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Amplifier power meter?

69.130.185.159
Posted on June 30, 2020 at 11:14:31
Mike K
Audiophile

Posts: 13973
Location: 97701
Joined: September 23, 1999
 I've been wondering how to measure the output power of my amplifiers,
and so I searched the internet for "audio power meters" and got
nothing. I queried PS Audio and their engineers know of no such device.
So ... how do designers and product testers (like John Atkinson) measure
the output power of an amplifier? And is there an inexpensive device
you can use for this purpose?

Lack of skill dictates economy of style. - Joey Ramone

 

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RE: Amplifier power meter?, posted on June 30, 2020 at 17:09:23
13th Duke of Wymbourne
Audiophile

Posts: 2794
Location: Orange Co., Ca
Joined: September 19, 2001
 Are you talking about a piece of test gear to do some characterization or are you just looking for some display meters to watch?

To measure power you need to know both the voltage and the current delivered to the load. I don't know if any commercial amplifier with meters actually do this - probably just measure the voltage and scale it to power into 8 ohms. I would assume there is a piece of test gear that would do this test.
Measuring the voltage is non-invasive and straightforward. Measuring the current is not necessarily so - one way is to insert a resistor in series with the loudspeaker and measure the voltage difference on both sides of the resistor. If that is unpalatable there are ways to measure current into a wire using pick-up coils or specific sensors but I have no experience of those.

Something to consider is the speed of what you will measure - if this is for music then there are weighting and meter ballistics to consider (see the recent review of the Classe power amp in Stereophile for some comment on that). There may not be a big detectable difference between a voltage meter and a true-power meter on music signals (?). For characterization using sine waves you could always measure the acrms voltage with a meter then measure the resistance of the load at that frequency and calculate power that way.

Regards
13DoW.

 

Thanks for the reply, posted on July 3, 2020 at 08:03:13
Mike K
Audiophile

Posts: 13973
Location: 97701
Joined: September 23, 1999
 I'm just looking for a display meter I can watch - apparently such does
not exist, except on a certain few overly expensive amps, like MacIntosh.

Lack of skill dictates economy of style. - Joey Ramone

 

I remember Radio Shack used to sell them..., posted on July 7, 2020 at 13:47:21
Jonesy
Audiophile

Posts: 3156
Location: Winnipeg, Manitoba
Joined: September 1, 2005
Contributor
  Since: March 1, 2018
 that was a long time ago.

This is the only one I can find, but you might have better luck.

There are a lot of VU meters out there, but it's tough to find one that shows wattage.

I didn't go through all the details on this example, but looks like some calibration has to be done. Actually I think calibration is a must for the meter to really do it's thing properly.

The meter example is for one only. Then it would have to be mounted into something, and wired up.

Back in the day I remember DIY kits or completely built LED meters. Unfortunately searches are not turning anything up.

Anyway, hope this helps. There definitely is/was such a thing.

Cheers!

Jonesy


"I know just enough to get into trouble. But not enough to get out of it."




 

Might need a driver board and DIY skills as well..., posted on July 7, 2020 at 14:57:10
Jonesy
Audiophile

Posts: 3156
Location: Winnipeg, Manitoba
Joined: September 1, 2005
Contributor
  Since: March 1, 2018
 going into unknown territory here with these meters. Not sure how simple it would be to set up.

These particular ones are definitely not plug and play.

Interesting though because it was something I always though would be cool to have.

Cheers!


Jonesy


"I know just enough to get into trouble. But not enough to get out of it."




 

RE: Might need a driver board and DIY skills as well..., posted on July 18, 2020 at 15:01:07
Mike K
Audiophile

Posts: 13973
Location: 97701
Joined: September 23, 1999
 Thanks for your efforts. I'd like something that's plug=and=play,
however, since I'm not electronically inclined.

Lack of skill dictates economy of style. - Joey Ramone

 

LOL! I'm getting tons of VU meter ads now in my browser..., posted on July 18, 2020 at 15:30:49
Jonesy
Audiophile

Posts: 3156
Location: Winnipeg, Manitoba
Joined: September 1, 2005
Contributor
  Since: March 1, 2018
 so inadvertantly keeping an eye out for you. All the plug 'n play one's are in dB needle or dB LED. Still no watts unfortunately. Did find an ad of the old Radio Shack one in watts. Memories!

Cheers!

Jonesy


"I know just enough to get into trouble. But not enough to get out of it."




 

RE: Amplifier power meter?, posted on July 23, 2020 at 15:16:42
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 There is enough ELECTRICAL variation among the various speakers that to get an accurate (say <10%) you'll need a real custom execution.

I suspect that bench testers have access to a good DUMMY LOAD (pure resistive), a scope and calibrated meters (volts and amps)
Too much is never enough

 

RE: Thanks for the reply, posted on September 2, 2020 at 18:49:47
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 Even the numbers read back on the Mac Meters is simply 'advisory'.

Might help if you had a (non existent) pure resistive speaker at 8 ohms.....
Too much is never enough

 

RE: Thanks for the reply, posted on September 2, 2020 at 23:38:15
John Elison
Audiophile

Posts: 23900
Location: Central Kentucky
Joined: December 20, 2000
Contributor
  Since: January 29, 2004
 Actually, the meters could be on the preamp if you knew the relationship between the output of the preamp and the output from the power amplifier. For example, you could use a test CD with sine wave tracks while measuring the power amplifier output when the preamp meters display a given decibel reading such as -30-dB. From that you can calculate the amplifier output when the meters indicate zero-dB.

The Mytek Brooklyn Bridge preamp/DAC/streamer has digital meters that could be used as described above. I recently replace my Pass Labs X1 line-stage with a Mytek Brooklyn Bridge. Here are two pictures of the display on the Brooklyn Bridge.

......

 

RE: Thanks for the reply, posted on September 3, 2020 at 00:03:24
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 Man, that's a LOT of work, but I think I get it.

Make up a spreadsheet in Excel, enter the first measured data, the relationship equation and CRUNCH.
Too much is never enough

 

RE: Thanks for the reply, posted on September 3, 2020 at 10:16:43
John Elison
Audiophile

Posts: 23900
Location: Central Kentucky
Joined: December 20, 2000
Contributor
  Since: January 29, 2004
 Well, first you need a preamp with meters!

Good luck,
John Elison

 

RE: Amplifier power meter?, posted on September 7, 2020 at 21:21:44
LtMandella
 plenty of examples how it's done on youtube.

need a load (very high wattage 8 or 4 ohm resistor), signal generator within audio band, and scope.

Send in the signal, increase the power until the scope shows clipping of the sine wave or unacceptable level of distortion.

am i wrong?

 

RE: Amplifier power meter?, posted on September 8, 2020 at 21:32:27
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 And that would be how many watts?

If I'm not mistaken, those who want a meter want WATTS as a visual....
Too much is never enough

 

RE: Amplifier power meter?, posted on October 2, 2020 at 12:06:14
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 One slight issue .

For a RESISTIVE load, you need a Mondo-Power resister. And a scope and a good, calibrated volt meter which will be accurate at various frequencies.

But thant? You have REAL speakers which are reactive. So your resistive measure just went OUT the window.

Perhaps make a Simulated Speaker Dummy Load....? At the LINK is one such load. Too bad that amp manufacturers could NOT care less. It is in their beat interest to continue with resistive measures which conceal potentially poor amp performance with REAL speakers.
Too much is never enough

 

RE: Amplifier power meter?, posted on October 4, 2020 at 15:10:49
John Elison
Audiophile

Posts: 23900
Location: Central Kentucky
Joined: December 20, 2000
Contributor
  Since: January 29, 2004
 > And a scope and a good, calibrated volt meter which will be accurate at various frequencies.

There are AC voltmeters designed for measuring AC across the frequency bandwidth from 10-Hz to 100-kHz or higher. They used to be called Vacuum Tube Voltmeters, but they're solid-state nowadays. I have one called an AC Millivoltmeter. It's the component in the middle of the picture to the right of the audio signal generator. I was measuring the output of a step-up transformer.


 

RE: Amplifier power meter?, posted on October 4, 2020 at 16:47:02
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 I built the Knight Kit VTVM a LONG time ago. Wish I still had it. 11meg input impedance killed ANY of the VOMs available at the time.

Do you worry about stuff like 'True RMS' or not?


Too much is never enough

 

RE: Amplifier power meter?, posted on October 5, 2020 at 20:08:37
John Elison
Audiophile

Posts: 23900
Location: Central Kentucky
Joined: December 20, 2000
Contributor
  Since: January 29, 2004
 > Do you worry about stuff like 'True RMS' or not?

Not really! I measure sine waves so it doesn't concern me. True RMS is only of concern if you're measuring odd shaped waveforms and you want to know the RMS value.

I guess it would be a concern if you used the meters to measure amplifier power with music waveforms.

 

RE: Amplifier power meter?, posted on February 19, 2021 at 11:27:12
Ugly
Audiophile

Posts: 2912
Location: Des Moines, WA
Joined: August 22, 2006
 Hall effect current sensors like those made by Allegro are pretty cool.

Sense voltage and current and you can calculate/display whatever you want.

 

RE: Amplifier power meter?, posted on February 20, 2021 at 00:32:15
andyr
Manufacturer

Posts: 12548
Location: Melbourne
Joined: September 2, 2000
 I presume you want to measure the amp's output power to just before clipping?

Which of course will vary, depending on the resistance across the amp's spkr terminals.

So which resistance value do you want to choose?

The way I did it for the amplifiers I've been building is as follows:
1. Either buy a 'power resistor' of the value that interests you or make one up from a collection of cheap 10w ceramic wire-wound resistors.

2. Say you choose 4 ohms. Using a sig-gen, feed a 1kHz signal into your amp, when the 4 ohm res is across the spkr terminals.


3. Connect a CRO to the spkr terminals ... turn up the signal until you see either the top or the bottom of the sine wave flatten out; this is clipping.


4. So, measure the peak-to-peak reading on your CRO screen, just before the sine wave flattens - let's say it is 20v.
Now:
* V = I * R ... so the output current flowing across the load resistor is 5a
* P = I^2 * R ... so the power at clipping, into a 4 ohm load, is 100w.

Andy

 

RE: Amplifier power meter?, posted on January 19, 2022 at 17:50:45
psn
Audiophile

Posts: 56
Location: Houston
Joined: January 6, 2007
 Get a bare meter
Look up its reference current (the max current to present full scale deflection (FSD))
Determine its connection polarity (so it does not move the wrong way)
Use a terminal strip with screw connections
Use resistor (appropriate for your amp's power)
Use a diode oriented so it prevents the signal polarity from driving the meter backwards

Experiment with very low power while confirming that only the unblocked polarity of the signal through the diode drives the meter in the full scale deflection direction.

Experiment with resistor values or do the math with the FSD current in mind - gradually increasing power

Use smart phone SPL measure and rated speaker sensitivity to approximate amp power output against meter deflection - adjust amp volume and resistor until deflection registration on face is near FSD at very loud or rated power - if printed scale figures don't match calculated power levels, print your own meter face scale or just live with it

Make copy for other channel


*** Cautions***

Start off with large value resistors and work down
The meters I used were scaled for 250W at "full 0dB" with the red zone extending to 300W, but they had a secondary scale maxing to 30W with 1W about dead center... that was the useful scale for my amp
I connected my meters to the "B" set of speaker outputs (so they were in parallel with the "A" set of main speakers), and set the speaker selection to "A+B" when I wanted to view the meter

No guarantees that you won't find a fire extinguisher to come in handy :)

 

RE: Amplifier power meter?, posted on March 22, 2022 at 13:07:06
gberchin
Audiophile

Posts: 33
Joined: January 12, 2008
 Maybe something like this?

 

RE: Amplifier power meter?, posted on August 21, 2023 at 22:37:29
ytb402
Industry Professional

Posts: 11
Location: Utah
Joined: August 19, 2023
Contributor
  Since: August 19, 2023
 You can make your own power measuring
meter. Let me find a good diagram for you.
Easy to do and cheap
CJL

 

RE: Amplifier power meter?, posted on August 23, 2023 at 09:57:58
ytb402
Industry Professional

Posts: 11
Location: Utah
Joined: August 19, 2023
Contributor
  Since: August 19, 2023
 Here is a simple testing circuit
see details at
https://www.circuitstoday.com/audio-wattmeter-circuit

Curtis
CJL

 

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