Watt meter

[jack6428]

Since these Altecs are so efficient, I suspect I am actually using very little watts from my power amps.

Does anyone know how to install a watt meter onto an existing power amp?

Also, I?ve heard that normally the noon setting on a volume control is generally one watt. Is this true?

Thanks again,
Jack

[Altec19]

Since these Altecs are so efficient, I suspect I am actually using very little watts from my power amps.

<font size="2" face="Verdana, Helvetica, sans-serif">Unless you are listening at "Space Shuttle launch levels" you are probably correct. I am assuming that we are talking about a traditional Altec two-way system with a 15 inch woofer and a compression driver. The smaller consumer products are not as efficient.

Does anyone know how to install a watt meter onto an existing power amp?

<font size="2" face="Verdana, Helvetica, sans-serif">Radio Shack used to sell a stand-alone watt meter that connected to the speaker lines. Maybe they still do. Crown also had an industrial grade model that occasionally shows up on e-Bay. I wouldn't add one permanently at the risk of interfering with the amp/speaker relationship.

Also, I?ve heard that normally the noon setting on a volume control is generally one watt. Is this true?

<font size="2" face="Verdana, Helvetica, sans-serif">No. Power = Current^2 x Resistance. The amp manufacturer has no idea what the resistance (actually impedance) of the wide variety of speakers that could possibly be connected to the amp might be. That makes it impossible to calibrate an absolute wattage level to a position on the volume control from the factory.

There is a lot to be known about this topic and the above just scratches the surface. Listen at a sane volume level to protect your hearing. Don't drive your amp into distortion to protect your speakers.

Later . . .

[GM]

Greets!

All power meters I've seen had four terminals (or wire leads), two for current, which you attach to one amp lead and the other to the same polarity speaker lead (series connection), and two for voltage, which attach to the two amp leads (parallel connection).

GM

[selmerdave]

Someone told me that you can calculate with P=E^2/R, using the DC resistance of the speakers. I thought there would be a problem with this method given that speakers are not purely resistive. Is this right?

Dave

[GM]

Greets!

Correct, but using a true RMS voltmeter with a peak hold feature allows you to calc the transient peak power, which is of far more use than the average power used:

dB SPL = 20*log10(E/2.83)

P = 10^(dB/10)

To determine the max low distortion SPL, an o'scope is needed to view how badly the amp clips on the fastest transients since the vast majority of all HE HIFI systems are grossly underpowered, so basically most folks would just be calcing their amp's clipping point on all but the most compressed material, such as FM and most of today's mainstream music CDs mixed for boombox, etc. playback.

GM

[selmerdave]

Thanks GM, I was thinking in terms of measuring speaker efficiency with a measured watt from the amp.

Dave

[Altec19]

A measured watt will work as long as the device making the measurement (and subsequent calculations) takes into account the entire frequency spectrum of the test signal being used. A DC parameter (zero hertz) doesn't really tell an accurate story about something happening between 20Hz and 20KHz.

[donaldpatten]

Jack
GM and others hit the nail on the head . All the meters on amplifers I have seen are volt meters. It is expersive to measure Volts, current, and phase angle with speaker loads and program ( music ) material. We did it with Hall effect devices.
In the real world : if you have an ampllifier that puts out 20 Volts at rated distortion ( clipping, 1 % ) and bandwidth ( 20 -20k ) it should deliver 100 Watts to 4 Ohms, 50 Watts to 8, and 25 Watts to 16 . Most amplifiers have current limiting to protect the output transistors .Say we limit the current to 5 amps max ( peak ),the 4 Ohm power is : 4*5=20 V peak=20*0.707 Vrms 14.14Vrms = 50 Watts. All amplifiers have a VI limit .Now a real speaker is not 8 Ohms , or 16 ; it varies from say 6 to 60 Ohms with freq. With our amplifier above at 6 Ohms we get 20V*20V/6=66.7 Watts if we do not current limit . Wiith a 5 Ampere current limit 5*0.707 )( 5*0.707 )*6 =75 Watts . So we only get the lessor Watts : 66.7 Watts. At 60 Ohms we get 20*20/60=6.7 watts. And at no freq do we get 100 Watts.
If a volt meter has power markings they would be for resistor loads , and not be exact for speakers.
Don