[sdiy] Muting circuits for line out

[Oren Leavitt]

I've done this line out muting retrofit a couple times.

Basically a simple relay circuit with a couple seconds delay at power-on 
and immediate turn off on power-down.

The muting circuit is powered by a separate rectifier with a very small 
filter cap so that it shuts off immediately on power-down, before main 
power rails fall.

- Oren

On 5/8/25 7:30 AM, Benjamin Tremblay via Synth-diy wrote:
> Hello,
> I have this lovely Univox JR-5 rhythm machine I have modded with knobs 
> to control the sound mix. The original circuit was high gain but 
> impedance sensitive so it summed current, not voltage and was very
> I added a simple op amp to handle the mix. When power is turned on there 
> is a horrible thump that sounds like it is 100x too loud.
> I have been looking for a simple muting circuit. When I was repairing a 
> Roland MC-909 I saw the muting circuit was little more than a pair of 
> BJTs shorting the signal path.
> I also saw this novel patent which attenuates the signal without much 
> voltage offset, maybe?
> US4288753A - Muting circuit with operational amplifier - Google Patents 
> <https://patents.google.com/patent/US4288753A/en>
> patents.google.com <https://patents.google.com/patent/US4288753A/en>
> 	favicon.ico <https://patents.google.com/patent/US4288753A/en>
> 
> <https://patents.google.com/patent/US4288753A/en>
> 
> I really just need to prevent that initial thump. I’ll pick up a relay 
> if that is best.
> 
> Is there an easier solid-state solution? I don’t mind adding another 
> buffer stage at the output.
> 
> Benjamin Tremblay
> 
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