Add a switch to allow for remote or battery power and battery charging.

[Dolphran]

I want to describe an easy addition to preamp/battery wiring that allows for remote power or battery power for the guitar, recharging of an Li-Ion 9V that may be tucked inside the guitar, and also gives an easy on off switch.

We'll use a small DP3T Slide Switch that can be mounted for easy access from the sound hole. ( E.g. https://www.amazon.com/gp/product/B07H3RPD23 )

The Switch connections look like this:

              * P+ (PVcc & BatPos+)       (Charging Position)
           /  |
Ring    *  -  * P+                        (Remote Power Position)
           \
              * n/c                       (TS cable Position [Battery powered])


              * Tout (Tip & Preamp Out)   (Charging Position)
           /
BatNeg- *  -  * n/c                       (Remote Power Position)
           \
              * Gnd                       (TS cable Position [Battery powered])

Tip and Preamp Output are always tied together (I'll call this Tout)
Preamp Vcc (input power) and BatPos+ (Battery Positive) are always tied together (I'll call This P+)
Sleeve is always attached to Preamp and Pickup Ground (Gnd)
Battery Negative is switched. It is not always connected to Gnd.

The three switch positions provide the following connections:

1) Charging Position. Ring goes to P+. Tout goes to BatNeg- (BatNeg- does not connect to Gnd), Charger goes between Ring(+) and Tip(-) [Charger does NOT connect to Sleeve!]. Sleeve/Gnd is floating. If a regular TS (mono) guitar cable is (accidently) plugged in while the slide switch is in the Charging Position, B+ is grounded but B- is on Preamp Out (battery is not shorted). Unpowered preamp sees -9V on its output. Not ideal, but likely survivable.
If a Powered (9V on ring) cable is (accidently) plugged in while in the Charging Position, the preamp will be powered by ring and the battery will be connected between P+ and the Preamp out. No idea how any given preamp might react to this. Could be bad. It is advsed to make sure you're not in the charging position unless pluggin in a charging cable (Charger+ on Ring, Charger- on Tip, and Sleeve n/c).

2) Remote Power Position. Ring goes to P+, BatNeg- is not connected (Ring powers preamp, battery is diconnected) This is the normal external power mode for the guitar using a TRS cable with 9V on Ring). The beauty of this mode is you can keep your guitar plugged in all the time (as I do) and never worry about killing you battery. The Preamp powers down when your floor board powers down. If a regular TS (mono) guitar cable is accidently plugged in while the slide switch is in the Remote Position, then the Preamp is just off. No sound, and no harm.

3) TS cable Position. B- goes to Gnd, Ring is n/c (Ring grounded by TS plug) This is the battery power mode for the guitar for using a standard TS (mono) guitar cable. Ring is not connected in this mode, so if a Powered (9V on ring) cable is (accidently) plugged in while in this mode, the preamp will still draw from the battery only.

When battery powered, switching from position 3 to position 2 acts as an on/off switch for the battery even with a guitar cord plugged in.

I'd recommend something like the Fishman Switchjack as the TRS endpin jack https://www.stewmac.com/electronics/...k-endpin-jack/
It provides a "Switch" connection so you still get the unplugged-is-off functionality.