Vactrol Vs. Relay

[NoisyVia]

Hi,

I'm buying parts for my first amp with channel switching. I'm trying to decide on buying vactrols vs relays.

Can we discuss the advantages and disadvantages of each and what your current preferences are?

My thoughts without having tried both is that relays are cheaper and simpler to implement but perhaps more susceptible to noise when using both poles. I also wonder if the mechanical action of a relay might make it more electrically transparent. I'm leaning towards buying relays and a power supply pcb and run it off heater voltage, and using only one pole of the relay and grounding the other to avoid crosstalk issues inside the switch.

The vactrols seem like they would be more robust, more complex implementation, but for affect on tone and noise immunity I have no ideas.

What do you think?

[R.G.]

There is a whole sub-technology of switches. Which is best depends on what you are switching and what your application needs.
Switch "goodness" can be evaluated on several factors. Here are some of them:
- on resistance
- off resistance
- ratio of off to on resistance
- distortion
- voltage tolerance
- maximum withstanding voltage
- smallest signal switchable
- control signal feedthrough; how much of the signal that makes it switch appears on the contacts
- many others.

If you're switching AC mains power, the withstanding voltage and on resistance matter a lot. You don't want your switches accidentally letting power through when they're supposed to be off.
If you're switching millivolt audio signals, you're probably more interested in the smallest signals you can switch, distortion, and on to off ratio and control signal feedthrough.
If you're switching line level audio, you're probably interested in off resistance or on/off ratio, and feedthrough.

So the question back to you is - what kind of electrical signal are you switching?

[NoisyVia]

It's about switching the audio signal path in a multi channel amp. I guess I'm now digging up some data sheets and trying to figure out the best on paper or at least what I think is the best.

[R.G.]

OK, line level up to tens of volts.

Does audio leakage in the "off" state matter to you?

[NoisyVia]

Yes it does. From what I've read the insertion loss with the vactrol shouldn't be too much to make it a bad choice but nonetheless minimizing the insertion loss would be a plus.

[R.G.]

Hard metal contact switches are the gold standard for on/off ratio. They're milli-ohms closed, many megohms into giga-ohms opened.

They only reasons anyone ever uses anything other than relays are:
- they are relative power hogs (the coils)
- they are prone to getting dirty, corroded, sticky, and other maladies of the contacts
- they are mechanical, therefore prone to mechanical shocks
- they are expensive
There are ways around these, of course. A good option is to use some of the miniature telecom-style relays. The Panasonic EA series have a DPDT variant with several coil voltages that only use 200mW or so coil power and have contacts designed for "dry switching" - signals not high enough voltage to punch through a layer of dirt on the contacts like AC mains power will usually do. These run to about $3 each, and other companies make very similar products.

Vactrols are possible, but I always wanted a higher off/on ratio (= lower off leakage) and didn't like the 1-2% distortion the photoconductors have. Also, many LDRs are slow at switching. That's good if you want a pop-free fade from one setting to another, bad if it takes a noticeable fraction of a second to switch.

I use CMOS switches a lot, but they're only good for signals below about 15V pk-pk because of power limitations. They're very, very off, but on is 50-250 ohms.