Signal path switching (with minimum distortion) - Page 1
Forum section: electronics / beginners

author: Sigmoid
author_rank: Frequent Contributor
timestamp: February 18, 2014, 07:03:30 pm
content: I'm thinking of what's the best way to electronically reroute a line level audio signal, say to bypass a filter, or include it in the signal path, based on a microcontroller action...I'm thinking of using MOSFETs, but I'm a bit unsure about it. I'd like to make an open signal path as identical to a single line of copper as I can get it.Could someone give me a link to a (reasonably good) circuit diagram that does something like this? Whenever I sit down to design it I get completely confused, after having packed the circuit with MOSFETs for pulling the filter circuit down if it's off, disabling the input and the output, switching over to the bypass line, etc... Plus MOSFETs seem unreasonably expensive.

author: Bored@Work
author_rank: Super Contributor
timestamp: February 18, 2014, 07:27:33 pm
content: Many semiconductor manufacturers offer integrated analog switches, including switches specifically intended for audio switching. I would have a look at these offerings.

author: kizzap
author_rank: Unknown
timestamp: February 18, 2014, 11:13:49 pm
content: Quote from: Bored@Work on February 18, 2014, 07:27:33 pmfor example, if your signal is in the realms of ±5V, try the 74HC4051:http://www.nxp.com/documents/data_sheet/74HC_HCT4051.pdf

author: fcb
author_rank: Super Contributor
timestamp: February 18, 2014, 11:26:49 pm
content: CMOS switches: 4066, 4016, 4051, 4052, 4053 - all good for audio.DG409/DG411 etc.. very good for audio (I use DG409's with +/-12v rails in audio).How to use them. Treat them as switches!

author: Fabiusp98
author_rank: Contributor
timestamp: February 19, 2014, 09:19:45 am
content: I suggest old style relays, since there is complete electrical isolation.But I'm a beginner and I can be completely wrong

author: JuKu
author_rank: Frequent Contributor
timestamp: February 19, 2014, 12:59:06 pm
content: Once, we caught ourselves making compensation circuitry for the compensation circuitry to overcome electronic switch deficiencies. Miniature signal relays are perfect.If you really want to use electronic switch and minimize distortion, look for crosstalk*, R match and R/f**, max. signal*** and distortion itself****.*: You want double stage switching. Have an on-off switch on each input. Switch on the signals you don't use. In another component, select the signal you want. Motivation: An electronic switch with less than -100dB leakage or crosstalk are rare.**: Not really distortion, but the switch R properties can be audible, depending on the circuit topology. The R will be frequency dependent, and all the switches will be different.***: Usually, the max signal is not that much over the rails. When the power to your switcher is off, the rails are at 0. Don't kill the switch by live input signals.****: That is not specified very often. If not, expect bad.In other words: Put a double relay in normally off position to each input, have your software or switches control them break before make fashion and you are good. You might save a buffer stage or two also. My favorite is Omron G6J series: You can get it with 5V coil voltage (no need to worry about relay switching peaks on your audio supplies) and it is small enough to fit behind a stacked RCA connector, making PCB layout modular.But it really depends on how good you want to make it. For reference, reading the DG409 datasheet, these are red flags for me, your requirement might be different:-Charge Injection != 0: It will pop somewhat.-OFF Isolation -75dB: Unusable for audio, if the other inputs are live-Internal clamp diodes on inputs: Good, doesn't need protection of its own, the ESD protection and this is likely enough.-Ron 40 Ohm, match to 15 Ohm: Only something to look for, but must take into account the potential L/R imbalance.-R heavily depends on signal voltage: harmonic distortion.Verdict: A DG409 in an audio pre-amp is likely the quality limiting component.

author: fcb
author_rank: Super Contributor
timestamp: February 20, 2014, 01:24:57 am
content: Switches like the DG series can be thoroughly excellent. They are used in some pretty high-spec audio analyzers (I know this field!), mixers, etc...What you listen to is likely to have gone through at least one DG series and a NE5534 at somepoint.As for charge injection issues, gain variation due to Ron, isolation, etc... that's down largely to how you build your circuits. There are some neat tricks to solving each of them (such as using extra switches to ground out nodes).As the OP mentioned he was interested in bypassing filters, I assumed he wasn't building something HiFi related, probably more in the electronic-music/processing side of things.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: February 20, 2014, 02:26:18 am
content: Thanks for all the infoQuote from: fcb on February 20, 2014, 01:24:57 amExactly.