I'm building an Es-12q hollowbody 12 string and I've designed a flexible wiring scheme that I'd like a sense check from the group on.
I'm aiming for:
Three position selector
Coil splitting on the bridge and neck pickups with push/pulls
A passive piezo bridge with an on/off using a push pull
A master tone
I've attached a schematic of the way I think it should be wired.
One drawing omission to start with, as the piezo volume pot back need its own ground connection. You are grounding the bridge post from the pot, but the pot rear itself lacks a ground.
As it stands, the piezo pot is switched and connected to the output of the 3-way switch, so the piezo signal can be quickly brought into play at the volume set on the volume knob. You could of course simplify this by doing without the switch and just using the volume knob, but that's 100% up to you. The piezo volume pot is wired so that it doesn't kill the signal from the magnetic pickups if set to zero, which is good.
However a few points.
1) I don't know what value pot you plan to use for the piezo bridge volume, but the way it's wired, it will always be in parallel with the humbucker pots. So assuming that you have 500k pots all round, for single humbucker operation, it will seem as if you have 250k pots fitted, and loose some treble. You always get this on a normal two humbucker guitar when two pickups are selected, but the brighter parallel sound compensates for the treble loss. So one option here is to use 1meg pots all round (recommended for passive piezo volume anyway). Or you could switch the output from the piezo pot rather than the input to it, so that the pot is only connected when the piezo signal is wanted, which will help compensate for the loss of treble.
2) Passive piezo elements need to run into at least a 5 meg or so (10 meg is the norm) input impedance in order to give a fairly flat and balanced response. Plugged into the 1Meg input of an electric guitar amp, you'll get a very mid-forward, muddy sound with not a lot of treble. You'll often find that passive piezos in this sort of circuit are wired to a TRS socket, so that a split lead can be used to feed the electric side to an electric amp and the acoustic side to an acoustic amp with a very high input impedance, or an external acoustic pre-amp before being fed to the PA or another amp. I believe you can get special switched TRS sockets that can detect whether a TS or TRS jack is inserted, and so be wired to mix the two signals for a TS jack, or keep them separate for a TRS jack. Alternatively, you could simply wire the piezo to a separate dedicated output jack to keep things very simple.
3) Depending on the size of piezo crystal used, the output from the piezo could be anywhere from a lot weaker than the magnetic pickups to a lot stronger, so that any balance with all pots set at 10 could be way out. It will be a pain if you can only turn the piezo volume up to 2 without drowning the humbuckers, or vice versa. A loud piezo is easier to deal with as you can wire a resistor on the input side of the volume pot and wire the piezo signal to that, in order to limit the maximum signal volume. You won't know what to fit though until you can compare the outputs.
A better alternative is to fit a proper designed for piezo preamp in the guitar that has both a maximum output level control and a 0-10 vol control. You can then match the output level with the humbuckers, have full range control over the piezo volume and the piezo signal will sound as natural as possible (well as natural as a piezo can)because the (correct) preamp will have that 10meg input impedance. It will still sound less than stellar through a standard guitar amp though, so some form of split output is recommended os you can fed an acoustic amp or a full-frequency range PA speaker. I know Fishman do their 'Powerchip Onboard' volume control and also do a 'smart switch' jack, but it's not cheap. Other preamps of various sorts are available at lower cost though. I know a bit about these things (which is pretty much listed above), but I have never installed this type of mixed system before, so can't give any better guidance.
And if the piezo output is significantly less than the humbucker output, then split outputs or an onboard preamp are the only real solutions. A piezo preamp output level will normally conform to a strong passive pickup signal level. It normally won't be significantly louder or quieter.
Simon, thanks! I figured you’d be able to supply helpful information.
Good catch on the ground for the piezo pot. That was just an oversight - I thought I’d put it in.
It’s 500k pots all around. I’ve tested the piezo tuneomatic bridge on another guitar (without using a pot) and the output is pretty hot. It’s actually six crystals - one in each saddle.
How would the wiring be done to switch the output on the piezo push/pull instead of the input? I think that makes more sense.
Where the green wire from the piezo bridge goes to the switch and then the tab on the pot, that green wire goes direct to the tab. The blue wire from the middle tab goes to where the green piezo bridge wire used to go on the switch, and the centre output from the switch then runs off to the 3-way selector switch centre tab.
Almost. Correct wires moved, but you're using two different sets of poles on the switch, so they'll never make contact. Either move one wire up or the other done so they are both on the same line of 3 contacts, or else link them across the two sets of poles for redundancy (as you've done on the volume pot switches) and you'll be fine.
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