Receiver Output Measurment

Discussion in 'GENERAL AV Discussions' started by ahdeeoh, Jun 30, 2005.

  1. ahdeeoh Enthusiast

    ahdeeoh
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    Does anyone know of a cheap way to measure (in watts) the output from a receivers speaker outputs using a constant 8 ohm load (not speakers). Hopefully with a decent amount of accuracy.
  2. Buckeyefan 1 Audioholic Ninja

    Buckeyefan 1
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    A huge 8 ohm resistor and a voltmeter - $8 at ratshack. You can get a decent 8 ohm resistor for under 15 bucks. Test each one at a time.

    If the meter reads 14.4 volts, square it, then divide by 8. That will give you the rms wattage. Don't clip your unit.

    Why are you asking?
  3. MacManNM Banned

    MacManNM
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    The above will work but you must not clip the amp. As soon as it goes into a clipping condition the measurement is inaccurate. You really need more sophisticated equipment to do anything that is worthwhile. What are you trying to figure out?
  4. ahdeeoh Enthusiast

    ahdeeoh
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    Receiver Output Measurement

    Trying to relate volume display on receiver to actual wattage put out. Yes I am aware that as speaker impedence changes with frequency so does output. Figure that given a specific, continuous impedence output should be as manufacturer states. Plus, why does an audiophile try to figure anything like this out, I'm obsessed.
  5. Francious70 Senior Audioholic

    Francious70
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    You don't need a resistor.

    Use a volt-meter and measure the max AC voltage at the speaker terminal.

    V^2/R

    Max voltage squared divided by the resistance (8 ohms)

    Paul
  6. gene Audioholics Master Chief Administrator

    gene
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    I thought I'd pop in since there were some interesting suggestions here, although not all were very accurate.

    You cannot simply measure the output at the speaker terminals unloaded. This will get you close to the rail voltage, but not any indication of true power rating.

    The very minimum you will need to measure unclipped power is as follows:
    1) 8 ohm non inductive power resistor
    2) Oscope
    3) Signal Generator

    Adjust the signal generator to around 1Vrms and set the frequency to 1kHz. Attach the resistor to the speaker terminal under test. Then increase the master volume level until you start seeing clipping on the oscope. Turn it down slightly until you no longer see clipping.

    Take the RMS Voltage reading of this waveform and turn off the source.

    Do simple math: V^2 / R

    Repeat at 1/3 octave intervals or at least 20Hz and 20kHz.

    ***It is recommended to monitor the line voltage with an RMS meter and make sure it doesnt sag. It should stay within +-10Vrms of 120V nominal.

    For more ellaborate tests given this test equipment, see our article:

    Basic Amplifier Measurement Techniques
    gene,
  7. Buckeyefan 1 Audioholic Ninja

    Buckeyefan 1
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    Volume display has little to do with output. The only time you may experience wattage at near max mfg. specs would be during extreme peaks/bursts with HT or some classical music.

    Older Pioneer receivers used to have V.U. meters that showed power output. I can remember playing AC/DC at insane levels with some old AAL speakers, and the V.U. meters would "bounce" from 25 to 35 watts. These "bounces" are extremely short periods of time. If you were to play white noise, or a full frequency bandwidth at 25-35 watts, you'd probably fry your speakers, as well as your ears.

    IMO, the more important parameter would be the unit's noise floor, or the dead period between notes. Near zero distortion between bursts/notes/peaks... really brings out the quality sound.
  8. ahdeeoh Enthusiast

    ahdeeoh
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    I do believe you guys are missing the point. I do not wish to find the absolute power a receiver will put out. What I want to do is equate the arbitrary volume number displayed with an output wattage. Perhaps having access to a sound pressure meter and the manufacturers data fo speakers it would be easier to determine this by sound pressure. I have seen many posts asking " What does the volume number on my receiver represent?". It would be great to find a way to equate any receivers volume display to it's output wattage (given a standard 8 ohm load). Maybe there are to many variables (receiver specs, speaker impedence etc.) to accuratley predict this.
  9. gene Audioholics Master Chief Administrator

    gene
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    you need a power meter to do this or you can stick an Oscope in parallel to your speaker load and guesstimate an average impedance, and calculate the power.

    This is of course a somewhat nebulous goal, since music is very peaky in nature and source material varies in loudness even at a fixed volume level.
    gene,
  10. MDS Audioholic Spartan

    MDS
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    I think the real question ahdeeoh is asking is something that I have always wondered as well so let me ask it my way and maybe Gene can solve the mystery once and for all.

    Some receivers automatically increase the level to its pre-determined 'reference' level when you use the test tones for calibration so you don't have to manually choose a point on the volume display (by convention you would choose 0dB). Given that the term reference level is meaningless until you actually calibrate and equate an output SPL with your chosen position on the volume dial, there has to be some electrical parameter they use for determining this point.

    So what does that point signify electrically? It can't be max power, because the volume knob can be increased beyond that point and I would assume the receiver manufacturer leaves a little headroom. Is it the point at which the distortion equals the rated distortion in the spec sheet or the point at which the receiver's output voltage is 80% (or some other arbitrary percentage) of its rail voltage?
    MDS,
  11. ahdeeoh Enthusiast

    ahdeeoh
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    Receiver Output

    Lets put it this way. Given an 8 ohm loudspeaker with a power handling capability of 60 watts RMS and a receiver rated 100 W per channel, min. RMS @ 8 ohms - 40 Hz to 20 kHz @ 0.8 total harmonic distortion and volume numbers 0-50 can you equate a volume number to 60 watt RMS output from the receiver. Therefore, equating other volume numbers to there output. If there is a way to do this, people would be able to make sense out of the volume display on their receiver. Obviously, given any speaker-receiver combination and a sound pressure meter you could determine SPL at you're listening position and tie that to a number on the receiver. This however would not tell you the power output of the receiver at that volume display.
  12. Buckeyefan 1 Audioholic Ninja

    Buckeyefan 1
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    The biggest variable is the speaker. Not all 8 ohm speakers are created equal. Amp mfg.'s have no clue what speakers will be running off their amp, nor do they know what equipment, mic or meter we're using to state this "rms" output power. Unless you are listening to white noise, there is no reason on earth to equate volume number to rms peak output. Your average rms on music may be 14 watts, with peaks to 60 listening to Phil Collins, and 25 watts average rms with peaks to 30 listening to Bach. There's no logic in it.
  13. ahdeeoh Enthusiast

    ahdeeoh
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    Thanks for the suggestions on how to do this but I have decided that I want more information about what is going on with the amps in receivers. After reading the post in this thread that said old Pioneer receivers had V.U. meters I got thinking. Most pro gear has V.U. meters on amps to give a visual indication of amp condition including clipping. Even Windows allows you to monitor system resources. As most new H.T. receivers are now just dedicated mini computers, why can't manufacturers provide us with a visual of amp conditions. I'm sure that most audiophiles would prefer processing power being dedicated to this instead of hall, theater, wide open field modes. It would be nice to have a volume indicator on one side of the volume knob with a percentage of amp power used indicator on the other. This way we could pick our reference volume level of 0db and watch as our amp cruised along at 15 watts and clipped on peeks. Perhaps the guys at audioholics could put some pressure on manufacturers for this feature. The first one to put this feature in a receiver might sell more product to those of us who are a little more analy retentive about our gear.
  14. Buckeyefan 1 Audioholic Ninja

    Buckeyefan 1
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  15. ahdeeoh Enthusiast

    ahdeeoh
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    What I'm looking for is a simple, yet accurate way of knowing a receivers power output with any source material. I believe my problem is wanting SIMPLE and ACCURATE as even manufacturers can't agree on how to measure power output.

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