RST Engineering (jw) wrote:

Joerg...

I'm about to do my first FET audio gate. Heretofore (for the last 30 years)
I've been relying on an opamp output at some positive voltage, a decently
high value resistor off the output (say, 100K or so), then a bipolar shunt
to ground, then another decently high value resistor to a summing junction.
I'd like to reduce the package count and not have an opamp (or it's cheap
cousin the cmos ibuffer with a feedback resistor) for each audio channel.


I'd check out the CD4051 or 74HC4051 series. Cheap, pretty good off
isolation, does the job, comes in TSSOP. I absolutely try to avoid any
DC shifts upon switching because that leads to nasty audio pops.


I'm leaning towards initial testing with a 2N7000 ... cheap, readily
available, predictable Vgs turnon voltage, and all the rest of it.

Comments?


Its charge injection is high and unless the audio signal level is in the
volt range you get the multiplier effect Vswitch/AudioRMS. Discrete FET
switches is what I had to design out of squelch circuits because the
popping was annoying. Nowadays the radio mfgs seem to finally have
understood that. Wonder what took them so long. Bought a Icom R1500 a
month ago an its squelch does not pop.

--
Regards, Joerg

http://www.analogconsultants.com