Relay Bypass w/Anti-Pop

[redbagy]

Hi! I'm currently comparing these two schematics for an 'intelligent relay bypass' with anti-pop.
https://www.coda-effects.com/2016/08/relay-bypass-with-anti-pop-system.html
https://mas-effects.com/relay.pdf

• Do you suggest connecting the optocoupler output grounding to the circuit output or jack output? When connecting it to the circuit output technically the effective capacitance of the optocoupler (~130pF or less) only comes into effect when the circuit is engaged and would not always be connected to the output jack (which is the case for mas-effects even in bypass).
• Would you suggest using an alternative to the optocoupler to ground the signal? Such as an NMOS for example. The main advantage of the optocoupler is that it isolates the control signal from the output signal.

I look forward to hear your thoughts - thanks! 

[bluelagoon]

Well to be sure the Optocoupler Source GND Pin3 whether connected to the output jack GND or Circuit GND will be exactly the same whichever you decide to choose, as they are both the same GND..

As for using an N Channel Mosfet such as a BS170, or 2N7000 or 2N7002 . Well some will say that its internal body diode, (parasitic diode) would always be hanging off your signal chain when a mosfet is used as a temporary mute switch, thus causing signal distortion degradation.. But having used extensively the BS170 or equivalent as a temp mute switch, I have found no real discernable difference or variation of any added distortion or signal degradation, so personally believe its a pretty negligible issue..

Given most purest will say it has to be an optocoupler to preserve your signal, just like they say a whole lot else, otherwise you wont have the ultimate. This case I begs to differ.

Your choice ultimately, . Why not try both approaches and let us know if any differences noticed.

[redbagy]

Well to be sure the Optocoupler Source GND Pin3 whether connected to the output jack GND or Circuit GND will be exactly the same whichever you decide to choose, as they are both the same GND..

Sorry - I meant to refer to the other pin i.e. the circuit signal or jack output signal not ground.

Regarding the nmos switching I found this article which explains it well https://sound-au.com/articles/muting.html#s4

[FSFX]

Given most purest will say it has to be an optocoupler to preserve your signal, just like they say a whole lot else, otherwise you wont have the ultimate. This case I begs to differ.

Both solutions of a MOSFET optoisolator (MOSFET relay) and that of two series BS170 or other MOSFET will still have the body substrate diodes so there should be no difference. One of these diodes is always reverse biased so it should have a very small junction capacitance with a value in the pF region, probably no more than the capacitance of the wiring or PCB track,  and the junction will have a breakdown voltage of at least 30 or 40 volts so won't clip a typical solid state guitar pedal signal. 

[redbagy]

Just to understand better - in this (https://sound-au.com/articles/muting-f4b.gif) case, the signal should not be higher than the Vsd forward voltage because the bottom AC waveform would conduct/clip even when the mosfet is off, right? Thus this can be solved by finding a MOSFET with a higher Vsd forward voltage for example?

In the case of the MOSFET optoisolator, the body didoes being back-to-back 'cancel each other out'?

[bluelagoon]

Yeah FSFX is correct, but from experience only using just a single BS170 mosfet triggered momentarily, gets pretty much same result as the purest approach of canceling the internal diodes out.
on paper might not add up, but like I said already, can't pick any sound degradation from a Single Mosfet internal diode hanging to ground.
Maybe an oscilloscope might give a better diagnosis, but just from hearing, no difference just using the single mosfet.
Choose your own Path.

[bluelagoon]

Hello FSFX, AS for the equivalent for the optoisolator and the 2 N Mosfets in series to cancel out the body diodes and present no clipping on the signal output. Was just curious wouldn't the same as shown in attached image of a single N Mosfet with just a single diode off the source pin provide the same cancellation of the mosfet body diode ?

[ElectricDruid]

Was just curious wouldn't the same as shown in attached image of a single N Mosfet with just a single diode off the source pin provide the same cancellation of the mosfet body diode ?

That'd just be a half-wave diode clipper turned on and off by a mosfet, wouldn't it?

The two-mosfets solution shorts out both diodes when the mosfets are on, so there's no clipping problem.

[bluelagoon]

Well No, I believe effectively the single mosfet and the extra diode is the same with 2 opposing diodes cancelling each other out when the mosfet is switched off,  as what you get with the 2 mosfets put together. See the following article

https://electronics.stackexchange.com/questions/79028/understanding-two-mosfet-with-sources-connected

But even though it works effectively to cancel out any body diode on your signal lead. I tried the extra diode opposed to the single mosfet body diode, and the switch clicking is much noisier than the 2 diodes put together, so will stick to the traditional, or just a single mosfet on its own.

[ElectricDruid]

Well No, I believe effectively the single mosfet and the extra diode is the same with 2 opposing diodes cancelling each other out when the mosfet is switched off,  as what you get with the 2 mosfets put together.

Yes, that's true. The extra diode lets the combination block both polarities just like two mosfets would do.

But the problem is when the mosfet is switched *on*, not when it's off. When that mosfet is switched on, you've essentially got just a diode hanging off your signal down to ground. That's half of a hard clipper, like in a Boss DS-1 or DOD250 or something. If the signal goes over 0.6V, it's going to clip. That doesn't happen with the dual mosfet because when it's on, both diodes are shorted out of circuit.

[bluelagoon]

Okay thanks Electric Druid, that clears it up, thanks for giving us some better understanding. Cheers

One thing Id like to add though is that since the concept idea for these switches is to just switch in and out momentarily in microseconds , when the footswitch or any other type switch is activated and the relay triggered,. So in effect the miniscule amount of time , like only 20  to 40 milliseconds that the switch is on, and the signal is grounded, it wouldn't much matter if that extra diode were hanging to ground, as it is only very momentary, and at the time period the signal is at ground potential its being muted, thus not allowing any clipping on the signal. hope you get my meaning. Cheers, again.

[ElectricDruid]

Still no, because unless the voltage rises above that 0.6V threshold of the forward voltage of the diode, the signal *won't* be grounded, since the diode won't conduct. Only "ideal" diodes conduct perfectly one way and not at all the other way.

Until the signal hits 0.6V, it can pass unmolested, as if the diode was a big resistor.

One note about the timings too. The datasheets for the relays in the two examples above give 3ms switching time for the FTR-B4CB4.5Z and 6ms for the NA-5W-K. So the microcontroller has to switch the mosfets on, then wait to make sure that's happened (another 2msecs at least for the TLP222A) and then switch the relay on, wait to make sure that has happened (10msec pulse is recommended by one of the datasheets) and then turn the mosfets off.
That's going to be about 12 msecs as an absolute minimum and the 20-40msecs you suggested is probably closer to the mrk for a realistic and robust solution. That's nowhere near "microseconds", and it's a perceptible amount of time. Luckily humans don't notice momentary silences half as much as we notices momentary noises (pops, clicks etc) so it's probably not a big deal. Still, I'd be happier at the "20msecs" end of the range than the "40 msecs" end, which I think might start to become obvious.

[bluelagoon]

Okay ElectricDruid, Thanks again for the lessons, just figured microseconds and milliseconds were same or near enough, my bad.
The 2 Mosfet idea works well and its peanuts for an extra one , with very minimal extra space requirement, specially when you shrink them down to SMD size and even smaller when you get the Dual mosfet in the one package. So all good. Cheers.