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Author Topic: LTSPICE – the Tone Control
Digital-
Larry

Posts: 192
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Post LTSPICE – the Tone Control
on: August 14, 2012, 11:59
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Wanted to show you something interesting about the basic tone control as we know it.

This schematic shows a single coil pickup going to a 500k load (i.e., a 500k volume pot turned all the way up) then to a series capacitor – pot load (a typical tone control).

Image

The value of Rt steps from 2.5k to 25k to 250k. At 250k, there is a peak at about 5 kHz. At 25k, there is no peak, but it starts rolling off around 1 kHz. At 2.5k there is again a peak, this time at about 800 Hz.

Image

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The first (higher frequency) peak is primarily the result of the pickup’s inductance, resonating with the pickup’s capacitance in conjunction with the cable capacitance. Bring down the value of Rt increases the tone control’s effect. At first it just flattens out the first peak. Then as the tone pot goes even lower and the tone control cap comes to dominate, a new peak is formed at a lower frequency.

Putting it another way – you want “woman tone”, you gotta turn it all the way down! Only half way down is just “girly tone”.
joe
Administrator
Posts: 224
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Post Re: LTSPICE – the Tone Control
on: August 19, 2012, 11:49
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LOL — I’ve never seen it analyzed like that, and I definitely couldn’t create an analysis like that! But empirical evidence (i.e., my ears) confirms everything you say. 🙂
bear

Posts: 153
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Post Re: LTSPICE – the Tone Control
on: August 19, 2012, 13:21
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Larry, very cool to see this modeled. A question: does tone-knob-coupled capacitance behave differently from cable capacitance (or equivalent capacitance to ground)?
Digital-
Larry

Posts: 192
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Post Re: LTSPICE – the Tone Control
on: August 19, 2012, 15:15
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Hi Bear,

I can think of no reason at all that tone-control capacitance should behave differently than cable or any other capacitance to ground. The only thing that could happen is that real-world things (non-ideal component behavior) start to appear. In passive audio filters, this is mostly due to inductors with any appreciable value having a non-negligible series resistance that un-sharpens any resonant peaks. Or if you want to use a 150 foot cable like Guitar Slim or… er.. (BB King?) you might get some series resistance or inductance there. Whether it has an effect depends on the other parts of the circuit.

Thanks for your interest and comments.
Digital-
Larry

Posts: 192
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Post Re: LTSPICE – the Tone Control
on: August 19, 2012, 15:19
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Quote from joe on August 19, 2012, 11:49
LOL — I’ve never seen it analyzed like that, and I definitely couldn’t create an analysis like that! But empirical evidence (i.e., my ears) confirms everything you say. 🙂

That’s good to hear! To be honest I was not aware of this phenomenon until I saw the simulation results.
Jeff_H

Posts: 47
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Post Re: LTSPICE – the Tone Control
on: August 20, 2012, 06:29
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With cable capacitance, the equivalent series resistance [ESR] goes up proportionally with the capacitance. This tends to keep the peak at about the same level, but just moves it down in frequency.
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