulate on an unconnected capacitor and when you switch to it, the
charge will dissapate with a pop!  Open the boxes and solder 100K ohm
resistors across all of your outputs and your inputs. - try it with a
few and see if this makes a difference.



> -----Original Message-----
> From:	Joe Grisso [SMTP:proteus at inconnect.com]
> Sent:	Wednesday, June 25, 1997 11:08 AM
> To:	Martin Czech; synth-diy at horus.sara.nl
> Subject:	Re: analog switches and thd
> 
> Martin Czech wrote:
> 
> > I want to avoide a lot of relays in an multiple input switching
> > matrix for an 8-channel audio system with 2x8-track, 2x-track, 2xCD
> > and 2xDAT
> > interface with independent level and EQ control for up to eight
> > spatial distributed
> > loadspeakers (is this called octophony ?).
> >
> > It's a DIY birthday present for a friend.
> >
> > Up to now, he has to run arround to regulate the sound at EVERY
> > active loadspeaker (Yamaha), and there's always some awful
> > switching noise during the performance, since he has no fade out
> > box.
> >
> > Real studio quality is not required concerning THD, since the
> > loadspeakers
> > will allready contribute some distortion, it's for live
> performances.
> > But too much noise would be awfull.
> >
> > Does anybody know, how much distortion (thd) is introduced
> > by using a cheap CD4066 CMOS-Switch instead of an expensive relay
> > @ Vdd=7.5V, Vss=-7.5V, Vsignal 1Vpp, signal current 100uA ?
> >
> > And what about noise ?
> >
> > Crosstalk will be no issue, I could use different ICs in such a
> case.
> >
> > Any experience out there ?
> >
> > m.c.
> 
> Any 40xx CMOS Switch have NASTY feedthrough, which when used as an
> audio
> switch, will induce a very loud "pop" into whatever it is driving. My
> suggestion would be to use a J174 JFET switch fed into the inverting
> input of an op-amp (5532 prefferably). This not only is a much quieter
> switch, but has more bandwidth, and can be amplified and trimmed with
> the op-amp.
> 
> Just my $.02 worth...
> 
> Joe
> 
>                          _____________
> ________________________/ Talon Audio \_
> Joe Grisso
> CEO/Head Engineer
> proteus at inconnect.com
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