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Author Topic: Electric Guitar Pickup switching  (Read 1204 times)
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Hi!
I am a relative newbie here. 

My project requires being able to route audio signals from the 12 pickup coils of an electric guitar.  In other words, I need to create some kind of switching network that can send or not the signal generated by each pickup coil along a wire.  The Arduino will select to let signal through or not.

I was thinking of using relays, but I have no idea if they will add noise or alter the signals, nor do I know which relays to get?

Or maybe there are other better components?

My questions:
  • What component to use as a digitally controlled audio signal switch?
  • Any other suggestions?

Thanks for this great forum!
Ciao,
Bob
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you could use 3 quad analog switches  CD4066
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The best answer depends on a number of factors:

1. How do you want the pickups to be connected when they are all active? In series, in parallel, or each one to a separate amplifier input?

2. What is the resistance of each pickup?

3. Can we safely assume that the peak signal voltage from the pickups is not more than +/- 0.5V ?

3 x CD4066 quad switch ICs may be suitable, but their on-resistance is 270 ohms typical @ 5V and 25C, which may be too high in some configurations (e.g. if you connect the pickups in series and use the switches to short out the ones you want to disable). Another possibility is 12 x 2N7000 mosfets, which have a muhch lower on-resistance.
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We can assume around 6 Kohm resistance per coil.

There is no fixed network layout, I want to be able to play around, selecting series/parallel, in phase/out of phase, for all pairs of coils. Yes, that means a big network if it were to be fully deployed, but at first at least I may reduce the complexity.

Output voltage is on the order of +:- 0.125V.

The on-resistance should be as low as possible, like a normal mechanical switch.

I have no way of judging what is the best, or even a good starting point, so I will just take all advice and try it out!

Thanks to everyone - more advice is always welcome!
Ciao,
Bob
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dc42s suggestion of 2N7000 is good, and they are cheap.

If you just want to experiment with the idea with 3 chips, you can get an idea of what you want to do, we have no idea of what music you play,  heaven forbid we suggest something that introduces distortion :-)
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Thanks Boffin1 !

I am trying to avoid any introduction of noise or "distortion" by the circuit.  That is the musician's choice, not a "feature/bug" of the software. 

I am trying to keep this circuit very very clean, otherwise it is a downgrade from the mechanical version already on board the guitar.

Ciao,
Bob
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Actually, what I really need is a digitally controlled DPDT switch capable of handling 0.125 Vac. 

Is the 2N7000 the right solution?

Thanks,
Bob
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I think 2N7000 is a good solution, provided you keep the "switch terminals" (i.e. source and drain connections) close to ground potential. Its on-resistance is typically around 2 ohms with 5V gate drive. The reason for limiting the input voltage to 0.5V is that it has a body diode that will start to conduct at about 0.6V.

One of them acts like a SPST switch. To make an SPDT switch you will need 2 of them, and either 2 output pins (one to drive each 2N7000 gate), or 1/6 of a 74HC04 to generate an inverted signal for the second gate. To make a DPDT switch you will need 4 x 2N7000 and either 2 output pins or 1/6 x 74HC04.
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@dc42

Thanks for that!  I guess I will run out of pins on my arduino Uno, unless I use some kind of shift register chip like a 74HC595 to get more pins?

Are there any subtle things I may be overlooking here? I can just use the 2N700 as a SPST switch and combine them to make other switches by interconnection the gate pins?

Cheers,
Bob
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Are there any subtle things I may be overlooking here? I can just use the 2N700 as a SPST switch and combine them to make other switches by interconnection the gate pins?

The source and drain terminals of the 2N7000 will act as a switch, controlled by the gate terminal, under the following conditions:

1. The source terminal is within 1V or so of ground potential.

2. When the switch if off, the drain terminal is no more than 0.5V below the source terminal.

3. You don't try to pass more than about 75mA through the switch when it is ON.

These conditions should be straightforward to achieve in the system you describe. If it helps, you can drive the gates of several 2N7000s from the same signal, if you always want them all to be in the same state at any given time.

I'm not sure what you expect to achieve by reversing the phase of a pickup signal, I doubt you will be able to hear any difference. I suggest you try it using a mechanical switch first.
« Last Edit: November 15, 2012, 01:21:48 pm by dc42 » Logged

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Hi Frog,

Important question: Do you need /want to be able to switch configurations during a musical piece?  If so, you have yet another dimension of the control voltage(s) being induced or capacitively coupled to the audio circuit.  The 100mv or so of the pickups doesn't give you a lot to play with for signal-to-noise ratios. 

What guitar has 12 pickups, BTW??

Maybe do this differently: Put a small board with low-noise opamps like NE5534 in the instruments and feed every pickup out a cable to an external mixer / patch bay.  BUT if the coils interact directly when you connect the, in series / parallel, that part would be lost. 

Hmmm....
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Hi King Terry of the Vermont Wood Forrest!

Oops, I guess I can't multiply. There are 3 pickups with 2 coils each, so a total of 6 coils, not 12 as I wrote earlier.

I'm just trying to reproduce the on board switching and pots in a way that the settings can be programmed.  I do not want to modify the coil output, or as little as possible.

I'm still searching for ideas as to how to replace the switches, I've already got a plan for the pots using LDR-LED setup.

All suggestions are welcome!

Thanks for your help!
Ciao,
Bob
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Quote
I've already got a plan for the pots using LDR-LED setup.

Bob, you can use LED-LDR with LDR with low ON resistance as switches too. And optical isolation means no ground / voltage swing issues..

I used to epoxy together my own in making mixer consoles..

Maybe...

...I have just about enough of my part of the forest cut and split and stacked to heat the cabin all Winter while I build Arduino stuff  smiley
« Last Edit: November 15, 2012, 10:22:54 pm by terryking228 » Logged

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Oh Terry, Great and Noble Forrest King!

I have seen your website and found the PC817 opto-isolator, but it looks to me, in my extreme ignorance, that ac current won't get through it when turned on?

I am feeling really ignorant, when I see all the replies suggesting various ways to build my circuit.

I am also realizing that I haven't been very clear in my questions.

My goal is reproduce as much as possible an electric guitar's mechaincal man-machine interface, which comprises pots & switches, by an Arduiono based programmable interface. Take a very simple circuit such as http://www.dimarzio.com/sites/default/files/diagrams/Eesqwiring.pdf. I would like to command the switching and the pots programmatically from the Arduino. In my naivety, I thought to use some MOSFETs to replace the switch, and to use 2 LDR-LED combinations to replace the pots. Then, I would use PWM on the LEDs to set the LDR resistance values.

I do not want to impact the signal coming from the pickup coil.

I have no idea what I am doing here, but I am prepared to hack at it!  I have already got an old sacrificial guitar and an Arduino and lots of tools since I am an RC hobbyist.  But, I am still trying to see what electronic components could be used to make all this work...

All your advice, and that over everyone on this forum is GREATLY APPRECIATED!

Thanks so much!
Ciao,
Bob
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The "Digital" opto-isolators like 4N35 and PC817 etc. will not work for audio circuits.

You need something that will be linear with a varying voltage applied.

FET's if biased properly can work.

Light Dependent Resistors work well also.  You can use them as volume controls in a voltage divider circuit, with varying LED (or other) illumination.  But you can use them as audio "Switches" too, if you switch the LED from OFF to Bright.

If you're into making your own for volume controls, then you could make switches too.  

Google "LED-LDR" and you'll find how-to, like:

http://www.silonex.com/datasheets/specs/images/pdf/104057.pdf   (A commercial unit)

http://home.online.nl/fvrijswijk/guitar_electronics_diy/optocouplers_and_measurements/optocouplers_and_measurements.htm DIY

If the coil Impedance (probably not the resistance) is 6K (did you say that?) then they should work well as switches.

Experiment!

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