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My situation is as follows:

I'm trying to convert a small guitar amplifier to dual power. Mains and battery. The power supply that comes in the guitar amp is a center tapped 120V to 14V supply. After doing some research online I found that I needed two 12V batteries connected in series with a center tap in order to make things work. I proceeded to wire this up by temporarily connecting the 3 wires from the batteries to where the power supply's 3 wires used to go and it worked fine. (I know it would probably be better if I bypassed the rectifier IC since I'm already 4V lower when running on the batteries, but it was nice being able to use the socket on the PCB where the original power supply connects)

The thing is I want to use a switch to alternate between the two power sources. I guess if I stuck a 3PDT switch in there I'd be able to switch out all 3 wires from each source and connection would never exist between them, BUT the problem is that the perfect size replacement rocker switch only comes in a DPDT instead of a 3PDT, so this brings me to my question:

Can I connect the center tap of the mains power supply and the center tap of my batteries to the PCB at the same time and use the DPDT switch to just switch between the two remaining wires of each power source?

Basically the center tap from both the batteries and the power supply would go straight to the guitar amp's PCB and the DPDT would switch between the +14V -14V of the power supply in one position and the +12V -12V of the batteries in the other position. Is this ok? I've attached a diagram of what I mentioned. Thanks for your time guys![wiring proposal]1

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  • \$\begingroup\$ No problem with a common 0V as both are floating so far. As long as switch can handle the big charge current. \$\endgroup\$ – Sunnyskyguy EE75 Jul 18 '18 at 4:52
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Without seeing the actual schematic, I'd say that there is not problem in doing that. One aspect I'd like to mention is that you probably don't want to switch the amp while it is on. Also, if there is no bleeder resistor for the filter capacitors (which discharge the caps when the amp is off), I'd add one. This way you can be moderately sure that your switch does not switch currents. Another potential problem is inrush current when you switch the amp on - again, without the schematic or at least a make/model, I don't know if the designers had taken any steps to limit this inrush current that can potentially overload the current capability of the switching contacts.

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