Route multiple analog (audio) signals using a single switch

Question

I'm working on a guitar wiring project (=> I'm confined by space and I'm working with high impedance, low voltage & current signal) - it's a router for pickup wires (there will be 5 "banks" that will connect the pickups in a certain way). There are 10 wires from the pickups + 2 (ground and hot lead from output jack).

These 12 wires will need to be switched by a 5-way switch (in each position, they will be connected to a single bank) - essentially, I need a 12 pole 5-way switch.

It doesn't seem like it's possible to buy these anywhere (for understandable reasons) - I was thinking about using a set of multiplexers - that would, however, require power source which I'd like to avoid.

What are some other possibilities? Does multiplexer work with analog signals without introducing delay or distortion?

Answer 1

Multiplexers work well if: 1) you have the Load resistance (impedance) much higher than the Mux resistance; do not expect switch/channel resistance to match between ICs, and perhaps not between switches within the same IC; thus with Rmux of 100 ohms, varying 50 ohms, to have 0.1dB matching (1%matching) you need at least 5,000 ohms load and probably should be higher.

2) you keep the voltage being switched near the middle of the MUX's VDD range; if +5 and -5 VDD (two supplies) then you should get good (low distortion) near GND, if the Load is very high.

3) the MUX control lines will inject trash into the signals being switched, because the MUX uses big FET transistors to achieve low resistance switching. Those big FETs have wide channels and thus large gates and thus large Capacitance from Gate to Channel (through which you signal electronics flow); thus you must use R+C low pass filters on the Select wires, to exclude MCU trash or just RFI pickup.

4) The MUXes have finite frequency response, unlike a metal-switch-contact where the wiring inductance is the limit; this MUX F3dB (-3dB) comes from the MUX resistance and the Mux capacitance; usual specs on frequency response are done in 50_ohm environment, with popular spec being "10MHz with 50_ohm load"; you can expect 1,000X lower response (10KHz) if you raise the load to 50K ohms.

5) any VDD trash (such as on +5 and -5 rails, or +15 and -15 rails) will end up on the LARGE analog-mux-FET gates, and thus inject charge into the FET channels, which is your signal_path. Assume 10pF capacitance from your VDD rails into the signal_path.

How severe is this? assume 0.5 volt ringing at 1MHz from a switching power supply. The impedance of 1pF at 1GigaHertz is -j159 ohms. The impedance of 10pF at 1MHz is (down 10X to 16 ohms, up 1,000X to 16,000 ohms) 16,000 ohms. If your load on the analog-mux is also 16,000 ohms, then you'll have 0.5/2 = 0.25 volts at 1MHz that ALSO comes thru/from that innocent analog MUX. Solution? lotta VDD filtering.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Don't give up on mechanical switches yet.

Figure 1. A selection of stackable switches (and some non-stackable).

Stackable switches generally have "decks" made of twelve outer contacts with 1, 2, 3, 4 or 6 wipers enabling switch configurations from 2-way to 12-way with any number of poles subject to the mechanical limitations of the frame.

The regular sized ones I'm familiar with (top right) may be too large for the guitar body but the PCB type pictured may work for you. There may also be a miniature version of the one on the top right.