How to test an amps true power output

[seamonkey]

People still assume you get double power going from 8 to 4 ohms.

If the voltage doesn't measure enough for a 4-ohm load you won't get the power out neither.
Since most amps are rated at 4-ohms these days. Measure the voltage and see if ohm law works out for 4-ohm. Ohms law of course is what some would call "RMS" power.

 

[StatesideRambler]

Ohms law of course is what some would call "RMS" power.

No one knowledgeable in electricity.

 

[seamonkey]

No one knowledgeable in electricity.

A true RMS Voltage reading on a DMM
Throw it into ohms law calculation for power and sure enough someone will say "Watts RMS"

 

[agedhorse]

Watts "RMS" is a legally defined term (by the FTC) which stands for (or is short hand for) power based on RMS voltage and current.

Ohm's law relates voltage, current, resistance/impedance which leads to power. Add in time and now you have energy.

 

[BogeyBass]

Root Mean Square is a different calculation than Ohms law.

You can get RMS from a meter.....but

Most people just use a scope under load
To test amplifiers. Likewise distortion analyzer to find actual distortion level.

Most mid-range to high level scopes already have this. Likewise RMS

Readings are taking..... under load

You can do RMS for a Sinewave , Squarewave, Triangle etc etc. If it makes you feel better.

You can put a car on jack stands and rev it in gear till you hit a Quarter Mile and tell everyone you Quarter Mile time.
My Geo Metro does 8 secs.....unloaded

 

[BogeyBass]

If the voltage doesn't measure enough for a 4-ohm load you won't get the power out neither.
Since most amps are rated at 4-ohms these days. Measure the voltage and see if ohm law works out for 4-ohm. Ohms law of course is what some would call "RMS" power.

You can put any voltage into 4 ohms.
We are trying to tell you. That if you apply rail voltage to 4 ohms. It will give you a higher power rating.
Devices loose gain under high current. So under load you can loose anywhere from 3 to 7 volts of voltage swing per rail or more.
If your good at designing amps even at 4 ohms you can swing close to the rails.
As mentioned even 20 years ago. Anybody that knows how to build a amp would use the well known higher performance output transistors. Depending on current needed you parallel more devices. Major drawback with many MI amps was using minimal output devices and depending on forced air cooling for Safe operating area. Eventually you have to use forced air cooling. But you can improve low impedance drive capabilities and have a more robust amplifier if your not a cheap turd and just add more output devices.

Even a turd amp will provide advertised power into 4 ohms unloaded. You don't seem to understand that almost any amp will do what is advertised according to rail voltage. Worthless information

This is why unloaded measurement is useless. The real challenge in design is maintaining SOA and voltage swing into low impedance loads. Rail voltage is useless information. I could apply rail voltage to a turd amp and get 2x power nevermind it will overheat in 5 mins lol.

People assume a weezy 200 to 300 watt amp is nothing. But even 300 watts into 4 ohms requires a reasonable amount of design consideration. Include inrush current, fault protection, DC protection etc etc. Even DC voltage at 200 watts will destroy many speakers instantly.

I understand what your saying but if you measure rail voltage on majority of amps over the last 20 or so years and apply theoritical power. It's going to match. Won't find any huge conspiracy theorist or cry murder. Over fake claims.

Another trick is using higher voltage. But the current capabilities won't match. So your method would mainly show you many amps can make almost twice advertised power. But under load the current of the power supply is no where close. Voltage is, current...no

 

[agedhorse]

In case you didn't realize it, we have already defined what happens with unloaded measurements. Those who care already understand this.

 

[seamonkey]

To the OP, or anybody wanting to measure the power capabilities of an amp.

Some manufacturers actually list the Voltage on the amp, along with minimal R (ohms) load.
See the Elf as an example

Plug those Elf numbers into an ohms law calculator and you'll get 200w
Load or no load, it's going to measure a max 28.3V on the output.
As others have stated don't try to use a oscilloscope to measure the voltage, but todays DMMs are all isolated grounds.

A low cost DMM, which has many uses. Checking cables, Checking for shorts, Intermittent connections, etc. Many of these DMMs also add true RMS to V these days or you can calculate RMS from the peak V.

Hook it up to the amp, and put in a 60hz from a mp3 player, your phone, or PC, as they all can drive amps to full power just like a bass would.

Then measure the Voltage and use Ohms law.

Then hook it up to a speaker and listen if it sounds good.

If an amp can't do the voltage on the output with no load, then it can't do it with a load connected. If you tried to hook up a 2-ohm load to the elf, it might sag the voltage for a short while but the amp would also shutdown. It won't help to hook up a speaker to test the voltage as speakers are reactive loads and you'd get unexpected readings. Current and Voltage are out of phase on AC (Audio signals)

Sure Manufacturer amp ratings are based on high power resistor loads with complicated test equipment. Those power resistors are useless to buy for an average person to buy.

Amplifiers are actually Voltage Sources. See: Voltage source - Wikipedia.

There is no Amplifier manufacturer that would design V to vary with R
That would sound horrible into a reactive loads like a speaker.
They will instead say what the low limit of R ohms is so the I (current) never goes too high to damage the V source, but all amps today also have protection to shutdown if too much current is drawn.

 

[Killing Floor]

I think there's a way to drop out the speaker and lick the leads. Never tried it. But you know someone has.

 

[Wild Rice Chris]

...You can put a car on jack stands and rev it in gear till you hit a Quarter Mile and tell everyone you Quarter Mile time.
My Geo Metro does 8 secs.....unloaded

Oh man, I gigged out of a 20 year old Geo Metro hatchback for a year and a half back around 2001. Double bass, electric bass, acoustic guitar, electric guitar, violin, andtand sax in the back, drummer in the passenger seat. Trying to reach 55 on the tollway was tough.

The double bass went from the tailgate to the radio dials, and I had to shift gears under the scroll. Strap my B-15N in the passenger seat and go!

Sorry for the derail!

 

[byacey]

You can put any voltage into 4 ohms.
We are trying to tell you. That if you apply rail voltage to 4 ohms. It will give you a higher power rating.
Devices loose gain under high current. So under load you can loose anywhere from 3 to 7 volts of voltage swing per rail or more.
If your good at designing amps even at 4 ohms you can swing close to the rails.
As mentioned even 20 years ago. Anybody that knows how to build a amp would use the well known higher performance output transistors. Depending on current needed you parallel more devices. Major drawback with many MI amps was using minimal output devices and depending on forced air cooling for Safe operating area. Eventually you have to use forced air cooling. But you can improve low impedance drive capabilities and have a more robust amplifier if your not a cheap turd and just add more output devices.

Even a turd amp will provide advertised power into 4 ohms unloaded. You don't seem to understand that almost any amp will do what is advertised according to rail voltage. Worthless information

This is why unloaded measurement is useless. The real challenge in design is maintaining SOA and voltage swing into low impedance loads. Rail voltage is useless information. I could apply rail voltage to a turd amp and get 2x power nevermind it will overheat in 5 mins lol.

People assume a weezy 200 to 300 watt amp is nothing. But even 300 watts into 4 ohms requires a reasonable amount of design consideration. Include inrush current, fault protection, DC protection etc etc. Even DC voltage at 200 watts will destroy many speakers instantly.

I understand what your saying but if you measure rail voltage on majority of amps over the last 20 or so years and apply theoritical power. It's going to match. Won't find any huge conspiracy theorist or cry murder. Over fake claims.

Another trick is using higher voltage. But the current capabilities won't match. So your method would mainly show you many amps can make almost twice advertised power. But under load the current of the power supply is no where close. Voltage is, current...no

What's a turd amp? Is this a new output topology?

 

[cdBuckeye]

To the OP, or anybody wanting to measure the power capabilities of an amp.

Some manufacturers actually list the Voltage on the amp, along with minimal R (ohms) load.
See the Elf as an example
View attachment 3937473

Plug those Elf numbers into an ohms law calculator and you'll get 200w
Load or no load, it's going to measure a max 28.3V on the output.
As others have stated don't try to use a oscilloscope to measure the voltage, but todays DMMs are all isolated grounds.

A low cost DMM, which has many uses. Checking cables, Checking for shorts, Intermittent connections, etc. Many of these DMMs also add true RMS to V these days or you can calculate RMS from the peak V.

Hook it up to the amp, and put in a 60hz from a mp3 player, your phone, or PC, as they all can drive amps to full power just like a bass would.

Then measure the Voltage and use Ohms law.

Then hook it up to a speaker and listen if it sounds good.

If an amp can't do the voltage on the output with no load, then it can't do it with a load connected. If you tried to hook up a 2-ohm load to the elf, it might sag the voltage for a short while but the amp would also shutdown. It won't help to hook up a speaker to test the voltage as speakers are reactive loads and you'd get unexpected readings. Current and Voltage are out of phase on AC (Audio signals)

Sure Manufacturer amp ratings are based on high power resistor loads with complicated test equipment. Those power resistors are useless to buy for an average person to buy.

Amplifiers are actually Voltage Sources. See: Voltage source - Wikipedia.

View attachment 3937474
There is no Amplifier manufacturer that would design V to vary with R
That would sound horrible into a reactive loads like a speaker.
They will instead say what the low limit of R ohms is so the I (current) never goes too high to damage the V source, but all amps today also have protection to shutdown if too much current is drawn.

To be clear, an oscilloscope can be used to measure the amplifier output voltage, assuming we are talking about a multi-channel oscilloscope. If CH1 and CH2 are being used, set up the measurement for CH1-CH2. All of the modern digital oscilloscopes I've used have this built into the menu system. On common 2 channel vintage analog scopes typically used by hobbyists, this is accomplished by enabling both channels, setting CH2 to Invert and the dual channel mode to ADD.

 

[agedhorse]

To be clear, an oscilloscope can be used to measure the amplifier output voltage, assuming we are talking about a multi-channel oscilloscope. If CH1 and CH2 are being used, set up the measurement for CH1-CH2. All of the modern digital oscilloscopes I've used have this built into the menu system. On common 2 channel vintage analog scopes typically used by hobbyists, this is accomplished by enabling both channels, setting CH2 to Invert and the dual channel mode to ADD.

That's just a differential measurement. What signals would channel 1 and 2 represent, and how would a differential measurement get you a measurement of voltage different than using a single channel?

 

[BogeyBass]

Oh man, I gigged out of a 20 year old Geo Metro hatchback for a year and a half back around 2001. Double bass, electric bass, acoustic guitar, electric guitar, violin, andtand sax in the back, drummer in the passenger seat. Trying to reach 55 on the tollway was tough.

The double bass went from the tailgate to the radio dials, and I had to shift gears under the scroll. Strap my B-15N in the passenger seat and go!

Sorry for the derail!

Lol all good.
Metros are the source of countless jokes.

Mine still runs. 52 mpg
Paid 450 dollars 18 years ago.
Never been repaired or broke down.
Just tune ups and oil changes

I've carried a 810 to multiple gigs on the roof.
I also checked the unloaded voltage of the stereo amplifier. And I'm confident it's still 40 watts per channel. Real mystery is why my alternator never died. Since I'm running a 2x12 sub at 400 watts. ( The roof flexes at half volume lol) it's a Metro so cabin gain is ridiculous

 

[jnewmark]

What's a turd amp? Is this a new output topology?

His favorite adjective for an amp that, in his opinion, sucks in most ways. I think.

 

[seamonkey]

To be clear, an oscilloscope can be used to measure the amplifier output voltage, assuming we are talking about a multi-channel oscilloscope. If CH1 and CH2 are being used, set up the measurement for CH1-CH2. All of the modern digital oscilloscopes I've used have this built into the menu system. On common 2 channel vintage analog scopes typically used by hobbyists, this is accomplished by enabling both channels, setting CH2 to Invert and the dual channel mode to ADD.

It was just a warning, Scopes probes that ship with scopes are grounded, and all these high power class-d amp heads are bridged. Ground + bridge -> Not good

 

[cdBuckeye]

That's just a differential measurement. What signals would channel 1 and 2 represent, and how would a differential measurement get you a measurement of voltage different than using a single channel?

For this measurement, CH1 would be connected to Speaker Out + (e.g., Speakon 1+ or Phone Plug Tip), and CH2 would be connected to Speaker Out - (e.g., Speakon 1- or Phone Plug Sleeve). Yes, the measured voltage would be the same as the single channel measurement if the CH2 probe is at Chassis Potential, with a grounded chassis. If the amplifier is internally or externally bridged, assuming Speaker Out - is at the same potential as the grounded chassis would be a bad assumption.

The point I was trying to make (and now realized I left out some important information) is that scopes can be used to make voltage measurements referenced against a potential other than ground, if you supply the scope with the reference potential. Affordable options exist for this, even at the hobbyist level; there is no need to resort to a 60 Hz measurement with a handheld DMM simply to get a pair of voltage measurement connections where the (-) lead is not permanently connected to ground.

 

[seamonkey]

Sure
Everyone can own a UL listed DMM for $40 bucks these days that can do RMS voltage.
It's just a quick, no-load test that people can do.

 

[agedhorse]

For this measurement, CH1 would be connected to Speaker Out + (e.g., Speakon 1+ or Phone Plug Tip), and CH2 would be connected to Speaker Out - (e.g., Speakon 1- or Phone Plug Sleeve). Yes, the measured voltage would be the same as the single channel measurement if the CH2 probe is at Chassis Potential, with a grounded chassis. If the amplifier is internally or externally bridged, assuming Speaker Out - is at the same potential as the grounded chassis would be a bad assumption.

The point I was trying to make (and now realized I left out some important information) is that scopes can be used to make voltage measurements referenced against a potential other than ground, if you supply the scope with the reference potential. Affordable options exist for this, even at the hobbyist level; there is no need to resort to a 60 Hz measurement with a handheld DMM simply to get a pair of voltage measurement connections where the (-) lead is not permanently connected to ground.

Gotcha, it wasn't clear that you were addressing BTL outputs.

 

[byacey]

For this measurement, CH1 would be connected to Speaker Out + (e.g., Speakon 1+ or Phone Plug Tip), and CH2 would be connected to Speaker Out - (e.g., Speakon 1- or Phone Plug Sleeve). Yes, the measured voltage would be the same as the single channel measurement if the CH2 probe is at Chassis Potential, with a grounded chassis. If the amplifier is internally or externally bridged, assuming Speaker Out - is at the same potential as the grounded chassis would be a bad assumption.

The point I was trying to make (and now realized I left out some important information) is that scopes can be used to make voltage measurements referenced against a potential other than ground, if you supply the scope with the reference potential. Affordable options exist for this, even at the hobbyist level; there is no need to resort to a 60 Hz measurement with a handheld DMM simply to get a pair of voltage measurement connections where the (-) lead is not permanently connected to ground.

If you have to explain all this to a person trying to do these tests, it's clear that they aren't qualified to do this.
An isolation transformer would be a better way to go.
Edit: No offence towards Andy; I see he had some posts interspersed in here.
The point I was trying to make is if a person isn't familiar with BTL outputs and how to use a scope, they really shouldn't be messing around with this.