Impedance and switching between series and parallel

I have two main combos that I use; my self-built SE-5a in a 2×10″ combo and my Fender Twin with the standard 2×12″.

I’ve got in mind modifications to both so that I can feed the SE-5a through the Twin’s 2×12″ and vice versa. The problem with this is that of matching impedance. The Fender Twin requires an impedance of 4 ohms whereas the SE-5A requires either 8 or 16 ohms.

All of the speakers in question are 8 ohm. For the SE-5a they’re wired in series for a combined impedance of 16 ohm and for the Twin they’re wired in parallel to give 4 ohms.

I needed to be able to switch impedance. After a few minutes of googling I couldn’t find what I was after so I sat down, worked out how to do it and thought it may be worth sharing for others in the same predicament.

With a DPDT (double pole double throw) switch I can make a pair of speakers switchable between series and parallel. With 8 ohm speakers this results in each pair being switchable between 4 and 16 ohms. Here’s the schematic.

Impedance Switching

Please note: I haven’t tested this yet. Once I’ve confirmed this is working I’ll report back but, for now, please treat this with scepticism.