Passive Piezo UST and volume pot?

[Sam Spoons]

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?

[zenguitar]

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

[Wonks]

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.

[Sam Spoons]

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.

[ef37a]

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.

Dave.

[Sam Spoons]

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.

[Wonks]

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.

[Sam Spoons]

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.

[Wonks]

Ah well, I'd probably throw them away and get some new ones that work then.

[Sam Spoons]

If they are the problem that's what I will do

[Wonks]

I was talking about the guitars.

[Sam Spoons]

So was I

[Wonks]

I'll bring a bin over.

[Sam Spoons]

[Sam Spoons]

Well I've just spent around £20 on half a dozen piezo UST elements and a preamp from China. I'll keep you posted.

[Wonks]

Last of the big spenders. Corbyn's got his sights on folks like you.

[Sam Spoons]

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

[Sam Spoons]

Just to recap and save you rereading the whole thread, I'm looking at replacing the Fishman Rare Earth sound hole pickup in my Brian Eastwood Custom with a piezo hopefully to make it work better with my Tone Dexter preamp as mag pickups are not recommended.

Anyway I'm resurrecting the thread as I had a little mind bomb earlier when I realised my NS Design Wav electric upright bass has what I believe is a piezo pickup under the bridge (the only others I can think of are B-Band type capacitive sensors but they'd need a polarising voltage) with tone and volume controls and a pizz/arco switch. It's passive, no preamp or battery, the pizz/arco switch just adds a second cap to the circuit (I'll try to draw it out tomorrow) and the volume pot measures around 1 Meg. It sounds great straight into the amp (in this case a little 1 x 10" Warwick bass combo). Granted it doesn't need much in the way of HF which may be why it works but a passive circuit would be much nicer on the Brian Eastwood and save the faffing with internal batteries (I'm not about to cut a hole in the side for a battery box) so I'm thinking I'll order up a 1 Meg pot and give it a try.

[Sam Spoons]

Ok, that didn't work, so far, as the cheapo Chinese piezo elements appear to be dodgy, lots of hum and low output so I'm going to try the similarly cheap and cheerful active pickup I bought. It has no hum and plenty of output and a volume control on a sound hole mounted edge wheel pot. I want to use the existing hole for the volume so I need a standard guitar pot with a 6mm split/splined shaft. The problem is that it's a A50k log and I can't find a UK supplier with a split shaft. Any suggestions?

[ef37a]

https://www.esr.co.uk/components/resistors-18.htm

Possible?

Dave.

[Sam Spoons]

Brilliant, ordered, thanks Dave.

I spent about ¾ of an hour searching for one, fine of you live in the US but none of the UK suppliers that Google 'supplied' had the combination of split shaft and 50kΩ log taper.

[ef37a]

Brilliant, ordered, thanks Dave.

I spent about ¾ of an hour searching for one, fine of you live in the US but none of the UK suppliers that Google 'supplied' had the combination of split shaft and 50kΩ log taper.

Glad to be of service. I just searched for "splined shaft potentiometers" and that site came up PDQ. Just lucky I guess.

BTW even though they ARE in America, Stewmac get guitar related parts here extremely quickly.

Dave.

[Sam Spoons]

I'm a step further down the road in that the piezo with the preamp sounded pretty bad* so I've had another go with the passive, better result this time, it's a funny install, the saddle is about 8mm wide to allow good intonation (initially done that way as I used a plain third but also 'cos I had planned to install an Ovation bridge and electronics at some point). The piezo goes in a deeper slot at the front side of the bridge slot and I think I didn't have a thick enough shim underneath it so the saddle wasn't making full contact with the element. I have sorted that and installed it without a volume control for the moment. It sounds much better than the active version and better matches the level of my Rob Aylward 'Selmer style' and my mandolin (both also passive).

Now the problem, neither the RA or the Mandolin hum but the new pickup does quite badly. Since the setup is very similar, passive piezo UST** wired straight to a jack socket the only thing I can think might be causing it is the open frame jack socket or the piezo element itself.

I will try swapping the socket for an end pin, enclosed/shielded type tomorrow.

* also the fitted/included volume control doesn't work well at all, no difference until the last few mm before off.

** the Mando and RA pickups are both encapsulated in the floating bridge, a Fishman and a BigTone respectively.

[Murray B]

I don't know if this is related to your hum issue Sam, but some time ago I had a new bone bridge made for my acoustic. I discovered at the first gig I'd picked up a hum when it was amplified Replacing the bridge with the original plastic version made the problem go away. I never really worked out what was going on but assumed it was due to the bone not being as good an insulator as the plastic was?

I can't speak to the science of the situation and I'm sure that others will have been able to use a bone bridge without any trouble... Anyhow just thought I'd share it just in case it's relevant.

[Sam Spoons]

Thanks Murray, the bridge on the guitar is brass but the shims are plastic, I tried to find a brass shim I had for it a fair while ago and see if earthing the strings helped, if I can't find it I'll try wrapping them in kitchen foil to make the connection when I next take it apart (probably tomorrow). Touching the sleeve of the jack socket reduces the hum but doesn't remove it entirely.