Multimeter to test power output at speaker terminals?

Discussion in 'Solid State' started by rushfan, Sep 16, 2007.

  1. rushfan

    rushfan Super Member

    Hi all,

    Is there a fairly simple method for using a multimeter to test the power output of a receiver or amplifier? I am hoping to discover a method that is as easy as checking DC offset.

    The output meters of my SX-880 display unevenly most of the time - the left channel is almost always the "louder" channel, as determined by the meters. I might be imagining things, but the left channel seems to have more presence. The difference is subtle and my eyes might be tricking my ears.

    I know that I could switch speakers, swap cables and do other kinds of tests, but I would like some kind of empirical evidence that a problem does or does not exist.

    Would playing a steady mono signal and measuring the DC voltage at the speaker terminals give me the answer that I need? I am just guessing at this point.

    EDIT: It looks like the idea above doesn't work. I played back a -20 dB 1 kHz test file and the multimeter hardly registered a thing. I had it on "VDC" and "200m". Was I doing something wrong? I could hear it loud and clear through the speakers. As I expected, the left channel seemed to be getting a little more power. If I am interpeting the meter correctly, when the left channel is outputting 0.1 watts, the right channel is putting out around 0.08 or 0.09. That was with the mode switch in mono (to negate uneven output from my PC's sound card, an Audiophile 2496).

    Thanks for your help!
  2. MunkeyQ

    MunkeyQ Super Member

    Wattage is measured into a load - and the multimeter doesn't present a load to the amplifier. I'm not sure if there's a way to do it easily...

    Maybe one of your speakers has a slightly lower impedance due to some crossover component drifting, so the amp puts more power into it. Hence why the meter indicates a higher wattage.
  3. Can't be done. You need a 8 ohm load and the ability to to measure AC rms voltage. A scope can be used with some math to get the power. Others will tell more,
  4. rushfan

    rushfan Super Member

    Interesting idea. I just recapped the crossovers a couple of days ago but this phenomena existed long before that.

    I should probably have a pro look at the receiver. In the meantime, maybe I could cheat a little by giving the right channel a little boost from the EQ, or by simply tweaking the balance knob a little.
  5. justw

    justw Well-Known Member

    Play a test tone, 1 kHz is standard, into an 8Ω load, or speakers. If you use speakers don't play so loudly that it damages your speakers. Set your voltmeter for AC Volts. Measure the volts across the speaker terminals. Power is: E²/R where E = volts RMS and R is resistance of load. Both channels should be the same at a given setting of the volume pot. Note: volume pots can tend to not track perfectly but small differences should be inaudible.

    You can pick up a couple of 10 watt resistors at the Shack. You can generate a test tone on your computer, or have someone do it, and burn to a CD.
  6. MOPAR3

    MOPAR3 Super Member

    Hi MunkeyQ to measure the output you read it as an AC signal. I have a meter that will measure and store the peaks and valleys so I can see the min and max of the signal. I also have a problem with the left channel being louder and it has followed through 3 systems and like 8 sets of speakers. My problem is not equipment its the room and perhaps how many recordings are mastered. It's driving me nuts and I hate to shift the balance because I don't want to run one channel harder than the other. When I was running my AV receiver was able to compensate for this with digital delay.
  7. rushfan

    rushfan Super Member

    I am going to give this a try. Thanks for the suggestion.
  8. MunkeyQ

    MunkeyQ Super Member

    Ah, interesting method there - thanks. I'll bear it in mind, I didn't think it was possible to do it easily.:D
  9. DSChamber

    DSChamber Well-Known Member

    Interesting about the LEFT channel "seemingly" getting more power than the RIGHT channel.

    I got the same dilemma with my Akai AM-2600... looking at the power meters I would say that the Left channel meter "bounces" more freely and hits peaks higher than the Right channel. Audibly I don't know if there is a difference, hard to say for sure...

    I checked all the voltages on the board and they are nearly mirror images of each Left and Right when idling (ie: powered up, no source input) but that could change once an input is applied and music is playing. Maybe one Power Transistor was breaking down during heavy load...

    Then I stumbled across something that I would guess consider "emperical" data.. lol... one evening I was in the basement messing around with the Akai, and I had the baby monitor with me... wife was gone out and the baby was asleep (3 floors up) ... I turned the sound up and I saw that the baby monitors' 1st light lite up... so I turned it down as I didn't want to wake the baby... then I thought "Hummm.... lets see what happens when I turn the balance all the way to the LEFT"... did that and turned the volume up until the 1st light on the monitor lite up. Then keeping the volume level the same on the amp I turned the balance all the way to the RIGHT.

    Sure enough when it was all to the LEFT I got a full "1 light", all the way to the RIGHT and I wasn't, it would partially light it up..

    LOL... I think I found a "poor man's" SPL meter... hahahaha

    Anyway..... I think I found the root of my problem... my Akai has a Tone Control - ON/OFF switch. Lots of vintage amps/receivers have this function. With it on you have control of the BASS and TREBLE.. with it off I guess the BASS and TREBLE get set to a "middle value"??

    I found that I could affect the Right channel output by flicking this switch a few times... turn it "on" and my power would drop, flick it "on/off/on/off" a few times and my power would return both by my ear and the meters... there is a "crack" sound when I flick the switch as well so I will be taking off the front panel and giving it a good cleaning...

    After looking at the schematic I can definitely see how this Tone Control could affect my power outputs to both channels.
  10. rushfan

    rushfan Super Member

    What about measuring the output from my components via the RCA cables? I guess that a hotter signal from the left side would account for this imbalance, though switching to mono output should negate this, should it not? Can I take a useful reading from the ends of the interconnects? If yes, is the tip positive and the ring negative?
  11. not if you're looking for POWER OUT. You must meas. at the spkr terminals.
    Yes the pin/tip is the signal.

    JustW, how do you generate a tone from a PC. Don't you need special software?
  12. ruesselschorf

    ruesselschorf Well-Known Member

    If you want to check volume tracking of balance and Volume pot, and don't have the equipment to make accurate measurements:
    connect ONE of your speakers as follows: + wire to red terminal LEFT and the - wire to the RED terminal of the RIGHT channel.
    Play a mono signal (a AM station from the tuner is mono) if you turn balance to the left or right volume will increase, in the exact center position, volume should be almost zero! when you play around with the volume pot, you may find that volume tracking is not at all perfect!
    Loudness and tone controls shoould be in the off/center postion.

  13. rushfan

    rushfan Super Member

    Wow - that sounds like an interesting thing to try.

    Not that I don't trust you, but I think that I'll use one of my Minimus-7's for that test! :para:

    Assuming that I find something wrong, is it correctable (like DC offset) by adjustment versus replacement/repair?

  14. whoaru99

    whoaru99 Part-time Iconoclast Subscriber

    Aren't the (-) terminals on many amps/receivers common to the left and right channels?
  15. MunkeyQ

    MunkeyQ Super Member

    In most of the amps I've seen inside the grounds are common for both channels. I don't see how his method could work - is there something I've missed?

    Oldhifiguy, you can indeed get software - there's loads out there. NCH's tone generator is a nice simple one.
  16. rushfan

    rushfan Super Member

    I'm not looking to measure the power output of the amplifier - it's the output voltage of individual components that I am after. If I discover that my sound card or CD player is for example, feeding 150 mV to the left channel and only 140 mV to the right then that might help to explain what's happening here.

    Also, the service manual for the SX-880 describes using a 150 mV 1 kHz signal to calibrate the output meters. If, after all is said and done, I learn that it is the meters themselves that are out of whack, I would like to be able to adjust them. It would be ideal to use the gain control for my sound card to generate that 150 mV signal since I have downloaded a 1 kHz test file for troubleshooting.
  17. whoaru99

    whoaru99 Part-time Iconoclast Subscriber

    Well, maybe I misread it. I would've sworn the original post said to connect from the + of one channel to the - of the other, but now it says to connect across the two + terminals...
  18. bastek

    bastek Well-Known Member

    Is a "True RMS" multimeter better for measuring power? Standard meters won't accurately measure 1khz AC, they are only rated at 60hz, no?
  19. ruesselschorf

    ruesselschorf Well-Known Member

    Yes, for my 'balance test', connect ONE speaker across the RED (+) terminals of left and right channel, leave the black (-) terminals untouched.

  20. Damage

    Damage Super Member

    I don't believe that to be a correct statement.

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