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How to test an amps true power output

#61
This is exactly the sort of thing that draws me to TalkBass, and the reason I have the admiration I do for Andy and all of professionals that are willing to step in and educate someone like myself.

There is SO much BS when it comes to gear, and yet, these were designed by rational engineers and/or artisans who had to know the technical end to produce the gear we use: There's no mystic aura involved. To try and understand the technicalities and the fabrication of these things is simply a must to steer past the junk and buy smartly designed and built gear; Lord knows in the early days I took a lot of bad advice and wasted money on poor choices as I had NO IDEA what made the wheels go 'round where the rubber meets the road.

Thanks again to all.
 
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#62
AudioTaper said:
Yeah, this was a fairly non-scientific. I was verifying operation and didn't monitor the input voltage. I measured the rails, I think they were +/- 72V.

The power output seemed low, but I wasnt pushing it as hard as I could have. The waveform wasnt as clean once I pushed it up into the 40s.

When I get more time I should set it all back up and use my Tektronix digital scope for a more accurate measurement. (Out of cal, but not out a lot)

Rated power is 250W into 8 and 400W into 4.
Click to expand...
With 72V rails, you should be measuring ~65Vpeak into 8 ohms, which would be ~46Vrms, which would be ~265W into 8 ohms which is well within the range of expectations.

If you really measured that far off and the waveform was not clean, there is something wrong with the power amp. It's not a design issue for sure.
 
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#63
beans-on-toast said:
Nice setup. Just a couple of comments. Your measurements should be good enough for your needs. They should be repeatable, which is good, but they may not be accurate because of limitations in the equipment. Just be aware of that.

If you haven't used heat sink compound between the resistor and the heat sink, it can help with thermal transfer. Since the assembly is getting very hot, the heat transfer compound may not help enough. Still, it's good practice to have optimal transfer. A resistor will change in resistance with temperature. Use an accurate ohm meter to verify the resistance at the operating temperature. This can slightly affect the results of your calculations if the true resistance is off from the nominal 8 ohms. If you are using different frequencies to evaluate your amp, ideally the load resistor should be non-inductive. These resistors read consistently over a range of frequencies.

A low noise at all frequencies signal generator is important. You don't want to be injecting noise at the front end with the sine wave.

You know how a scope probe can be adjusted for a square wave. Similarly, BNC terminators and cable compensation adjustments (like with a scope probe) can be used when using T-junctions to connect multiple pieces of equipment. This will ensure that there is minimal signal loss and the levels are correct.
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I'm using a Bergquist thermal gap pad HC 5.0 between the resistor and the heatsink. The entire heatsink gets hot, I dont think heat transfer is an issue. I had rigged up some fans, I just need to use them next time.

I was sure the resistor was non-inductive, but the datasheet is saying otherwise. It was a leftover part we had at work that my boss let me take home, Ohmite TAP600. I'll have to get the non-inductive version and try this all over again.

Signal generator is a BK digital cheapo. I have a nicer analog Wavetek, I'll dig that out for next time.

Maybe the boss will let me sign out one of the LeCroy scopes for the weekend? HA, fat chance of that....

Here I thought I had a decent setup... sounds like it needs a lot of work...
Thanks Guys!!
Ha, I'm learning something today
 
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#64
agedhorse said:
With 72V rails, you should be measuring ~65Vpeak into 8 ohms, which would be ~46Vrms, which would be ~265W into 8 ohms which is well within the range of expectations.

If you really measured that far off and the waveform was not clean, there is something wrong with the power amp. It's not a design issue for sure.
Click to expand...
I'll regroup and do it again. Or buy a TT-800. Either way, thanks for the help.
 
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#65
AudioTaper said:
Yeah, this was a fairly non-scientific. I was verifying operation and didn't monitor the input voltage. I measured the rails, I think they were +/- 72V.

The power output seemed low, but I wasnt pushing it as hard as I could have. The waveform wasnt as clean once I pushed it up into the 40s.

When I get more time I should set it all back up and use my Tektronix digital scope for a more accurate measurement. (Out of cal, but not out a lot)

Rated power is 250W into 8 and 400W into 4.
Click to expand...

If you have 72 volt power rails and the waveform gets ugly in the 40s then you got a problem.

Good news is if they claim 250 watts at 8 ohms. Depending on how good the amp swings you could have 260 to 280 watts at 8 ohms with 72 rails. So the 250 watt rating would seem accurate, possible very clean rating.

I'd rerun the test. But you mentioned the output section being hot before repairs.
I'd check bias and bias circuit for possible very high bias. Or one side of the output section is biased very high from another faulty device. To loose that much of the rail. Or if you losing continuity on one of the output transistor. If you just swapped some.
Or I'm hoping this isn't a floating load or bridged. Need be careful with ground and the scope with floating loads
 
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#66
AudioTaper said:
I'm using a Bergquist thermal gap pad HC 5.0 between the resistor and the heatsink. The entire heatsink gets hot, I dont think heat transfer is an issue. I had rigged up some fans, I just need to use them next time.

I was sure the resistor was non-inductive, but the datasheet is saying otherwise. It was a leftover part we had at work that my boss let me take home, Ohmite TAP600. I'll have to get the non-inductive version and try this all over again.

Signal generator is a BK digital cheapo. I have a nicer analog Wavetek, I'll dig that out for next time.

Maybe the boss will let me sign out one of the LeCroy scopes for the weekend? HA, fat chance of that....

Here I thought I had a decent setup... sounds like it needs a lot of work...
Thanks Guys!!
Ha, I'm learning something today
Click to expand...

Ask your boss for a suitable spectrum analyzer. :laugh: Always nice to see what is going on with harmonics.
 
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#67
This scale is from a meter that my dad found in a factory being demolished. Yes, they used that much juice, which they generated on-site.

upload_2020-8-6_19-16-45.png
 
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#70
BogeyBass said:
If you have 72 volt power rails and the waveform gets ugly in the 40s then you got a problem.

Good news is if they claim 250 watts at 8 ohms. Depending on how good the amp swings you could have 260 to 280 watts at 8 ohms with 72 rails. So the 250 watt rating would seem accurate, possible very clean rating.

I'd rerun the test. But you mentioned the output section being hot before repairs.
I'd check bias and bias circuit for possible very high bias. Or one side of the output section is biased very high from another faulty device. To loose that much of the rail. Or if you losing continuity on one of the output transistor. If you just swapped some.
Or I'm hoping this isn't a floating load or bridged. Need be careful with ground and the scope with floating loads
Click to expand...
The issue was in the preamp. If you are familiar with the Eden WT series, you'll know what this mess is
20200806_200507.jpg

Changed that out to chassis mount resistors. I also replaced the filter caps and fixed the grounding issue common to the WT power supply. Caps were working, but one had a buldged top. Its 20 years old. No work was done on the output section.
 
#71
beans-on-toast said:
Aluminium smelters consume a lot of power. Some are built next to hydro generating facilities. Recycling saves a lot of power.
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My family visited Iceland, and learned that their two biggest industries are tourism and aluminum smelting. They ship the bauxite from Australia.

The meter I showed was from a chlorine plant, but used a process that had gone obsolete.
 
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#72
The only thing about wattage is that a watt meter isn't actually very useful for audio measurements. Usually you're measuring power into a dummy load, so if you know the resistance of the load, you're good. Measuring power delivered to a speaker isn't all that useful because it depends on the impedance curve of the speaker.
 
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#73
seamonkey said:
It can't be done. There are no two manufacturers in the world that agree on what "true power" is. It's going to be really hard for anybody to measure what it could be unless you can find a standard.

But ... Let's assume though you have a dmm that can measure volts rms and you have a solid state amp. With absolutely no load connected, just hook up the meter put a 60hz tone in, and measure the max voltage you can get out. With just that voltage you can figure out what the potential max power would be. Make up a ohm of say 8 ohms, then use ohms law to calculate the power at 60hz. Repeat at different frequencies and I doubt you'll read much difference in voltage, but you can stop at 1khz.

Search ohm law calculator on the webs, put the voltage in, and 8 for resistance and although you measured at 60hz, you'll get what the potential equivalent of power would be. Some might call this "RMS" and that would be same power a DC circuit would give.

There's no way in real life an amp could sustain that level of power for any amount of human measured length of time. It'd decay in the blink of an eye's time, but manufacturers only rate their power for 30ms, so a voltage only reading actually gets pretty close to what a manufacturer might rate their amp.
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Wouldn't you need some kind of a load on that?
 
#76
This explains how behringer gets 1000 watts from 330 watt class D modules. When everyone else gets a more realistic 500 watts lol. Depending on what load you make up. I'm sure they are all correct
 
#77
bobyoung53 said:
Wouldn't you need some kind of a load on that?
Click to expand...

No load, just open. Just test the voltage the amp can put out.

If the amp can't do the V part of Ohms law it can't do the P

https://ohmslawcalculator.com/ohms-law-calculator

Let say you have an amp that's rated 1250 W
Put 1250 in the Power (Watts) box
Make up a load like 8 ohms and put that in the load box.
Calculate will show 100v in the V box.

It's a simple concept, but if the amp claims to be 1250w and 8ohms, it better put out 100v with no load connected otherwise it won't be able to do 1250w with a 8ohm load connected.
 
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#78
seamonkey said:
No load, just open. Just test the voltage the amp can put out.

If the amp can't do the V part of Ohms law it can't do the P

https://ohmslawcalculator.com/ohms-law-calculator

Let say you have an amp that's rated 1250 W
Put 1250 in the Power (Watts) box
Make up a load like 8 ohms and put that in the load box.
Calculate will show 100v in the V box.

It's a simple concept, but if the amp claims to be 1250w and 8ohms, it better put out 100v with no load connected otherwise it won't be able to do 1250w with a 8ohm load connected.
Click to expand...
Correct, this will give the open circuit, no load power POTENTIAL if the voltage is assumed to be into a load.

In practice, the actual maximum power will be lower because of the power supply and output device saturation voltages that I mentioned earlier. How much lower depends on the amp's specific architecture.
 
#79
" no load test" not much different than looking at a piece of paper and doing the math already shown to calculate theoritical max power. Depending on the voltage shown on data sheet.

I understand the concept. And how you could do the test quickly to see what the voltage was on the amp.

Complete turd amp will look beautiful with no load. But your basic concept is achievable just looking at the data sheet

Devices loose gain under load, and reach saturation. Power supplies drop voltage under load. Older unregulated supplies would have higher voltage unloaded.

Just wouldn't want people getting upset or find out how difficult even 4 ohm loads are yet alone 2 ohm. And definitely no you can't apply unloaded voltage to a 2 ohm calculation and expect amazing power levels. Doesn't work that way. I don't think most understand how difficult even 4 ohms is and how semi conductor devices behave under load.

You'll just get amazing ridiculous expectations like mystical 300 watts coming from 100 watt power supplies. Cause guys start applying unrealistic voltages to low impedance.

If you look at most typical MI amps and their ratings for the last 20 or so years. In general usually the rail voltage matches claimed power. Even turd combo amps do exactly as claimed according to the schematic. Not much different than a unloaded test. People still assume you get double power going from 8 to 4 ohms. Don't happen
 
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