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Acoustic guitar volume control

Postby GRAHAM99 » Sun May 12, 2019 8:45 pm

I have an acoustic guitar with a Fishman Rare Earth sound hole pick up.it has a volume control on it but it is not very accessible.
I have thus installed a separate volume control using a pot mounted on the top of the guitar.
I wired it in the normal voltage divider fashion.
When I increase the volume there is a bad hum which goes away when the pot is fully opened.
I have another guitar with the same pick up on it and when I wired up a pot on the output of this guitar I had the same issue.
Any ideas on this?
Thanks.
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Re: Acoustic guitar volume control

Postby DC-Choppah » Sun May 12, 2019 9:17 pm

The resistance of the pot you added, along with the pickup capacitance/inductance is changing the resonance of the circuit.

Check out 'Treble Bleed' circuit for one alternative way to add a volume control. This is the way to make the volume work without changing the tone too. You just add a certain capacitance into your circuit.

On my acoustic guitar and other instruments, what I do is use a high impedance input (1 - 10 M Ohm) volume pedal. This allows me to change the volume of the acoustic guitar, or bass, or violin, etc. without changing the tone of the pickup.
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Re: Acoustic guitar volume control

Postby GRAHAM99 » Sun May 12, 2019 9:21 pm

Thanks for that.
The Fishman pick up is active, does this make any difference?
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Re: Acoustic guitar volume control

Postby DC-Choppah » Mon May 13, 2019 1:43 am

Indeed. You don't want to use the normal passive pickup pots for your active pickups. You will need a much lower value pot than you would use with a passive pickup.

The output impedance of your active Fishman magnetic pickup is 200 Ohms according to a Fishman manual I found online: https://www.fishman.com/wp-content/uplo ... _guide.pdf

Seymour Duncan says to use 25k pots for volume control on their active pickups which are also low impedance like this.

I would send a note to Fishman and tell them what you are trying to do and ask them to recommend a pot value that will sound good and give you a usable range of motion. They will probably tell you that you need an active control circuit.

There is actually a reason they put that volume nob where they did on your pickup. That is probably the location (in the circuit) where you need to be to control the volume without changing the tone.

I wonder if you can take the pickup apart and get access to the wiring of the volume control and move it to your desired location instead?


BTW: Tell us what value pot you used and got the feedback problem.
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Re: Acoustic guitar volume control

Postby CS70 » Mon May 13, 2019 9:50 am

DC-Choppah wrote:The resistance of the pot you added, along with the pickup capacitance/inductance is changing the resonance of the circuit.

Check out 'Treble Bleed' circuit for one alternative way to add a volume control. This is the way to make the volume work without changing the tone too. You just add a certain capacitance into your circuit.

On my acoustic guitar and other instruments, what I do is use a high impedance input (1 - 10 M Ohm) volume pedal. This allows me to change the volume of the acoustic guitar, or bass, or violin, etc. without changing the tone of the pickup.

Gotta love this forum, great info and I wouldn't have had a clue!
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Re: Acoustic guitar volume control

Postby Sam Spoons » Mon May 13, 2019 9:56 am

I have a rare earth in one of my acoustics with a volume pot on the guitar and it's one. I can't remember the value but it'll probably be a standard electric guitar pot unless Fishman recommended a value in the manual. It's had two different REs in it, a Blend and a humbucker, both have been fine but they are older ones and neither has an onboard pot.
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Re: Acoustic guitar volume control

Postby GRAHAM99 » Mon May 13, 2019 12:18 pm

Replaced the 500k with a 25k pot and hey presto works fine now!!
Thanks for all the help.
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Re: Acoustic guitar volume control

Postby Sam Spoons » Mon May 13, 2019 1:25 pm

:thumbup: :thumbup: :thumbup:
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