Hi there, big fan of my PS100 particularly for live use. Recently I’ve begun experimenting more with smaller amps that I love but don’t have the juice to hang live on their own. This has led me to wonder how I can establish a “safe” range so that I’m not accidently blowing speakers by goosing the volume of these smaller amps.
Specifically, I’ve got a vintage vox AC10. 10w, sounds great but not loud enough. I run it into an AC30 2x12 with vintage silver bells, I’d like to know where the danger zone comes into play.
Intuitively, having gigged an AC30 often, I know how loud that amp is and I don’t feel like I’d be cranking the AC10 up with the PS100 to ever be louder than that but… I’m not satisfied with that assumption. I’ve got a signal generator, I’ve got an oscilloscope, multimeter and an OX box for a load so I decided to test it.
What I want to know is is this the proper way to do this? Obviously playing a guitar is different than a constant 1khz sine wave pummeling the front end, so perhaps there’s a better way to do this and at least identify roughly where I should be safe and where the no go zone is.
I think you have done this fine; you are basing the AVERAGE POWER on RMS voltage and dividing it by the resistance, this is correct. On average you will dissipate that amount of power.
This is how amplifier and speaker power ratings are defined. If you are using this with a vintage speaker rated at 30W then you do need to be very careful, so I’d probably mark 30W “sine” position and another at 20W and run it around the 20W mark! Just to be safe.
The power is really a function of the area under the curve; it’s the amount of energy transferred to the speaker per unit time. It is waveform dependent. The way different waveforms are handled is by defining the crest factor but this works for periodic signals. Guitar signal is not really periodic. It will have a very high initial value during the pick attack and then a really rapid decay.
If you really want to get nerdy, this is what I would do:
Connect a 100W 8 Ohm power resistor to the amp output, send a Sine wave through it at 30W until the temperature is stable, and record the temperature and call it T1. Make sure that the signal generator output is in the mV range, like a guitar.
Record yourself using a looper pedal and repeat the above; don’t change the amp settings. Now measure the temperature of the resistor and call it T2.
Measure the ambient temperature too, call it Ta
The power dumped into the resistor by the looper pedal/guitar signal (i.e not Sine wave input) will be:
P_2 = P_1 \times \frac{T_2 - T_a}{T_1 - T_a}
(Note P_1 = 30W)
I am guessing this will be much less than 30W probably between 5W and 15W assuming:
Looper pedal output is the same RMS magnitude as the signal generator