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.

The differences between guitar and speaker cables

As part of the build of my amp I was “forced” to investigate and understand the differences between a normal guitar lead (connecting instrument to amp) and a speaker lead (connecting amp to speaker). I suspect that, like many guitarists, I’d just use whatever came to hand, which is usually a guitar lead.

I think most of the confusion is caused by the fact that they both use the same connector – the ubiquitous 1/4″ mono jack plug/socket. If from the start different connectors had been used then it just wouldn’t be an issue.

At the heart of it is that the signal between guitar and amp is very different to that between amp and speaker.

The signal to the amp is a very low current and voltage when compared to the signal from the amp.

The former is susceptible to the noise of external interference (mobile phones and, no doubt, your local taxi company). Whereas the “pumped up” signal leaving the amp has no such problem. The shielding to reduce noise can increase the capacitance of the cable which, in turn, will reduce the treble being passed through.

Cable resistance is nowhere near as significant when dealing with the low power signal from the guitar. When you start pumping more through then this becomes significant and can lead to cable cores heating up and, in extremis, melting.

To summarise, the guitar cable needs to be well shielded to protect against noise and low capacitance to protect against treble loss. The speaker lead needs to be low resistance to cope with the extra power passing through.

Bottom line is that you do need different cables for these different jobs and you should understand why. For more reading, have a look at this article.