Balanced Audio Muting Circuit (Relay vs Optocoupler)

[redbagy]

Hi,

I'm building a balanced audio mute circuit. Till now I tried using a latching DPDT relay that when activated shorts Pins 2 and 3 together to Pin 1. I used a latching relay because it is supposed to have less mechanical noise. However I can hear a tiny mechanical click when unmuting the audio (this is more evident when no audio is present). For reference the circuit is the top one in this link (https://europe1.discourse-cdn.com/arduino/optimized/4X/5/d/3/5d3149c78303e01fbe67fc1001638bc72a048f3e_2_581x500.jpeg).

Thus I tried building something like this (https://mas-effects.com/relay.pdf) where I connected two optocouplers outputs to both pins 2 and 3. When activating the optocoupler this shorts the signal to Pin 1. This worked to a certain extent, because instead I am getting very low volume. Any idea why this is happening? Shouldn't I get a complete mute?

Thanks

[ElectricDruid]

The idea of the optocoupler in the Mas effects circuit is that it drops the volume just before the relay switches, so that any click made by the relay switching can't be heard. As such, it doesn't have to be a *total* mute. If it provides -40dB for a little click, that's probably plenty. The timings are important though - it needs to come on ahead of the relay switching, and then only go off once the relay has switched. It's all a matter of milliseconds, but it needs to be right.

Whether you could do a complete mute with an optocoupler, I don't know. Perhaps.

Is shorting pins 2 and 3 together and to ground a good idea? What's driving this signal? Can it cope with having its output shorted to ground? Just opening the output would be enough, no?

[redbagy]

Is shorting pins 2 and 3 together and to ground a good idea? What's driving this signal? Can it cope with having its output shorted to ground? Just opening the output would be enough, no?

If you refer to the first circuit that I posted, in fact the relay opens the output and shorts pins 2 and 3 (they don't necessarily need to be grounded). So the equipment driving the signal does not have its pins 2 and 3 shorted. I think that pins 2 and 3 are shorted to ensure a complete mute at the receiving end (perhaps they might pickup some noise?).

From further reading I also found out that the optocoupler resistance (in relation to the signal output resistance) plays a part in determining the volume attenuation.

The relay makes more of a very short tick rather than a pop so I can probably live with it.

Thanks!

[ElectricDruid]

If you refer to the first circuit that I posted, in fact the relay opens the output and shorts pins 2 and 3 (they don't necessarily need to be grounded). So the equipment driving the signal does not have its pins 2 and 3 shorted. I think that pins 2 and 3 are shorted to ensure a complete mute at the receiving end (perhaps they might pickup some noise?).

Ah, ok, I didn't understand the situation accurately then. Thanks.
Yeah, shorting pins 2 and 3 together would make sense, since it's a differential signal. If they're tied together there's no difference, so no signal.

From further reading I also found out that the optocoupler resistance (in relation to the signal output resistance) plays a part in determining the volume attenuation.

Yes. Effectively it's a voltage divider with whatever the source impedance is. To get plenty of cut, that source impedance needs to be many times bigger than the optocoupler's On resistance. Like 10K in series or something.

The relay makes more of a very short tick rather than a pop so I can probably live with it.

Fair enough.

The relays I have make a *mechanical* noise, so you can *hear* them tick, but they don't seem to make any significant *electrical* noise, so that tick doesn't come out in the signal. So in this case, opto muting wouldn't help either.

[R.G.]

Reed relays are very quiet, both electrically and mechanically. I have used them for muting balanced signals before.

[MrStab]

Is it philosophically "true" bypass with the photorelay's off-state capacitance, low though it may be?

[MrStab]

Jury's still out on my experiments with this sort of thing - an analogue approach to temporary mute upon any state change. Does seem to make some minor difference, with photocoupler between hot and cold (more so when a single-ended output is plugged in), but increasingly less with high-amplitude signals. Have made other switching noise reduction changes in the circuit in question since my last experiment, so may have more conclusive results in time. Wasn't worth scoping until then, just went by ear and DAW waveform.

Rationale being that TQ2 relays have a ~4ms max set time, whereas inverters and schottkys can act much faster. RC discharge is ofc a limitation. Microcontrollers push you into EMC compliance territory. RC delay before relay drivers could help make this more worthwhile, but pushes component count up too much.

https://tinyurl.com/ypczuc8z

[R.G.]

When I did my tinkering, analog methods were too fussy for best results. What worked GREAT was feeding the state-change switching info into a PIC and having the PIC then manage muting and the actual switching. This let me get the muting done just before the actual switches (of whatever kind) did their thing, and get un-muted as quickly as I wanted.

As a nod to doing it all-analog, you can do much the same thing with a string of CMOS-inverter-based half-monostables to generate the string of logic transitions at intervals. This gives you the ability to detect the user switch request, fire the muter, wait until it's muting, change the switches, wait til they're done, then un-mute.

I found that there was way too much variation in switch thresholds using non-Schmitt trigger inverters; at least with Schmitts, you get some boundary on exactly where the delay chain inverters switch and don't have to go tinkering the RC inverter delays every time you get a new batch of CMOS inverters.

One nice thing is that if you can implement this all in a single hex inverter, all the inverters on that chip are part of the same bit of the silicon wafer, so they all switch consistently for a given chip.

[bluelagoon]

Sounds Good RG, But could you be so kind as to give us some schematic ideas on how your logic transitions at intervals works using the Hex Schmitt trigger while incorporating  the muter circuit configuration. Just a rough out approximation circuit diagram, perhaps with a formula for attaining specific mute times. I know its asking a bit extra, and I get where you are at with using such a circuit, as have done similar myself in the past, Just would hope to see an approximation of how a pro as yourself would get it sorted.

Also just as an addendum, Could someone please clarify, does using a Schmitt trigger CMOS get you into the same EMC compliance territory as using a PIC Micro Controller with frequency implications above the acceptable 9MHz ?
Cheers

[redbagy]

Hi all - in my case I'm now going to use a DPDT relay to open/close the series feed lines coming from the output driver and this optocoupler (AA36) to short Pins 2 and 3 together. This optocoupler has an on-resistance of 1Ω max. so I expect much better results with a typical line level output resistance of 470Ω. The output capacitance is around 470pF which should be fine as well.

[MrStab]

a string

With the standard 6-inverter CD4049/69 or 74HC04-alike, using the GeoFex non-latching pulse arrangement, there's 2 inverters available for the latch, 1 for coil A pulse, 2 for coil B pulse (inc. inversion) and 1 inverter spare. So there is at least some scope for separation of events with jellybean parts.

The line for me was the 2nd set of resistors needed post-RC delay to prevent cap discharge from damaging driver NPN bases, but maybe not needed.
Still, arrays or bust for most of this.

...OR WE COULD JUST INSIST UPON BUFFERED SWITCHING AND FREE OURSELVES FROM THIS TYRANNY!!!!