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Is solid state relay capacitance bad for audio?

Posted: Wed Dec 13, 2017 3:00 am
by Duckeng
Regarding MOSFET optocoupler relays like the Panasonic AQW212, am I correct in understanding that the capcitance between the terminals and ground when the switch is open ("off") is around 100pF when the potential across the terminals is small?

If you look at the datasheet for AQW212( it says it has low Coff but if you look at the chart that seems to not be the case. With a small voltage the capacitance rises sharply.

Consider the following example where the audio signal may be routed through a SS relay driven stepped potential divider like a volume control in the nearly off position:

I've been searching for a good pro-audio signal routing method that could work with somewhat arbitrary circuits like old mic preamps running on 24V or maybe even guitar tube amps that might switch 70V signals.
In this example, it so happens the circuit calls for a large resistance so as not to load the collector of a transistor amplifier perhaps. This arrangement conspires to produce high series resistance and low signal level which when coupled with the multiple drain / source capacitances of each device could result in high frequency loss.

Is this understanding correct? Or could a MOSFET based device be used to build something like a stepped attenuator?

I have also looked at devices like the Analog Devices iCMOS analog switches but these too have high junction capacitance. It seems the lower the on resistance, the higher the capacitance.

If only there were a mechanical relay with many positions like an old telephone exchange stepper switch but the size of a regular relay. Meaning a SP12T relay that can ratchet in either direction.

Re: Is solid state relay capacitance bad for audio?

Posted: Wed Dec 13, 2017 6:47 am
by mediatechnology
I experimented using the higher current Panasonics for a floating switchable pad in a mic preamp and the off-state capacitance was too high.

Re: Is solid state relay capacitance bad for audio?

Posted: Mon Jul 09, 2018 2:14 am
by steveu
Given what you are trying to do, mechanical relays are probably the only way, and even then you may get noise and crosstalk. A binary tree will give you N+1 selection from N cascaded relays. There are Mux chips but opto or not, there are real limits to V/I linearity, voltage range, resistance and loss and bleed. Even with mechanical relays, each input needs both series switch and a shunt mute contact. Classic JFET circuits add 1/2 the audio to the control voltage to reduce distortion. There is no semiconductor that is linear over decades of range much less bipolar. Back-to back, you just get a S curve. One device that can be useful is LDRs. You can build your own couplers with a LED and the LDR in heat shrink.

Re: Is solid state relay capacitance bad for audio?

Posted: Mon Jul 09, 2018 11:19 am
by JR.
For modest voltage swing and current, CMOS transfer gates are reasonable (and cheap). There are dedicated electronic switches for audio but you need to evaluate for the specific application.


Re: Is solid state relay capacitance bad for audio?

Posted: Mon Aug 06, 2018 9:36 am
by Colin
As you show the circuit, the roll off frequency is around 1.59MHz. Worst case roll off would be when the pot is set at its half way position, or 460k +1k =461k Divide this by two because the resistors are effectively in parallel and you get the Thevenin equivalent source resistance of 230.5k ohms. That would make the roll off frequency 6.9kHz, which would only be suitable for a telephone circuit... If you chose a pot value more typically associated with transistor circuit impedances, like 10k for example, then the worst case roll off would be around 300kHz. If I was designing the earlier stage and expecting a pot, I'd ensure the source impedance was suitable for a 5 or 10k pot.

I've not used the AQW212 before, but an audio engineer I trust said that they have similar distortion issues to using dual JFET switches, plus the effect of the body diode is likely to cause some other forms of non-linearity, so I'm not optimistic that they would be very useful.

I've worked with program switchers using the following switch elements: 1. Dual JFETs, one in series and one shunt with a passive distortion null used on the series JFET (the shunt JFET kills the audio when on, so distortion nulling is not needed). 2. CMOS bilateral switches 3. Relays

My experiences are as follows:
1. Selected JFETs are fairly simple, silent in operation and highly reliable
2. CMOS bilateral switches work well enough, but I've seen enough apparently spontaneous failures to make me suspicious they are vulnerable to Electric Static Discharge. ESD can occur without immediate effect, and switch failure can be up 2 or 3 weeks following the exposure so it is difficult to connect cause and effect.
3. Relays. If you select proper signal relays with gold contacts, not the types rated for AC or DC current, then they will be nearly perfect switches and highly reliable. After ten years in service, I would suspect that you may need to change a relay every now and again for contact problems. I highly recommend that program switchers be installed with jack fields, so if they are in a working studio, problems can be immediately patched around until it's convenient to make repairs. Only down side is that relays can make an acoustic noise when they operate and that can be a problem if you have open microphones. You can easily put them into a heavy box to suppress noise. Lead boxes were an older solution that that problem, but if workplace health won't permit lead, then a steel box with foam padding will suffice. The smaller the relay, the smaller the click. Noise and crosstalk can occur, but keeping the matrix inputs and outputs to balanced lines at 600 ohms will avoid most of it. A bit of shielding, balanced lines, shielded and fed directly to each relay would prevent most of that problem.


Re: Is solid state relay capacitance bad for audio?

Posted: Mon Aug 06, 2018 11:40 am
by mediatechnology
Thanks Colin for joining us and an informative first post.

I tried the AQWs but never measured the distortion since the capacitance alone for me was a deal-breaker.
As a relay, not for audio, they rock for ease-of use.

Like yourself I prefer low-level sealed gold relays.

You didn't mention BS170/BS250 (N/P) enhancement mode MOSFETs in a "solid-state relay" configuration.
Have you any experience with them in this application?

An unbalanced series-shunt configuration requires 4 and capacitance, though much smaller than the Amp-rated AQWs, is still higher than one would like.
Distortion, at first glance seemed do-able but high-level THD required gate bootstrapping.
I think I have U-pad drawing using then with 4 BS170's switching the legs and 2 BS250s switching the shunt.
The gates were driven by an LM393 comparator output similar to the internal threshold of a DG-series.
I built one but never fully characterized it for distortion.

If you've ever used enhancement-mode MOSFETs I'm curious as to how well they performed.
I didn't spend a lot of time on my pad.