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External Battery for the Variax 300/500/700

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I have a battery powered Busking rig and I'd like to add one of my Variax guitars to the rig. 

I'm looking at using a battery pack that has 9VDC and 12VDC outputs.
I think I can use the 9VDC output to power the Variax. 
My idea is to make a box with a TSR connector to go to the variax with these connections:
Sleeve - Ground(Power and Guitar Signal)
Ring - 9VDC
Tip - Guitar signal

A TS connector for the output of the circuit
Sleeve - Guitar Signal Ground
Tip - Guitar Signal

A connector for the 9V Battery pack connected to both grounds and the ring of the TRS connector for the guitar.

Has anyone done this before? I'd rather check the feasibility before I just go for it. I'd hate to jack up one of my Variax guitars.

I'm attaching a schematic I worked up

Thanks for any assistance!!

Dave Nyberg

Variax power block.jpg

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I've done some similar experiments, powering a Variax 600 from a 5-volt power bank with an adjustable DC buck that gives me the correct supply voltage. In my case, the DC buck has a 9V battery clip that plugs into the guitar replacing the 6-AA cell battery pack. Works just fine.


Your solution is conceptionally-similar, simulating power on the ring of a TRS plug on an XPS footswitch. I have one concern, which may or may not represent an actual problem. Psarkissian may weigh-in with an official position,  which would  probably be a 'Don't do it', but here's my concern:


When you connect the TRS plug from your Variax to J1, the tip - which carries the output signal from the guitar electronics - will be briefly connected to the 9V battery voltage from J3 as the plug is inserted into the jack. The output electronics of the Variax may be such that this isn't a problem. For example, if it's a capacitor-coupled connection, the brief DC voltage connection to this pin would be blocked. An output buffer could also provide similar protection. The bottom line is, I don't know what's connected on the Variax innards to the Tip of the TRS plug, so I'd be careful. 


If I were to proceed with a connection like this, I would assure that the 1/4" TRS guitar connector to the Variax was fully inserted before any connection was made to the battery at J3. 


Just one guy's opinion...



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