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So I tried to use one of those widely available solid-state relay boards to switch some 12V power, and I found out the hard way that they are not designed to switch DC current. They use some sort of variac that only works when AC voltage is applied. Do I have that right??

So I got this other project where I need to multiplex some audio signals in a mixer board, and I was already planning on using those solid-state relays before I learned all this. If I send audio through these relays, is it going to work as planned? Will I suffer any noticeable delay, signal degradation or attenuation?

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    no, it will sound terrible because of the internal capacitance, junction voltage drops, the SSR's lock-on state, and DC biasing (half-wave). they make cheap audio muxer ICs like the 74HC4051 that would be perfect. If you want cheap and available, BJTs can be used to some extent. – dandavis Mar 6 '17 at 20:35
  • That's a nifty little breakout board. And much less bulky! I like it. – LegitimateWorkUser Mar 6 '17 at 20:39
  • a breakout board will be 10X as expensive as the bare DIP, but that's still not much ;) – dandavis Mar 6 '17 at 20:44
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A solid-state relay seems to be the totally wrong tool for the job. The image you posted shows it is intended to switch 240V AC. So it is designed to switch something like a 240V light on and off.

Relays in general are things designed to connect one thing to another (eg. power to a light). They just aren't designed to switch audio signals.

I need to multiplex some audio signals in a mixer board, and I was already planning on using those solid-state relays before I learned all this

The expression "when the only tool you have is a hammer, everything looks like a nail" springs to mind.

There are chips designed to multiplex analog signals (as I describe on this page). That would be closer to what you need.

However that chip (the 74HC4067) is a "choose one input from many" sort of design. It isn't going to let you fade one audio signal in or out (neither would the relay for that matter).

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  • "Relays in general are things designed to connect one thing to another (eg. power to a light). They just aren't designed to switch audio signals." I absolutely agree for SSRs. But, for an electromechanical relay why not? There's plenty of relay switching action happening in Audio Video Receivers during source selection. – Enric Blanco Mar 7 '17 at 11:38
  • @EnricBlanco: for turning a source on or off they would be fine, but for mixing they are useless. – dandavis Mar 7 '17 at 20:37
  • Yeah, I'm not mixing, I'm selecting signals by turning them on and off. – LegitimateWorkUser Mar 7 '17 at 20:51
  • And contrary to Nick's assertion, any type of relay is a reasonable go-to for switching signals on and off. dandavis's recommendation to use the 74HC4051 is a much more practical solution though. – LegitimateWorkUser Mar 7 '17 at 20:56
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    The link I gave specifically mentions the 74HC4051 multiplexer / demultiplexer. In fact it was about it. I don't know why you said the "74HC4051 is a much more practical solution" when that was the exact solution I offered to you. The 74HC4067 is its 16-channel cousin, which would be useful if you need to multiplex 16 signals. – Nick Gammon Mar 7 '17 at 21:40
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No. SSRs are completely wrong for switching audio.

Consider using the CD4066 chip. But be aware that will mean you'll need to ask a whole bunch of new questions I'm sure.

I hope this answer helps you.

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