Simple audio muting circuit

John Pieper
 

Hi all, this is John, AD0RW, new to the group. I have built my BITX40 board into a full-featured transceiver,

adding CW (a must for me), AGC, XIT/RIT, built-in keyer and SWR monitor, among other things. One of the

last issues I needed to nail down was the PTT thump, which was a real annoyance. I came up with a circuit

that does the job nicely and is much simpler than the muting board that was offered for the 3B/3C. Here it is:

The concept is simple: put a short across the LM386 input very quickly when PTT goes low, before the relays

have time to close causing the audio noise. C1 and R2 hold the muting on for a short time after PTT goes high

to cover the relays' opening. R2's value was arrived at by experiment, and a 470k should work just as well

but may be easier to find. R3 and R4 are only needed if you have a CW sidetone that you need to inject during

transmit. The adjustable sidetone level is valuable because with this arrangement the volume control has no

effect on it.

With this addition, the rig is now a real pleasure to operate.

73, John

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