Resistance Is Futile

Marshall Amp & homemade speaker cabinet

Younger son swapped his Fender amp for a Marshall, but he only got the head (the amplifier part), not the body (the speakers), so he took the money he got from selling his textbooks back to the bookstore (sneaky devil) and bought a sheet of plywood and a couple of Jensen speakers, and then I assisted him in putting together a speaker cabinet. If I am standing there watching him and telling him what to do, he does pretty well, but left alone with his friends he starts racing ahead and taking shortcuts, which compromises the quality of the construction. You can see how the speaker holes are not perfectly round. However, it works, and according to my son, it works well, much better than the old amp.

The output connections from the amplifier are the same as for the inputs: quarter inch phone jacks. We were originally just going to use a guitar cord to connect the speakers, but then I realized there is some actual power in the output, and the skinny wires in a guitar cord would not carry the load. I had some speaker wire that was pretty hefty, so we used it instead.

I got to thinking about this later and started wondering just how much current we were dealing with, so I did a little calculating. The speakers are rated at 50 watts each and have an Resistance of 8 ohms each. They are connected in parallel, so the resistance of the pair should be 4 ohms and the total power capacity should be 100 Watts. So how much electricity is flowing in these speaker wires? We start with a couple of basic formulas:

Watts = Volts x Amps

Volts = Amps x Resistance

Put 100 Watts in the first formula,
and then solve for Amps gives us:               Amps = 100 / Volts.
Substituting that and Resistance = 4
into the second formula gives us:                 Volts = (100 / Volts) x 4
Rearranging and combining terms to
make it pretty gives us:                               V = 400 / V
Multiplying both sides by V gives us:          V^2 = 400
Take the square root of both sides
and we end up with:                                   V = 20
So we have 20 Volts and 100 Watts.
Put those back in the first formula
(Watts = Volts x Amps) and we have:      100 = 20 x Amps
Divide both sides by 20 and we have:           5 = Amps

So we are using 20 Volts of electricity to push 5 Amps of current through the circuit to deliver 100 Watts of Power. This is not enough current to run a toaster or a microwave, but it is enough to run five 100 Watt light bulbs, so the guitar cord, which only needs to carry a signal, which has almost no power, would have been a poor choice to use to connect the speakers.

Update October 2016 replace missing picture.