I have an electric guitar fitted with a Fernandes Sustainer system. It's powered by an onboard 9 volt battery, and I would like to power it from a DC converter. I thought of changing the instrument 1/4in mono jack output to a stereo one, and making up a "stereo" lead to take the signal from the guitar using tip (of plug) as positive, and the sleeve for negative. The positive feed from the DC converter would use the ring, with the negative sharing the sleeve. Obviously, at the amplifier end, I would have another stereo socket to split the signal from the DC voltage.
Please excuse my extreme ignorance, but would this work, or would the shared sleeve cause problems?

 

Assuming the sustainer is fully floating or has its negative supply connected to local ground, then in principle you should be OK.

Make sure you have separate connections for the pickups/amp and the positive supply (we don't want you frying the pickups - though I doubt that would happen anyway). You'll need to make sure the 9V supply is "clean", because there will be capacitive coupling to the signal path. Even if it is clean, make sure you have a big decoupling capacitor across the supply - possibly connecting to the supply via a resistor (of value sufficiently low that there isn't significant drop due to load current).

Then the only other concern is the effect of capacitance on the signal. You'll be increasing the capacitive load on the pickups, and this could affect the sound (tending to make it sound "thicker").

Good luck, and make sure you don't damage your guitar!

 

How long will this cable be. The longer cable the higher risk for picking up noise. Very often in signal cables the shield is only grounded in one end.
I think you will experience more noise and somewhat degraded sound with your setup. I think noise pickup and sound quality is the reason why they selected to use a battery. And not some external power. But feel free to try by all means. Before your start. Check out how the cable structure for your guitar cable looks like. I think they have used a shielded twisted pair type cable. The twisted pair are used for signal. And the shield is connected to ground in one end of cable.

 

Thankyou both for your thoughts. I think I'll carefully build it, and it if affects the sound, I'll revert to batteries!

 

Another issue concerns plugging in live. If you insert or remove the jack with the amplifier running, there are likely to be more and louder crackles and bangs than if the plug were carrying only the sound signals. It is possible that these noises might even be sufficient to cause damage.

Any wear on the sleeve contact leading to raised and variable contact resistance could also generate noise under normal operation.