AUDIO WATTMETER

Here’s an easy way to measure an amplifier’s output power without trying to convert voltage to power measurements. Resistor R1 provides the load for your amplifier and should be rated at least twice the maximum amplifier power output; for example, if your amp puts out 25 watts, R1 should be rated at least 50 watts. The meter scale must be hand calibrated, and will take some time and effort, but once done it’s done for good. Remove the scale cover from meter M1 and borrow an AC variable auto-transformer, or connect a 1000 Hz signal generator to the amplifier output. Connect the output of the auto-transformer (or amplifier) to binding posts BP1 and BP2, and connect an AC voltmeter (VOM) across the binding posts. Set R2 to off – full counter-clockwise if correctly wired. Adjust the auto-transformer (or amplifier) output until the AC meter indicates 20 V rms – the voltage for 50 watts across 8 ohms. Adjust potentiometer R2 for a full scale indication on meter M1. Seal R2’s shaft with a drop of Glyptol or nail polish. Reduce the voltage across the binding posts in accordance with the table shown and mark the meter scale accordingly.

Parts List For An Audio Wattmeter

BP1, BP2 – Insulated binding posts

C1, C2 – 100 uF, 50 VDC

D1, D2, D3, D4 – Diode, HEP-134

M1 – 0.1 mA DC Meter

R1 – 8 ohm, 100 watt resistor, see text

R2 – 1500 ohm linear taper potentiometer