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How to use 20K pots for hi-Z guitar pickup?
How to use 20K pots for hi-Z guitar pickup?
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Old 24th January 2019, 02:18 AM   #11
dotneck335 is online now dotneck335  United States
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Originally Posted by JohnDH View Post
One jfet transistor makes a very good buffer, with low current draw. I have a few of these in guitars and stomp-boxes. I think the values here will drive a 20k load ok, with moderate input.
I think this circuit still leaves you with >20KΩ of source impedance, which really isn't much of an improvement. Use the OPA145 in a single-supply source-follower arrangement and it will have close to zero source impedance.
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Old 24th January 2019, 03:32 AM   #12
JohnDH is offline JohnDH  Australia
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Originally Posted by dotneck335 View Post
I think this circuit still leaves you with >20KΩ of source impedance, which really isn't much of an improvement. Use the OPA145 in a single-supply source-follower arrangement and it will have close to zero source impedance.
Actually its effectively much less, controlled by the resistance between drain and source. It works out around 2k. If the load tries to drag the output down, the gate to source voltage reduces and the jfet switches on more to compensate.
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Old 24th January 2019, 04:51 AM   #13
JMFahey is offline JMFahey  Argentina
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That FET buffer will work very well, and will properly drive a 20k pot.
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Old 24th January 2019, 06:30 AM   #14
voltwide is offline voltwide  Ireland
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agreed. I Use this circuit for decades inside my axe.
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Old 24th January 2019, 09:34 AM   #15
Kay Pirinha is offline Kay Pirinha  Germany
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Originally Posted by voltwide View Post
Yes, a buffer is the way to go. With the buffer installed you can parallel some resistor to your pickup-coil to dampen hi-frequency resonant peak.


Well, this peak is no bug at all, it's a feature that significantly forms the guitar's sound! So, if any active electronics is put between the PU and the guitar cable, the missing cable capacitance should be substituted by a discrete capacitor. Otherwise the axe will sound flat and boring.
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Old 24th January 2019, 06:21 PM   #16
voltwide is offline voltwide  Ireland
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The peaking is a matter of taste, too much of it gives you sound like a wah-wah. Parallel-caps are the same - I suggest more capacity (around 1~2nF) when playing heavy distorted and less (220p~470pF) for the cleans sound with a strat. Values for the bass might differ.
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Old 24th January 2019, 09:24 PM   #17
dotneck335 is online now dotneck335  United States
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Isn't that what a tone control does?
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Old 24th January 2019, 09:49 PM   #18
voltwide is offline voltwide  Ireland
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I do not think that the traditional tone stack can afford this. A parametric equalizer could do the trick - but this is way to complicated for the average joe. And besides it makes a great difference inserting the tone control before or after the overdrive stage.
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Old 25th January 2019, 04:14 AM   #19
PRR is offline PRR  United States
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Originally Posted by voltwide View Post
I do not think that the traditional tone stack can afford this...
I think Dotneck meant the knob on the guitar. A variable resistor and a modestly large cap. Zero-R it puts a big whack on the treble much as I understood you to be suggesting.
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Old 25th January 2019, 09:05 AM   #20
JohnDH is offline JohnDH  Australia
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Quote:
Originally Posted by voltwide View Post
The peaking is a matter of taste, too much of it gives you sound like a wah-wah. Parallel-caps are the same - I suggest more capacity (around 1~2nF) when playing heavy distorted and less (220p~470pF) for the cleans sound with a strat. Values for the bass might differ.
That's right, and just to add to tbat:

A normal guitar is significantly affected by the capacitance of the cord, which interacts with the pickup to make a treble peak at say 3-4khz, with a fall above that. A buffer stops that interaction, giving a flatter, more hi-fi extended treble response. It can sound great, but it may sound different. So to reproduce it with a buffer, a very small cap can be added across the buffer input, to ground. Its worth trying, but it may be preferred without. The change is not the same as a standard tone control, so it can't be recreated that way.
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