Voltage reading from audio amplifier - Page 1
Forum section: electronics / testgear

author: Johnnydiaz
author_rank: Newbie
timestamp: March 21, 2014, 10:17:53 pm
content: Hey guys long time lurker first time poster. Love the forum and would like to thank all the pros am learning alot. I would like to measure the voltage coming out of my audio amplifier. I have a full range speaker which is 800 watts at 8ohms. That being the case I would like to know which meter I need to measure the voltage on the output stage of my amp. I have a fluke 289 but the reading aren't consistent. I have been recommended to use an hp 400e but wanted to get more feedback on this. I'm feeding the amp pink noise with no load.

author: tszaboo
author_rank: Super Contributor
timestamp: March 21, 2014, 10:48:11 pm
content: Try a 1Khz sine wave. That is consistent. fluke 289 is fine. Your DMM is true RMS, so it is very easy to determine the peak to peak value from that if you are looking for that.

author: Johnnydiaz
author_rank: Newbie
timestamp: March 21, 2014, 11:45:00 pm
content: Quote from: NANDBlog on March 21, 2014, 10:48:11 pmI could possibly use the 1khz for the mid-high box but what about the subwoofer which lives in the 35-100hz.Also I would like to get the rms value.

author: tszaboo
author_rank: Super Contributor
timestamp: March 22, 2014, 08:19:45 am
content: The multimeter reads the RMS value. And unless you have a active crossover, or an amplifier with really wrong characteristics, the RMS value will be the same for frequencies between 30-20000Hz.

author: peter.mitchell
author_rank: Super Contributor
timestamp: March 22, 2014, 08:57:20 am
content: you may also want to measure it into a purely resistive 8 ohm load, so you can tell when it starts clipping.

author: HKJ
author_rank: Super Contributor
timestamp: March 22, 2014, 12:23:32 pm
content: Quote from: Johnnydiaz on March 21, 2014, 11:45:00 pmThe rms value of what?Music is varying in amplitude all the time, that means the rms value will go up and down.To get a stable value you need a source with a stable signal. Usual a sinus is used, but that might easily damage your loudspeakers.With a loudspeaker on the output, you can measure the voltage, not the power. The impedance of loudspeakers will usual vary with frequency, making it impossible to calculate the power from the voltage.Note: Fluke 289 is a true rms multimeter, like it says on the front. It will always give rms reading on AC ranges.

author: commongrounder
author_rank: Frequent Contributor
timestamp: March 22, 2014, 03:27:16 pm
content: There is a procedure used by manufacturers and servicers to accurately measure amplifier output power. You need to hook the amp up to as stiff a power source as possible (DC for car, or AC outlet). The amp output needs to be fed into a standard (non-inductive) load resistor that can handle (at least for short periods) the full anticipated output of the amp. This would probably be 8 ohms for home audio amps, and either 4 or 2 ohms for car and PA amps. You would connect your meter in AC volts RMS mode across the amp speaker terminals. An oscilloscope is also connected across the amp terminals, to detect the onset of clipping (beware how the scope is grounded relative to the power supply for the amp, if the amp is in bridging mode. A bridged amp is hot on both terminals and will fail if either of the terminals is grounded to the chassis ground of the amp. I use a differential probe in my shop and an isolated input scope in the field). You then connect an audio signal generator (can also be a CD player or computer with clean sine tones as files or tracks) with a range of audio frequencies of interest, typically 20hz to 20khz. With all of this set up, you can fire it up and increase the signal drive until the amp reaches the clipping point as shown on the scope display. Take your voltage reading. Do this across the frequency range and you can plot the maximum power output by doing the math with the RMS voltage rating and load resistance (example: 22.5volts across 8 ohms = 63.28 watts. Ohms law, Volts squared divided by Resistance equals Watts). Pink noise is useable for equalization tasks, but not for power measurements.Best of Luck!