Mux help with switching low V audio signal??

Discussion in 'The Projects Forum' started by D4RKH0R53, May 2, 2012.

  1. D4RKH0R53

    Thread Starter New Member

    May 2, 2012
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    Im in desperate need of some help i'm trying to modify my guitar and the poor thing has been in pieces for over a month while i make up my mind what I'm going to do with it.
    Basically I have 2xdouble humbuckers totaling in 4 separate pickups with 2 leads each. whether I hook those leads up to the output in series, parallel, mixed with extra cap or res, etc can greatly change the tone of the instrument.
    I want to be able to get every possible combination of pickups plus a couple of tone enhancing circuits added as options and to do this a normal selector switch wont work.
    So I really wanted to use a digital means of switching the analog signals. Eg. 2 mom' push buttons that scroll through all possible combos or presets.
    I've gotten as far as to purchase a couple of 4051 analog multiplexers.
    Does anyone have knowledge of circuits that are appropriate or could be adapted for this purpose? Any help would be much appreciated, thanks.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    I've been planning to rework my guitar pickups as well but have not found the time.
    I don't know if analog multiplexers will work in this case. The voltage out from a pickup is very low, about 50mV rms. You are likely to introduce noise and 60-cycle hum.

    My plan would have been to put SMD preamps with about 100-1000 times gain and to digitize the signal right at the pickup.
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,173
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    Have you tried a 4066 quad bilateral switch IC?
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,018
    3,235
    The 4051s will work if you have one output and you want to select various inputs (or vice versa). It should work for the low level signals if you place the circuitry in a metal box and ground the box, using shielded wire and connectors for the inputs and outputs.

    Note that for normal AC signals, which go above and below ground, you will need a plus and minus supply voltage to power the chip. For example Vdd= +3 to +5V, Vss = 0V (ground), and Vee= -3 to -5V. The value of Vdd minus Vee must not be more than 15V total.

    Two or three AA or AAA cells should work for each voltage. The circuit draws only a very small leakage current so the batteries will last for more than a year even if you leave it on continuously.

    You could also use a 9V battery with two 100kΩ resistors in series across the battery as a voltage divider to generate a pseudo ground at 4.5V for the muxes. Add 0.1 μf caps from each side of the battery to the pseudo ground point. Tie the pseudo ground to your instrument signal common. A 9V alkaline should run continuously for about 6 months and a lithium 9V should run over a year.

    To control the muxes with PB switch you could use a 4-bit counter, such as the 4510 or 4516. You will need some simple logic gates to detect the last switch position and reset the counter back to the start. You will connect the lower 3-bits of the counter output to the mux address line inputs.

    You will need some sort of debounce circuit (Google) between the PB switch and the counter clock input to avoid multiple clocking.

    Make sure all unused input on all the CMOS circuits are connected to ground. If using a SPST PB switch connected to Vdd for the clock then you need to add a 10k resistor to ground at the clock input to provide the logic low.

    The counter would be connected to your Vdd plus voltage and ground.
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You will need a lot of chips! I rewired one of my guitars with 2 armstrong humbuckers (that come with 4 wires and a separate shield) and used a few DPDT switches.

    There's no point reversing the 2 coils within a humbucker because of the phase cancellation, but just to get all the basic variations you need a DPDT on each HB to switch between parallel and series wired, and another DPDT to reverse the front pickup to phase it against the back pickup. That's 3 DPDTs and to do it with analog multiplexers would need 3 quad chips. If you want to be able to run just a single coil on the pickup (I did) that is another DPDT. My guitar has 4 DPDT switches in a row which is compact and allows instant control.

    I don't think you will be happy long term with the hassle of stepping through pickup selections with buttons, it will be painful especially if like most people you want to switch tones from verse/chorus.

    Also I've used quad analog switch ICs for guitar audio before, and due to the very high gain of guitar amps (especially if you use a distortion pedal) they leak sound through even when "off" so it won't switch as well as DPDT switches either.

    Also you don't need to switch parallel/series, sometimes it gives better tone variation to just turn one coil off, ie switch between humbucker and a single coil.
     
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