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Author Topic: Using LTSpice to model pickup wiring effects
Digital-
Larry

Posts: 192
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Post Using LTSpice to model pickup wiring effects
on: August 13, 2012, 11:10
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First of all, great forum!

I was recently scheming to change my guitar's wiring and wanted to do something a little special. I'm going to tell you about LTSpice. This is a very powerful, yet easy to use circuit analysis program (Windows only, sorry) that you can download here: https://www.linear.com/designtools/software/

After installing LTSpice, I went looking for the circuit equivalent for a guitar pickup.

I found a few different ones online. At random, I chose this one:

https://www.muzique.com/lab/pickups.htm

Let me stop to catch my breath here before moving on!
Digital-
Larry

Posts: 192
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Post Re: Using LTSpice to model pickup wiring effects
on: August 13, 2012, 11:45
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Here's the model which I entered into LTSpice showing a single pickup model with a load of a 500k pot, turned all the way up, and a 100 pF load (the cable). The values for the inductance, pickup capacitance, and resistance, were chosen somewhat at random from various spec sheets.

Image

I can also add a step parameter to the loading cap, so you can see the effect of cable capacitance. As the loading capacitor decreases, the resonant peak goes up in frequency. At the same time, the height of the peak starts to decrease. There is a point of diminishing returns because you can't go below the 270pF of the pickup itself.

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Digital-
Larry

Posts: 192
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Post Re: Using LTSpice to model pickup wiring effects
on: August 13, 2012, 12:58
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Still with me? Great. Now here's the schematic for a series-humbucker connection using the same parameters as the single coil pickup model.

Image

At low frequencies, the line crosses the axis at +6 dB instead of 0 dB (the humbucker is louder). It has a resonant peak a little lower than 4 kHz.

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Digital-
Larry

Posts: 192
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Post Re: Using LTSpice to model pickup wiring effects
on: August 13, 2012, 13:11
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I'm going to add the capacitor used in the AES-800 position 2. This is connected between the point where the two coils of the pickup join together and ground.
Jeff_H

Posts: 47
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Post Re: Using LTSpice to model pickup wiring effects
on: August 13, 2012, 13:39
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Larry - great post.

To answer one question "Why is this position not humbucking?", the simple answer is that if the two coils have different loads then they will not completely cancel the hum or noise. The 3rd control on the AES800 is called "Coil Cut" by Yamaha. If you roll it off, it will cut one coil of the bridge and let some hum through.

I'd like to see more folks using simulation programs for this kind of stuff!

Thanks, again,
Jeff
Digital-
Larry

Posts: 192
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Post Re: Using LTSpice to model pickup wiring effects
on: August 13, 2012, 14:22
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Thanks Jeff.
Jeff_H

Posts: 47
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Post Re: Using LTSpice to model pickup wiring effects
on: August 13, 2012, 14:28
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By the way, with SPICE you can simulate the ability for the circuit to reject noise by inverting the voltage source to the 2nd coil of a humbucking pickup.

Anywhere that you see output, the circuit isn't rejecting noise!
Digital-
Larry

Posts: 192
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Post Re: Using LTSpice to model pickup wiring effects
on: August 13, 2012, 14:45
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Remove the voltage source from the pickup at the far right of the "coil-cut" schematic. This is what would happen if you took the magnets out of that coil. It would no longer generate a voltage from a nearby vibrating string. And, this is more or less what a "Varitone" circuit is - a separate RLC load hanging on the end of whatever pickup combination you have selected.
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