# measuring watts

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#### sunseeker

How does one measure the watts that a speaker is drawing from an amp in real time? What type of test equipment does one need? Thanks

#### seanzozo

If you know the minimum impedance of your speakers you can easily get an rough upper limit on peak power using a scope to determine max voltage.

then power = v^2/R

(I just confirmed yesterday that I rarely use more than 2 watts peak, Ive got 12db of headroom for my headroom )

Sean

#### Ron E

Maybe you should explain why you want to do this. There is very little reason to do this and it could cause you to ruin either the amp or the speaker or your test equipment. Most amps are not made to run at continuous full power output for extended periods and will get really hot even if run at one third power for any length of time.

Well, to measure actual amplifier power output, you take a true RMS voltmeter and put it in parallel with the speaker, and a true RMS ammeter and put it in series. Then you measure both current and voltage and multiply them. Note that very few ammeters have negligible resistance and almost none of them will read much over 10 Amperes.

If you want to compare amps, a much safer way to do it is with an appropriately sized dummy load. I think you will find that measuring amplifier output voltage is much more simple. Then you can estimate the output power as Power = volts^2/dummy load resistance.....

#### sunseeker

I currently have a pair of Acoustic Research AR1's that are 95db at 8ohms. The speakers have a pair of 500watt amps driving the 15" woofers. I would like to know how much power the speakers are drawing from my current ss 75 watt amp when spl is 85db at 1m. I want to go to set amps but where i live there are no options to try different ones. If I know what I need in watts it would make it easier to make a choice and a place to start.

#### sunseeker

Here is the statts on the speakers.

3 way sealed
18hz - 23khz
crossover at 100hz and 3.2khz
95db @ 2.83 volts rms and 1m
500 watt sunfire amps driving the 15" woofers

Any thoughts on a SET for this speaker? I currently use a BC 21 for the pre. Thanks

#### seanzozo

Ron-

I don't think sunseeker wants to know the max power his amps can generate but wants to know the max power that they ARE generating under normal use. For that he wants ballpark figure and so can estimate the R and use V from either a RMS Voltmeter or an oscilliscope. Its good enough to help determine wheter he can go low power SET.

Sunseeker-

The 95dB doesnt mean a whole lot without reference to the impedence of the speaker. If the impedence doesn't dip much below 8 ohms and is above 8 in the bass region then you should be able to go with a low-power (probably not flea power) amp. But lots of that depends on your listening preferences.

Sean

#### sunseeker

seanzozo thats what I am looking for. I am tring to find out what the lowest power I can get away with. I unfortunatly listen to a very wide selection from pipe organ to soft jazz and some Pinetop Perkins To my 50's rock, hay Iam retired. I have been putting this 2 channel system together for the last 18 months and have to do it with used eqipment. I thought I might try and put my hardest piece of music on and find out what it will demand and then I can go from there. I was hoping to find an easy way to find out what the speakers draw from the amp. I have used my volt meter when playing pipe organ music that is getting down to the bottom peddle plus full mid and highs. I am showing 1 volt, I am not sure I am doing this right, the amperage is .8, that was the peak draw,(not sure I did this right). If I am right is this less then 1 watt? Iam over my head on this and just trying. Thanks

#### Ron E

In this case, it is very simple mathematically.

You want 85dB, your speakers are rated 95dB. That means you use 0.1 watts on average - typical music has a crest factor up to 20dB which is a factor of 100 - so a 10W amp will be more than sufficient to drive 85dB average.

By all means, try out the V^2/(guess)=power estimate. I say guess because speakers are not resistors.

I thought the AR-1's were 4 ohm with a series resistor or something like that. 8" midrange and tweeter along with 15" bass spkr.....? Now what you really need is a Klipschorn

I have run my old Cerwin-Vegas with a walkman, although they sounded a bit compressed

#### sunseeker

The speakers have a 1" titanium dome tweeter and 2 5 1/4" mid/bass drivers plus the 15" woofer. I have driven the speakers with a 22 watt intrigated amp and that was more then enough power. But amp only had 2 inputs, I need 4. I all so like to use seperates and mono blocks. Sound with the tube was nicer then my current ss amp, but we use what we have sometimes. I thought that 8 to 10 watts was what I was going to end up with. I was hoping to stay away from a 300b set because of tube cost. Now I will start looking at different tube amps and try and come up with 2 or three choices. Thank you all for your help.
mike

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