Audio DPDT & Muxers in DIP packages? - Page 1
Forum section: electronics / projects

author: Mark Scuder
author_rank: Newbie
timestamp: March 02, 2016, 02:17:18 am
content: Hello. Long time Dave watcher, first time poster.I am in the design phase of a uC-controlled electric guitar switching system and attempting to source the parts I need for the build.A little background: Electric guitar pickups vary, but they are all one or two coils of copper wire around a magnetic core, which creates an AC current when strings are strummed or plucked. Most electric guitars have up to three pickups, and each pickup may have two individually-addressable coils. Many various wiring mods are available, but they're semi-permanent; they're either soldered in place or to all manner of switches. I would like to see if I could create a system for my guitars that would allow me to assign any combination and modification of pickups I wish to a position on the guitar's 5-way blade switch, even if those wirings would be impractical or impossible to wire by hand.Electric guitar is funny. Since it uses unbalanced audio cables, the guitar has its own "ground," which is essentially all the grounds of all the switches, pots, pickups, and any other metal parts tied together with wires (potentiomers' metal cases are soldered together, for instance, and to the shield of the output cable, and to any other metal parts). That's ground potential in an electric guitar. When you introduce DC, you have to do one of two things: bias and un-bias the audio, carrying DC through the signal paths, or to use optoisolated switches and/or solid state relays, so that the DC ground for the uC and solid state switches is separate from the guitar ground. (This is is why I am not using bare MOSFETs or 4066ICs.)I have a proof-of-concept working with a set of guitar electronics, grounded to itself as described above, and a couple of Omron G3VM-21 Solid State SPST relays. Toggling a GPIO line on an Arduino closes the switch, and sound flows from the selected pickup to the output. Works really great, actually. But I need more than SPST switches.I was able to test the G3VM-21's so easily because Omron still makes them available in DIP, so I can use them on breadboard and protoboard. However, for all the possible guitar wiring mods, I also need a bunch of DPDT solid-state switches and/or relays (guitarists may recognize DPDTs as necessary for series/parallel wiring, phase reversal, and coil cut/coil tap). I haven't been able to find any optoisolated DPDTs at all, and I haven't been able to find much of anything in DIP. I have quite a few samples from Maxim Integrated in SOIC format which are useless due to my big, shaky hands - I find it impossible to "dead bug" an SOIC just to get it in a breadboard. I even corresponded with Maxim today and they swore they had certain chips available in DIP and a quick browser of their website shows otherwise.Another problem I'm running into is that a lot of the DPDT solid state switches are designed for some sort of USB application I'm not completely understanding. Sure they have 900MHz of bandwidth but if they're designed for DC signals only they won't work right.I would rather not manually build and manage DPDT switches out of 4 or more SPST switches; I am of the mind that such a simple product should be available for purchase.The relays only have to be able to handle about 1VAC on the switch side and should have a low on resistance.I also need a good "multiplexer," but all the ones I've seen allow you to only close one of, say, 8 switches to the common pin. I need one that allows me to open all, none, or some of the switches and sum them into the common.Again, I know I can do this all with SPST SSRs but in my head it doesn't make sense to do all sorts of gymnastics on my board to kludge together what should be available in an IC.I wonder if someone can help me find DPDT solid state relays for switching < 1VAC, and many-to-one multiplexers with multiple switch capability, made for audio applications, isolated from DC currents and DC ground, that are still available in both DIP and SMT, for prototyping and production, respectively. I don't understand why this is so hard! Thank you.

author: uncle_bob
author_rank: Unknown
timestamp: March 02, 2016, 02:39:39 am
content: HiThe conventional way to do this is to use good old magnetic relays. They have built in isolation, are cheap, and come in all sorts of contact arrangements from a bazillion different sources. Relays with built in drive will *always* cost you more money. Since you have a bunch of relays in the design, that's going to add up quick.Driving a relay is generally as simple as a single transistor and one (maybe) resistor. The transistor is < 5 cents and the resistor(s) is less than a penny. Getting the relays for under $2 (and maybe under $0.50) pays for the transistor pretty quick.Bob

author: jaycee
author_rank: Regular Contributor
timestamp: March 02, 2016, 02:44:58 am
content: What about standard DG408/409 type multiplexers? They're easily available in DIP - Maxim and Analog Devices make them. I've used these in audio preamplifiers as the input switching stage with no problem.

author: Mark Scuder
author_rank: Newbie
timestamp: March 02, 2016, 08:05:41 pm
content: Quote from: uncle_bob on March 02, 2016, 02:39:39 amDo they make magnetic relays in a form factor similar to the solid state relays (6.5x4x2.1mm)? Would the relays be affected by magnetic fields surrounding the pickups? Is the switch path completely isolated from the DC side?

author: Mark Scuder
author_rank: Newbie
timestamp: March 02, 2016, 08:08:16 pm
content: Quote from: jaycee on March 02, 2016, 02:44:58 amSimply because months of Googling did not bring them up. Do you know for a fact that the DG408/409s have independently operating switches? Can I have more than one (of eight or whatever) closed, summing more than one audio source into the common? When I think "Multiplexer," I think something that takes more than one signal and sums them into one common. Most of the "multiplexers" I've found are simply 8-position switches, and as useless for this application.

author: uncle_bob
author_rank: Unknown
timestamp: March 02, 2016, 11:18:18 pm
content: Quote from: Mark Scuder on March 02, 2016, 08:05:41 pmHiThey make magnetic relays in an enormous range of packages. The DC magnetic field around one will in no way be affected by the pickup's field. A quick search of mouser.com turns up > 30,000 different models. Like any component, they each have their own package size and shapes. Sorting reed relays by price show these as the winner:http://www.mouser.com/ProductDetail/Coto-Technology/9007-05-01/?qs=sGAEpiMZZMv4tz1TW%2fArE8beE9u84hIiEue4OitBme8%3d$1.03 for one piece, $0.85 ea if you buy 100.Sorting low signal relays shows:http://www.mouser.com/ProductDetail/TE-Connectivity-PB/V23105A5001A201/?qs=sGAEpiMZZMs3UE%252bXNiFaVOJsSk6%252bB0OozTMt7CRvnlE%3dat $1.55 for one and $1.38 at 100Consider that they call 3 amps a "small signal". Your guitar pickups will never hit that unless something shorts out.Bob

author: Mark Scuder
author_rank: Newbie
timestamp: March 04, 2016, 03:45:31 am
content: With 30,000 parts to choose from, do you have a recommendation for a good part, not just the cheapest?

author: MarkF
author_rank: Super Contributor
timestamp: March 04, 2016, 05:02:16 am
content: Quote from: Mark Scuder on March 02, 2016, 08:05:41 pmI would bemoreconcerned with the magnetic field of the relay getting into the pickup during switching.

author: uncle_bob
author_rank: Unknown
timestamp: March 04, 2016, 12:41:35 pm
content: Quote from: Mark Scuder on March 04, 2016, 03:45:31 amHiThe nice thing about relays is that they all pretty much just work. Both of the ones I came up with should do what you want to do. The only issue might be coil voltage depending on your power supply. The other issue would be the need for DPDT or 3PDT or something even bigger than that. The reed relays are cheap and low power, the bigger unit is essentially indestructible. (if you put a 10A surge through the reed relay you will likely weld the contacts).My recommendation -- pick at most 3 models and try to work around those. That will let you buy in slightly larger volume. It also will give you fewer spares to stock if you have a problem.Bob

author: uncle_bob
author_rank: Unknown
timestamp: March 04, 2016, 12:43:45 pm
content: Quote from: MarkF on March 04, 2016, 05:02:16 amHiYou see them used in the front ends of all sorts of high sensitivity instruments. To make the relay as efficient as possible.(lowest current to the coil), they work pretty hard on confining the mag field. That said, no, I would not glue the pickup directly on top of the relay coil.Bob