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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 20th January 2019, 08:17 AM   #1
BassAmp is offline BassAmp  United States
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Default How to use 20K pots for hi-Z guitar pickup?

I have a somewhat unique guitar that was made for me eons ago. The builder incorporated some very cool push-lock volume and tone pots that retract and become flush with the face of the body. He had to build a special framework inside the body to mount them on. They are 20k Audio taper pots to go with the active EMG pickup that was installed. I really can't use conventional pots on this guitar.

I have come across a different pickup that I would like to try, but it is passive and 250K Audio taper pots are recommended.

I don't think I can source 250K push lock pots to replace what is currently in the guitar. So my question is whether or not there is a way to use the pre-existing 20K pots.

The best idea I have come up with is to put a 9V buffer circuit between the pickup and the pots. I think this might work, but I am concerned about adding a whole bunch of top end on an already bright pickup. Is there some sort of impedance lowering circuit I could incorporate?

Does anyone have any better ideas or suggestions?

Thanks
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Old 20th January 2019, 01:59 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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In a tow words, you can't, unless you use a buffer to reduce the impedance as you suggest.
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Old 20th January 2019, 06:53 PM   #3
BassAmp is offline BassAmp  United States
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Well, I guess that settles it. Thanks for the input
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Old 20th January 2019, 08:14 PM   #4
voltwide is offline voltwide  Ireland
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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.
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Old 20th January 2019, 08:40 PM   #5
dotneck335 is offline dotneck335  United States
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Best low-current draw (required for on-board usage) buffer is an OPA145 IC. Great specs for noise, distortion, slew rate and <0.5 ma Iq.
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Old 23rd January 2019, 05:37 AM   #6
BassAmp is offline BassAmp  United States
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Thanks for the suggestions.

I searched the entire internet, from beginning to end, for an OPA145 based guitar buffer but I didn't fare too well. Unfortunately, coming up with a design on my own is WAAAY over my head. Stupidly, I studied music most of my life and not electronics. Duhhh...

If anyone has something or could point me in the right direction, I would appreciate it.

Meanwhile, I found one of these in my parts bin. I completely forgot I had it. It is a dual channel (magnetic/piezo) buffer. I think it ought to work using only the magnetic channel. However, I think I may run into another problem. Because the input impedance is so high on this buffer (1MΩ), won't I introduce a ton of high end into the signal? This seems consistent with what Voltwide said above. Maybe not, I don't know. I would suspect this is designed not to altar the character/sound too much. I have reached out to the mfr, but it will probably take a while to get a response. Could someone help clear this up for me?

If I am correct in my assumption, could someone suggest a remedy so that the pickup would sound as natural as possible?

Thanks

Last edited by BassAmp; 23rd January 2019 at 05:44 AM.
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Old 23rd January 2019, 06:42 AM   #7
JohnDH is offline JohnDH  Australia
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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:


Click the image to open in full size.

There are many tweaks and variants possible to optimise it for a particular use.

Last edited by JohnDH; 23rd January 2019 at 06:47 AM.
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Old 23rd January 2019, 07:20 AM   #8
voltwide is offline voltwide  Ireland
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If your preamp has 1MOhm impedance you are fine. Take it and later on you may switch some resistor of your taste (100~500k) parallel to the pickup. Btw I studied all my life electronics, but not music. Shame on me!
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Old 23rd January 2019, 07:55 AM   #9
BassAmp is offline BassAmp  United States
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JohnDH, Thanks for the schematic. I'll check it out. This might be a good project.

Voltwide, thanks for the confirmation. I am going to install this buffer and see what happens.

Quote:
Originally Posted by voltwide View Post
Btw I studied all my life electronics, but not music. Shame on me!
This may be true, but you invented the CLS-222. I think you probably know a few things about music.

Last edited by BassAmp; 23rd January 2019 at 08:05 AM.
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Old 23rd January 2019, 07:52 PM   #10
JohnDH is offline JohnDH  Australia
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A couple of things about the jfet buffer above:

Its very tolerant of different jfets, and jfets often have considerable variation even within the same type. Many may bias better if R8 is reduced from 2.2M to 1.8M or 1.5M. The aim is to pick a value where the source voltage (where jfet meets C2), is around 5.5V. Its not super critical but it allows the best undistorted max voltage swings.

With the 20k load, it should be possible to deal with up to +/- 3V swings before clipping, which is pretty high (I checked with a Spice run)
If the buffer is directly connected to a pot, the 3 3M resistor R11 can be omitted.
Current draw is <0.3mA
Frequency response is flat down to bass low E (eg 40hz), with overall a small reduction of about -0.8db when driving that 20k load.

But is the tone pot also loading the output, so there's overall a 10k effective load?

Last edited by JohnDH; 23rd January 2019 at 07:57 PM.
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