Re: speaker patch bay question

Posted by reddesert on Jul 12, 2018; 10:40pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/speaker-patch-bay-question-tp43433p43442.html

Let's suppose your peak signal is 100 W into an 8 ohm speaker.  Power is P = V^2 / R = I^2 * R.

Solving for V and I, we find that V = 28.3 volts and I = 3.5 amps.  (This is consistent with Ohm's law: I = V/R.) This gives you an idea of what we are up against, since the output voltage isn't incredibly high, but the output current is really large.

For example, a normal Alpha rotary switch like this: https://www.mouser.com/search/refine.aspx?Ntk=P_MarCom&Ntt=146305454 is rated to 125 VAC and 0.3 amps. So it would handle the voltage just fine but the current is way over the switch rating.

In fact, the power for 0.3 amps into an 8 ohm speaker is just 0.7 Watts! Realistically the switch rating is probably conservative, and amps are rarely run at maximum power, so you might not fry the switch with a 1 watt amp, but you could probably fry it with a 100 W amp even if it was only turned up to 2 or 3.

You could look for a rotary switch that's rated at 3-4 amps or better. Mouser seems to have some that only cost several bucks each (and some that are really expensive). I would buy a non-shorting, break-before-make switch for the amp outputs, so that you never connect an output to another output.

Another simple solution could be a regular patch bay: run the inputs / outputs to a box that has a series of 1/4" jacks on the front, and to use a short section of speaker cable with 1/4" plugs to manually patch the desired amp to speaker.  You have to be careful to not patch an output to an output, of course.