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where to place mute circuit?
where to place mute circuit?
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Old 31st May 2018, 02:15 PM   #1
jimmy audio is offline jimmy audio  United States
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Default where to place mute circuit?

Where is the best place to implement a relay based mute circuit? right at the input jacks before the summing amps/volume control or after the summing amps/volume control right before the amps(6x)?
Relays (3x dpst) will short the inputs to ground. Kinda worried about shorting the op amps outputs(JRC 2068dd) to ground. Thanks for the help most folks eyes glaze over when speaking electro-lingo.(new word!) and having a community online is awesome
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Old 31st May 2018, 03:34 PM   #2
Mooly is offline Mooly  United Kingdom
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where to place mute circuit?
I would always suggest at the output of the opamp but you must include a low value series resistor from the opamp to the relay. Make sure the feedback network connects before the resistor. Your audio output is taken from the other end of the resistor.

Doing it this way ensures that the opamp can stabilise its DC conditions correctly, and placing the relay at the output ensures silent operation. Muting the inputs probably wouldn't achieve much. Resistor can be as low as 22 ohm or so.
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Old 31st May 2018, 04:35 PM   #3
jimmy audio is offline jimmy audio  United States
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Thanks Mooly! The low value resistor acts as a load for the op amp? looking at Elliot sound circuit they use 100 ohm would you recommend less? So the relay should be shorting the inputs to the power amps?
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Old 31st May 2018, 05:33 PM   #4
Mooly is offline Mooly  United Kingdom
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where to place mute circuit?
Not so much a load, more a means of isolating the opamp output from the short to ground that the relay introduces.

Having no resistor would mean that at the instant the relay opens there could be a loud bang as the opamp output would produce a massive spike as it biased up correctly.

A low value resistor is sufficient to allow the feedback network to establish the correct DC conditions. No hard and fast rules on value and a 100 ohm is fine. In practice you might find you could go as low as just a few ohms without problems. Its up to you. If the chip is driving a capacitive load (long cable) then 100 ohms is sensible. If its all internal PCB print of a few cm length then you can go lower.

An empirical approach is good here. 100 ohm will be fine as will 22 ohms. When you reach a value that causes problems, increase it perhaps by four times.

If the chip is driving six amp inputs in parallel then 100 ohm might be a wiser choice.
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Old 1st June 2018, 01:32 AM   #5
Monte McGuire is offline Monte McGuire
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where to place mute circuit?
Why such a low series resistor value? When in mute, the op amp will be driving a screamingly low value, which will surely send the op amp into current limit, dumping hash-like signal related junk into the power supply rails and testing the thermal capabilities of the IC. It seems a lot safer to use some 10kΩ tackdown resistors on either side of an SPST series switch between the amp and the output jack. That way, no DC pops will happen during switching, and the load can always be driven cleanly.
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Old 1st June 2018, 06:13 AM   #6
PaulBysouth is offline PaulBysouth  Australia
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Here is a better way to mute a preamp output.
Choose a value of resistor to suit the amplifier in use.

PB
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File Type: jpg Preamp-Mute.JPG (76.2 KB, 221 views)
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Old 1st June 2018, 07:10 AM   #7
Monte McGuire is offline Monte McGuire
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where to place mute circuit?
Looks good! No unnecessary loading of the output amplifier, and only one resistor vs. my suggestion for two. The DC 'thump' will happen regardless of any of these muting schemes if the output amplifier has a significant DC fault - that is a problem that needs to be solved outside of the mute circuit. A truly minimal circuit - thanks!
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Old 2nd June 2018, 04:19 PM   #8
jimmy audio is offline jimmy audio  United States
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Thanks guys will use the circuit. works well breadboarded no pops whatsoever. Laying out the board. Thanks to all awesome community of all things audio
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Old 2nd June 2018, 05:06 PM   #9
Fast Eddie D is offline Fast Eddie D  United States
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I use a shorting relay on the output with 47 ohm series resistor between op amp and relay. It's all controlled by a humble 555 timer. Turn on delay of about 1.5 seconds, and "instant" off.

It works perfectly - never a pop, click, or thump.
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Old 2nd June 2018, 05:21 PM   #10
Mooly is offline Mooly  United Kingdom
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where to place mute circuit?
Pleased to hear you have a working solution
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