watt meters

[vizion]

hello,

does anybody know of old style watt meters instead of the typical VU meters ?

thanks.

[Steerpike]

Do you mean REAL Wattmeters? A true wattmeter has two coils, one for current and one for voltage.
Now a lot cheaper to compute the wattage electronically.
Most amps just use a voltmeter, calibrated in watts for a typical load

[Geek]

Originally posted by Steerpike
Most amps just use a voltmeter, calibrated in watts for a typical load

Which are really inaccurate, since a speaker is anything but a resitive load and rated impedance is only at one frequency.

[zigzagflux]

I would use an analog multiplier, feed into a uA or mA movement.

[nitrate]

I would like to find a good way of implementing a true watt meter too. I dont wanna go with the complication of using two coils but if anybody knows a simple work around please do tell. I have an ancient true watt meter i picked up off the car boot. It's huge and must be worth a fortune. That has seperate coils in it but unfortunatly the scale is calibrated to 600 Watts. Not much use for me unless i'm using it to verify total power consumption from the mains. There must be a cheap way of sorting this out so we can display true watts on a cheap moving coil voltmeter.

Leigh

[planet IX]

Here is a nice one:
http://surplussales.com/Meters/MtrRF-wats.html
But its not really a bargain...

[Yvesm]

Originally posted by Geek


Which are really inaccurate, since a speaker is anything but a resitive load and rated impedance is only at one frequency.

Right !
Not to tell about multi ways enclosures

Be does'it really matters ?

Do you wanna know how many watts are effectively transfered to the loudspakers or how many watts the amp would be able to push in a know load ?

Indeed, I just need to check if the amp is working correctly and the voltage developed across various resistors is all what I care off

Matching the right loudspeaker with the right amplifier (and vice et versa) is a completly differrent story

Yves.

[Steerpike]

As mentioned, you could use an analogue multiplier, but you'd need more than just that, because real power is
P = V*I*Cosine(phi)

If you just multiplied the current and voltage, (no cosine term) you'd get the apparent power - which is not the real power because it doesn't take into account the ratio of speaker inductance to resistance. (or capacitance where/if that exists)

SOMEWHERE I have a circuit that does it - I'll have to dig it out.

The cheapest (and smallest component count) way is to use an embedded microcontroller (eg, Pic or atmel) but that assumes you can write code & have a programmer.

[jackinnj]

since energy has become so expensive, firms like Analog Devices have developed entire series of chips which accurately measure power. i think that ADI has about 40 chips in this arena.

[Steerpike]

I have not looked carefully at any commercial power measuring chips, but are they not primarily intended for 50Hz/60hz AC mains. They may not have the bandwidth to cope with audio. (just a guess)
Certainly the circuit I have - as it stands - uses a PWM conversion at around 1khz, so it would need to be modified to cope with audio.

Thinking more about it, a useful audio power meter would have to have a bandwidth in excess of 20kHz, since when amplifiers go into clipping, they generate a lot of ultrasonic harmonics (the tweeters are always the first to burn at parties).