Using analog switches - what to look out for? - Page 1
Forum section: electronics / projects

author: david77
author_rank: Frequent Contributor
timestamp: February 21, 2012, 09:32:09 pm
content: Hi guys,I've had an idea for a new project in my head for quite some time and am presently researching how to go about it.The main part of this project has to do with switching high quality audio signals.I'd usually use small high quality signal relays for things like this but this time I'm running out of space and the cost for 40+ relays is quite prohibitive.So I'm thinking about using analog switching IC's instead of mechanical relays.I've not yet decided what type to use, however the MAX313 looks quite good, I recon.Whatever I end up using it has to be available in a DIP package.As I don't have any experience using these things can anyone share some of his insights intoanalog switches and audio signals? What to look out for, where's potential for nasty surprises, etc.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: February 21, 2012, 09:44:45 pm
content: Don't know about the MAX313, but I've used the MAX333 before and it worked well. Inputs are only up to the supply rail, which is +/-18V max, so it's not quite pro audio spec, but it works OK in consumer audio applications.

author: david77
author_rank: Frequent Contributor
timestamp: February 21, 2012, 10:06:27 pm
content: The highest audio level I have to expect is about +6dBm (1,55V) as a safety margin however I want to design to allow for +20dBm before anything goes into clipping. The audio electronics power supply will be +/-15V so I think I'm allright there.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 22, 2012, 12:12:37 am
content: I used 4066's in a project once,& discovered that the unused sections still drew current,even with the control pin returned to zero volts.They got quite warm!I had to put a negative voltage on these pins to turn the rotten things off.

author: Unknown
author_rank: Unknown
timestamp: February 22, 2012, 12:18:47 am
content: Quote from: vk6zgo on February 22, 2012, 12:12:37 amOscillator functionality was provided at no extra charge.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 22, 2012, 01:25:31 am
content: Yeah,Unc,they may well have been oscillating,but I was "under the hammer" to get the project finished,& the -ve bias fixed the problem!They didn't use it in the end,anyway!

author: david77
author_rank: Frequent Contributor
timestamp: February 22, 2012, 08:40:21 am
content: I've thought about using 4066's but they are probably not the best choice for what I want to do.

author: JuKu
author_rank: Frequent Contributor
timestamp: February 22, 2012, 09:50:47 am
content: Lookout for:-protection: the max. signal levels are usually referred to supplies, and this applies also when supplies are off. In other words, if you connect inputs directly to the switch, a hot signal on the input may kill your IC without protection.-charge injection spec: If not about 0, the switching is not quiet.-break before make operation-Ron accuracy: depending on your configuration, one channel with 30 ohms and another with 50 might be an audible balance error-leakage from one switch to another, leakage from a switch that is supposed to be off-distortion and noise: for high quality audio, it is easy for an IC switch to be the limiting factor.I recall an instance when we found ourselves designing a compensation circuitry to overcome deficiencies of the compensation circuitry for overcoming deficiencies of an analog IC switch. We came into our senses and decided that relays are actually quite good. Hint: if you look around, miniature relays might not be that expensive after all (<1$ per stereo pair in small quantities).

author: david77
author_rank: Frequent Contributor
timestamp: February 22, 2012, 10:44:43 am
content: Thanks for your hints. So basically what you're saying is: For high quality audio stick to tried and tested relays, right?

author: JuKu
author_rank: Frequent Contributor
timestamp: February 22, 2012, 12:25:57 pm
content: Pretty much. Of course, it depends on what you mean by "high quality".

author: david77
author_rank: Frequent Contributor
timestamp: February 22, 2012, 12:40:27 pm
content: By high quality I mean broadcast or studio quality or at the very least proper stereo hifi.Nothing to do with audiophoolery.

author: JuKu
author_rank: Frequent Contributor
timestamp: February 22, 2012, 01:23:34 pm
content: Nothing basically wrong with IC's, just watch out those "cotha's".

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2012, 05:19:31 am
content: Some Broadcast Audio gear uses discrete FETs for audio switching.See what other people have done---there is nothing wrong with "cheating!

author: david77
author_rank: Frequent Contributor
timestamp: February 23, 2012, 07:42:33 am
content: Now I think it's time to get the breadboard out and do some experiments, I've actually found a couple of DG409's in my junk boxes yesterday. Should be good enough to play around with.The discrete FET idea is also worth looking into, thanks!