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Passive Piezo UST and volume pot?

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Passive Piezo UST and volume pot?

Postby Sam Spoons » Mon Nov 25, 2019 11:26 pm

I currently have Fishman Rare Earth pickups on a couple of guitars. I like them a lot but I've been so impressed with the results from passing my passive 'Big Tone' pickup on the Rob Alward Selmer through my Tone Dexter that I'm considering fitting passive USTs to those two guitars (a GA sized Brian Eastwood and an '80's Mountain D45 clone). I'd like to retain the volume control on the Eastwood (if for no other reason than to plug the hole), what value pot would you suggest for a passive piezo UST?
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Re: Passive Piezo UST and volume pot?

Postby zenguitar » Mon Nov 25, 2019 11:54 pm

The problem here is that the volume control presents the load to the pickup, and piezoelectric pickups need to see a very high load. Decent pre-amps for piezos are generally 10-20 MOhms inputs.

I would just wire the pickup straight to the output and plug into a suitable pedal/amp. Then I would put a small, dummy pot on the guitar to disguise the hole and treat it to a nice wooden knob.

But, to be fair, I’ve never been a fan of onboard controls and preamps on electrified acoustics.

Andy :beamup:
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Re: Passive Piezo UST and volume pot?

Postby Wonks » Tue Nov 26, 2019 8:55 am

Wot Zen says.

Maybe you could fit an on/off push-button switch instead so you can mute the guitar quickly if necessary.

Otherwise you could fit a basic piezo preamp in the guitar and then you can put a pot on the output.

But I'd keep it simple, external preamp and find a suitable grommet to fill the hole.
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Re: Passive Piezo UST and volume pot?

Postby Sam Spoons » Tue Nov 26, 2019 11:04 am

Thanks chaps, it was the high impedance that was exercising my (few remaining) grey cells. I do like having a simple volume control on the guitar so might look for a simple preamp.

It was the fully passive 'Big Tone' in the Aylward that works so well with the Tone Dexter though so some experimentation might be in order. Will try the D45 first as it has no such complications.
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Re: Passive Piezo UST and volume pot?

Postby ef37a » Wed Nov 27, 2019 12:28 am

You could use a dual FET input op amp like the TL072 but better and have a 10k log pot between stages. The op amp can have a 20M input Z or much more.
The first stage has unity gain so its input overload point is the same as its output and all determined by the supply voltage. The usual 9v will give you about 3V rms out, about +12dBu, same as many AI line outs. The second stage could give a little gain if needed. You could even configure the second stage as a tone control.

If the guitar already has a jack replace it with a TRS type and use the usual centre contact/sleeve power switching system used on pedals.

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Re: Passive Piezo UST and volume pot?

Postby Sam Spoons » Wed Nov 27, 2019 1:13 am

Thanks Dave, sounds like a plan, especially since I played the Eastwood in rehearsal tonight and discovered that the jack was giving no output. I'll probably still try to fix that (dodgy socket or dead battery most likely) and try the passive piezo in the D45 to see if it helps the Tone Dexter to get the best sound.
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Re: Passive Piezo UST and volume pot?

Postby Wonks » Wed Nov 27, 2019 8:00 am

I just fixed a friend's no-output B-band system by adjusting the amount of jack poking out of the rear of the body. I noticed that if pushed nearly all the way in, there was sound but all the way in gave silence. So I just adjusted the internal nut on the barrel so that about 1mm less of the barrel jack poked out the end, so when the end cap/strap button was screwed over the top and the jack fully inserted, full contact was once again made.

All probably due to wear in the jack socket as the unit wasn't loose at all, and it is a 15 year old guitar. At some time in the future the barrel jack socket will need replacing, but for now, it's working reliably again.

So if it's not the battery, just unscrew the strap button cap (which should allow another 2-3mm of inwards movement) and see if you can get a signal at some position. If you can, then strings off, remove the external jack fixing nut, pull out the barrel socket, adjust the internal positioning screw in the desired direction, replace the jack socket (sticking a small screwdriver down the endpin hole from the outside can make it easier to find the hole and replace the jack). Screw down securely, replace the button cap and hopefully away you go.

If you can't get an output regardless of jack position, then it's time to replace the barrel jack. Best to use some heatshrink over the terminals as they are very close together and you don't want an internal short due to the wires moving around.
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Re: Passive Piezo UST and volume pot?

Postby Sam Spoons » Wed Nov 27, 2019 11:25 am

Thanks Wonks, many years ago I made a device for fitting jacks into guitars, basically all the ¼" diameter bits of a plug soldered to a couple of feet of wire coat hanger. Works a treat.

The jacks on both these guitars and open ones as they are fitted to the lower bout, Les Paul style. I prefer that to end pin jacks as the latter preclude the use of low profile guitar stands. The Eastwood has a reinforcing pad inside.
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Re: Passive Piezo UST and volume pot?

Postby Wonks » Wed Nov 27, 2019 11:27 am

Ah well, I'd probably throw them away and get some new ones that work then. :D
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Re: Passive Piezo UST and volume pot?

Postby Sam Spoons » Wed Nov 27, 2019 11:28 am

If they are the problem that's what I will do :D
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Re: Passive Piezo UST and volume pot?

Postby Wonks » Wed Nov 27, 2019 11:56 am

I was talking about the guitars. :D
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Re: Passive Piezo UST and volume pot?

Postby Sam Spoons » Wed Nov 27, 2019 3:02 pm

So was I ;)
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Re: Passive Piezo UST and volume pot?

Postby Wonks » Wed Nov 27, 2019 3:14 pm

I'll bring a bin over.
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Re: Passive Piezo UST and volume pot?

Postby Sam Spoons » Wed Nov 27, 2019 3:33 pm

:bouncy:
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Re: Passive Piezo UST and volume pot?

Postby Sam Spoons » Thu Nov 28, 2019 9:52 pm

Well I've just spent around £20 on half a dozen piezo UST elements and a preamp from China. I'll keep you posted.
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Re: Passive Piezo UST and volume pot?

Postby Wonks » Thu Nov 28, 2019 10:00 pm

Last of the big spenders. Corbyn's got his sights on folks like you. :D
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Re: Passive Piezo UST and volume pot?

Postby Sam Spoons » Thu Nov 28, 2019 10:18 pm

Yeah, life's a bitch innit.....
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