ProSoundWeb Community

Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Audio Measurement and Testing => Topic started by: Nathan Riddle on November 09, 2019, 11:36:14 pm

Title: How to test amps properly?
Post by: Nathan Riddle on November 09, 2019, 11:36:14 pm
tl;dr -> Can someone do a simple writeup of the most important features to look at in an amplifier?


So I've always wanted to learn more about testing amps. Then this thread popped up. (https://forums.prosoundweb.com/index.php/topic,172356.msg1588407.html#msg1588407)

When I say learn more, I'm saying I don't want to continue reading about this, I want to put some stuff into motion.
What excites me is that amps don't make noise (much) unless they explode :P
I've wanted to test more speakers, but I don't have an easy close place to take speakers to and blast music.
I have a number of various amps from low-end Behringer to high-end LineaResearch.

Anyone care to walk me through the process?

Materials:
Software: Smaart, REW, RITA.
Interface: RME/UMC404
Hardware: Linear-X
DUT: D50, NU4-6000, RMX2450, XTI-4002, PLD4.5, iTech8k, DNA20K4

Resources:
I know there is CAF  (https://caf.prosoundtraining.com/the-caf-reference/)
Data-Bass (https://data-bass.com/#/articles/5cc0bc36a75a260004255c88?_k=wcadkc) has some good testing methodologies, but they are for subs not amps.

Personally I'm most fond of this style:
http://diy.poweraudio.ro/albums/userpics/10001/BENCH_COMPARISON_TEST.pdf
I can't find/remember the source.
But this method of testing makes sense to me and is easy to read/understand.

Other resources:
https://www.prosoundtraining.com/2010/03/11/big-dummy-load/
http://www.prosoundtraining.com/site/wp-content/uploads/2014/01/L1109_Amped_Up.pdf
Title: Re: How to test amps properly?
Post by: John Roberts {JR} on November 10, 2019, 10:00:21 am
I don't know how good your search skills are, or the old forum data here, but Bink did a very extensive amplifier testing "shoot-out" a number of years ago right here.

There was much discussion about difficulties getting objective results.  Hint- it isn't trivial.

JR
Title: Re: How to test amps properly?
Post by: Nathan Riddle on November 10, 2019, 12:29:08 pm
I don't know how good your search skills are, or the old forum data here, but Bink did a very extensive amplifier testing "shoot-out" a number of years ago right here.

There was much discussion about difficulties getting objective results.  Hint- it isn't trivial.

JR

Thanks John,

I think this is what you were referring to: http://forums.prosoundweb.com/index.php?topic=70922.0

Finding the actual files and testing procedure is proving to be more difficult.

---

Let's set some parameters.

I don't want to exhaustively test equipment.
I don't want to spend too much money.

I know I'm not at the same level as anyone of these guys or y'all here.
My goal is to do some simple tests that help me grow in my skill and understanding of the equipment we use.
I dream of being mentored by someone like Langston, Bink, JR, et alia.

Just get a little dirt under my fingernails ;)



---

Other resources:
http://forums.prosoundweb.com/index.php?topic=74914.0
http://forums.prosoundweb.com/index.php?topic=90765.0
https://forums.prosoundweb.com/index.php?topic=76266.0
https://forums.prosoundweb.com/index.php?topic=97418.20
http://forums.prosoundweb.com/index.php?topic=73706.30

This guy is amazing... GEQ testing too.
https://web.archive.org/web/20090317091210/http://binkster.net/geq_shootout.shtml
Title: Re: How to test amps properly?
Post by: John Roberts {JR} on November 10, 2019, 01:41:43 pm
Thanks John,

I think this is what you were referring to: http://forums.prosoundweb.com/index.php?topic=70922.0

Finding the actual files and testing procedure is proving to be more difficult.

---

Let's set some parameters.

I don't want to exhaustively test equipment.
I don't want to spend too much money.

I know I'm not at the same level as anyone of these guys or y'all here.
My goal is to do some simple tests that help me grow in my skill and understanding of the equipment we use.
I dream of being mentored by someone like Langston, Bink,

Just get a little dirt under my fingernails ;)



---

Other resources:
http://forums.prosoundweb.com/index.php?topic=74914.0
http://forums.prosoundweb.com/index.php?topic=90765.0
https://forums.prosoundweb.com/index.php?topic=76266.0
https://forums.prosoundweb.com/index.php?topic=97418.20
http://forums.prosoundweb.com/index.php?topic=73706.30

This guy is amazing... GEQ testing too.
https://web.archive.org/web/20090317091210/http://binkster.net/geq_shootout.shtml
I worked for years as product manager over all power amps for Peavey. It is all too easy to draw incorrect conclusions from flawed testing.

Modern amplifiers are quite good (better than decades ago) so I wouldn't expect to easily find objective differences between them in casual testing that would represent actual real world performance. Bink and others attempted to use rigor and discipline in their old tests.

This has been pretty much beat to death here over the decades.

JR
Title: Re: How to test amps properly?
Post by: Nathan Riddle on November 10, 2019, 03:07:09 pm
I worked for years as product manager over all power amps for Peavey. It is all too easy to draw incorrect conclusions from flawed testing.

Modern amplifiers are quite good (better than decades ago) so I wouldn't expect to easily find objective differences between them in casual testing that would represent actual real world performance. Bink and others attempted to use rigor and discipline in their old tests.

This has been pretty much beat to death here over the decades.

JR

1) I respect you, your experience and what you're saying. Thank you for your help.
2) I am listening to you, but sidelining my request due to a lack of rigor & discipline isn't fair. I want to get started. The intellectual journey is long and full side quests. Help me get the basics?
3) I agree the statements have been made about modern amps ad nauseam. I disagree that it wouldn't be helpful to ME to learn some simple tests that are useful.

Perhaps let me rephrase.

JR, would you kindly tell me the first 3 simple tests you would do if you had two amps on the table and wanted to compare the major differences.
Given the following test equipment: o'scope; dual FFT; and a 1ch 4kw 2ohm load bank; w/ voltage & amp probes rated for 4kw.
Title: Re: How to test amps properly?
Post by: John Roberts {JR} on November 10, 2019, 03:49:28 pm
1) I respect you, your experience and what you're saying. Thank you for your help.
2) I am listening to you, but sidelining my request due to a lack of rigor & discipline isn't fair. I want to get started. The intellectual journey is long and full side quests. Help me get the basics?
3) I agree the statements have been made about modern amps ad nauseam. I disagree that it wouldn't be helpful to ME to learn some simple tests that are useful.

Perhaps let me rephrase.

JR, would you kindly tell me the first 3 simple tests you would do if you had two amps on the table and wanted to compare the major differences.
Given the following test equipment: o'scope; dual FFT; and a 1ch 4kw 2ohm load bank; w/ voltage & amp probes rated for 4kw.
I would plug them in and see if they make sound.

As long as modern amps are operated in their linear region (not clipped or over loaded), they should exhibit pretty much the same transfer function (sound the same).

JR 

Title: Re: How to test amps properly?
Post by: Frank Koenig on November 10, 2019, 07:14:18 pm
To test modern amplifiers to their full capability you're going to need a way of generating tone bursts. The amps can put out far more power for some tenths of a second than they can continuously, which is just fine for (most) music. If you try to run them to full output into a load for longer than that they'll go into thermal limit, pop breakers, and will undergo a lot of unnecessary stress.

The tone bursts don't need any sort of fancy window function for the envelope, a rectangle will do. Some "function" (sine, triangle, square, pulse) generators have a gate input that can be driven from a separate pulse generator. That's what I use. Alternatively you could use an "arbitrary waveform" generator, or a computer audio interface (sound card) along with some programming to generate sound files with the waveform you need. There are no doubt other ways, too. I suppose you could build a gate using an analog switch, or even a small, fast relay.

If you're just looking for clipping a digital scope will show that. If you want to do detailed distortion measurements you'll need to use a computer audio interface and analyze the recorded waveforms somehow, or an expensive analyzer such as Audio Precision.

For load resistors look for heating elements that have the right resistance. You can submerge them in water to extend their power handling and reduce the increase in resistance as they heat up.

A trick, that I've mentioned before, is to make a high-power voltage divider so that you can run the amp at full tilt boogie while critically listening at a pleasant level.

--Frank

It occurred to me that you could use the gate function in a digital mixer with the side chain coming from a pulse generator to generate tone bursts. Many ways...

Another thought. Don't forget to examine distortion at very low power levels. Some amps might do fine near full output but exhibit a dead-zone or "crossover distortion" (a terrible term) near zero. This might matter more than you think even for live sound. A dynamic program that hits peaks of several 100 W might have quiet parts that are 10s of mW. This was a common problem with old class AB continuous (non-switching) amps, both vacuum tube and semiconductor, if the output stage bias circuit got wonky.  --Frank
Title: Re: How to test amps properly?
Post by: Nathan Riddle on February 05, 2021, 01:22:02 am
Welp, time to resurrect an old thread :)

Thanks Frank for the wonderful and insightful info! Definitely using some of your ideas here!

--

I'm moving forward with amp testing, I've done some minimal stuff previously, but I plan on doing a good bit of robust testing this Sat on a slew of amps.

Primarily looking at a few metrics that are important to us (all tests at 2/4/8 loads):
-FreqR, unloaded,2,4,8 ohm loads
-PhaseR, unloaded,2,4,8 ohm loads
-OutputPower (OP) vs Time (ms to 100s)
-Sine 20/1k/10k Hz OP @ 1% THD
-DIM
-InputPower (IP) + OP vs Time
-Subjective listening (subs) [4xTH118]
-Subjective listening (mains) [TD1]

In my research I had a few questions.
Mostly going off of Bink's amp comparison studies.

---

How to 'emulate' voltage sag (96vac)
-long extension cord?
-apply load at source
-variac (need a 5kw model)

Output Power Vs Time:
-Distortion limit?
-Bandwidth or sine?

Dual tone burst for intermodulation testing (DIM)
-which two tones?

Peak output power
-What is the source? (sine, bandwidth?)

Output power @ 1% THD
-which freq would you use? (I'm considering: 20hz, 1kHz, 10kHz)

---

Frank, I'm also trying to figure out your resistor padding.
I think I'm overly complicating things and considering an O-pad resistor network so the amp still sees an 8/4/2 ohm load while subs/mains get 10-30dB of gain reduction.
https://chemandy.com/calculators/matching-pi-attenuator-calculator.htm
Title: Re: How to test amps properly?
Post by: Steve M Smith on February 05, 2021, 03:12:39 am
Modern amplifiers are quite good (better than decades ago) so I wouldn't expect to easily find objective differences between them in casual testing that would represent actual real world performance.


Agreed.


I think the only differences you are likely to be able to easily detect is what they do when pushed up to and past their limits in terms of distortion, actual power output and how any automatic protectrion works.




Steve.
Title: Re: How to test amps properly?
Post by: Nathan Riddle on February 05, 2021, 08:56:48 am
No offense to anyone here, but I'd prefer answers to my questions.

I will be doing the tests with or without help.
Might as well help me use the best testing strategy.
Title: Re: How to test amps properly?
Post by: John Roberts {JR} on February 05, 2021, 10:45:46 am
No offense to anyone here, but I'd prefer answers to my questions.

I will be doing the tests with or without help.
Might as well help me use the best testing strategy.
Good luck... I have already given you my best advice.

JR
Title: Re: How to test amps properly?
Post by: Tim McCulloch on February 05, 2021, 01:08:48 pm
No offense to anyone here, but I'd prefer answers to my questions.

I will be doing the tests with or without help.
Might as well help me use the best testing strategy.

Then the fast, simple answer is "maybe."

You can thank me later.
Title: Re: How to test amps properly?
Post by: Chris Grimshaw on February 05, 2021, 01:15:00 pm
In my research I had a few questions.
Mostly going off of Bink's amp comparison studies.

---

How to 'emulate' voltage sag (96vac)
-long extension cord?
-apply load at source
-variac (need a 5kw model)


Either a long extension lead (which will give a "squishy" mains supply) or a variac (low, but more solid mains supply) would be fine. If I was running these tests, I'd use a 50m 13A (240v) extension reel, mostly because I have one to hand.

Output Power Vs Time:
-Distortion limit?
-Bandwidth or sine?

I'd say keep it at 1% THD (although the soft-clip circuits on most amps will do that for you, give or take). I'd use a sine tone, to give the amps maximum pain. After all, power vs time is a strenuous test where you're mapping the absolute maximum the amplifier can put out.

Dual tone burst for intermodulation testing (DIM)
-which two tones?

Something over a decade apart. I'd say 50Hz and 1kHz would be fine, but there are lots of examples online.

Peak output power
-What is the source? (sine, bandwidth?)

Burst sine, or maybe pink noise.
Either way, high crest factor is key.

Output power @ 1% THD
-which freq would you use? (I'm considering: 20hz, 1kHz, 10kHz)

I don't care about 20Hz for PA systems. 40Hz is a more sensible number.

Frank, I'm also trying to figure out your resistor padding.
I think I'm overly complicating things and considering an O-pad resistor network so the amp still sees an 8/4/2 ohm load while subs/mains get 10-30dB of gain reduction.
https://chemandy.com/calculators/matching-pi-attenuator-calculator.htm

Conventional wisdom states that resistance between the amplifier and speaker will result in degraded performance.

However, if you were to put a 0.22ohm resistor across the speaker terminals, then the speaker's effective driving impedance becomes 0.22ohm, which is fine. With that in place, you can put an 8/4/2ohm resistor in series, and achieve a useful amount of attenuation.

Doing it properly, you'd have 1/2, 1/4 or 1/8th Ohm resistors across the speaker for 8/4/2ohm in series. In all cases, the speaker will see 1/16th of the driving voltage, giving 24dB of attenuation.

Chris
Title: Re: How to test amps properly?
Post by: Steve-White on February 05, 2021, 01:17:45 pm

Agreed.

I think the only differences you are likely to be able to easily detect is what they do when pushed up to and past their limits in terms of distortion, actual power output and how any automatic protection works.

Steve.

Fact.  Today's amps, at the mid to high end are all similar within the price range they reside.  The Ford -vs- Chevy -vs- MOPAR days are long since gone.  Now it's as much about the business model and proper power matching to application.

What the testing essentially shows is simply where the manufacturers make their compromises in the development and execution of design relative to performance.
Title: Re: How to test amps properly?
Post by: Steve-White on February 05, 2021, 01:30:11 pm
No offense to anyone here, but I'd prefer answers to my questions.

I will be doing the tests with or without help.
Might as well help me use the best testing strategy.

Everything thus far has been a meaningful response to your questions.  Attitude is as important as content.  :)

I know a few have and would venture to guess most that have replied have the working knowledge and understanding in real world application that you are seeking - your objective is to elicit them sharing it.

The best testing strategy:  Test test test, then operate the amps in systems - that's where the most "testing" should take place.  Playback, live sound, basic PA, etc.  Gain real world race track experience to apply what you see on the dyno.  Once you have gone through a pile of broken components, both amps and drivers - you will know how to setup and operate for optimum results from what gets hauled around in the truck - which will translate into being both competitive and surviving in the real world.

Rock on brother - don't be deterred.
Title: Re: How to test amps properly?
Post by: Russell Ault on February 05, 2021, 04:39:21 pm
{...}
Frank, I'm also trying to figure out your resistor padding.
I think I'm overly complicating things and considering an O-pad resistor network so the amp still sees an 8/4/2 ohm load while subs/mains get 10-30dB of gain reduction.
https://chemandy.com/calculators/matching-pi-attenuator-calculator.htm

Perhaps I misunderstood his suggestion, but I'm pretty sure he meant using a heating element instead of a speaker, since a heating element is basically just a very-high-power resistor (which, importantly in my mind, won't exhibit much frequency-specific impedance fluctuation).

As an example, a heating element rated at 1800W at 120V is 8 ohms, two in parallel would be 4 ohms, etc.

-Russ
Title: Re: How to test amps properly?
Post by: Brian Jojade on February 05, 2021, 07:18:33 pm
What are you hoping to learn from this testing?  While there are a million specs you can test against, there are only a few that matter.

And in terms of live sound, there are 3 things that matter.

1. Does it make sound?
2. Does it sound good enough? (Both in quality and total output)
3. Is it still making sound?

Yes, you can go on and on about subtle differences in the quality of each amp, but if it fails any of those 3 tests, it doesn't matter.  Now, if you're actually engineering the amp, then more attention to detail is necessary.  As the end user though, it's mostly a waste of time.

Reality today is it would be pretty tough to find ANY amp from a reputable manufacturer that doesn't produce decent quality sound.  You may be able to see some differences with detailed measurements, but they would be pretty tough to pick out in a blind side by side test.  Note, this is comparing just the amplifier section, not DSPs that are often baked into amplifiers these days.

So, for testing purposes, the things that really matter to test are how the amp might behave in real world situations.  One of the biggest that you'll find differences is how the amps behave with crappy power.   Low voltage and voltage fluctuation are probably the most common scenario you'll run into.  Constant low voltage can be fairly easily tested with a simple variac. 

However, constant low voltage is a fairly rare scenario.  Most of the time, your meter will read an acceptable voltage, but it can swing wildly once load is applied. This can get tricky to measure reliably, as the swings can be fractions of a second.  To get an accurate graph, you need a way to map voltage over time.  Additionally, you need a way to vary that voltage both by draw of the amp and by draw of something other than the amp.  You may find different behavior depending on where voltage drop happens.

On less reputable manufacturers, you may find that they exaggerate their specs in one way or another.  Back in the day, amps used to be rated on how much power they could deliver with a full sine wave 24X7.  Reality is, in most cases amps don't ever need to do that.  They need to be able to deliver bursts of sound.  The shorter said burst needs to be, the bigger your rated power can look.  Some 'cheaper' brands take this to an extreme and the burst isn't even enough to make a single sinewave at low frequencies.

Being able to measure that would be useful.  You rarely will find that detail on a spec sheet.

The last thing that's important is that the amp keep making sound after a life of abuse on the road.  This can be much harder to assess from a brand new amp.  You can look inside and kind of guess based on general construction methods on how it might hold up, but at the same token, some high end designs have shown to develop problems young in life.  This isn't necessarily something you'd be able to test for, but would need to collect historical data to determine the life expectancy of the product.
Title: Re: How to test amps properly?
Post by: Nathan Riddle on February 05, 2021, 08:32:31 pm
Either a long extension lead (which will give a "squishy" mains supply) or a variac (low, but more solid mains supply) would be fine. If I was running these tests, I'd use a 50m 13A (240v) extension reel, mostly because I have one to hand.

That's great, I want a variac; but a 5kva variac is expensive and I didn't order it in time...
100ft 16awg @ 120vac/30A = 96v/20% perfect!
200ft 16awg @ 240vac/15A = 216v/10% not quite 20% but enough!

I'd say keep it at 1% THD (although the soft-clip circuits on most amps will do that for you, give or take). I'd use a sine tone, to give the amps maximum pain. After all, power vs time is a strenuous test where you're mapping the absolute maximum the amplifier can put out.

I'm curious about peak output power vs input voltage (basically looking at the amp's soft-limiting curves).

Something over a decade apart. I'd say 50Hz and 1kHz would be fine, but there are lots of examples online.

Thanks, after a bit more research I'll do the SMPTE RP120-1983 IMD test, 60Hz & 7kHz 4:1 ratio. Using ARTA.

Burst sine, or maybe pink noise.
Either way, high crest factor is key.

I should be able to do an EIAJ 8/32 burst easily on my scope and capture the output to see how the amps handle burst power.
Maybe I can use a 20ms burst (EIA RS-490) to see as well as that's a common spec.
But more recently 200ms burst power is making more and more sense for today's programme material.

I don't care about 20Hz for PA systems. 40Hz is a more sensible number.

Fair enough, I wanna say split the difference? But IMO 20Hz speaks about the lower limit of the amps capabilities which should speak very well for true numbers. That and it lines up with Bink's shootout tests so we can make some sort of comparison.

Conventional wisdom states that resistance between the amplifier and speaker will result in degraded performance.
However, if you were to put a 0.22ohm resistor across the speaker terminals, then the speaker's effective driving impedance becomes 0.22ohm, which is fine. With that in place, you can put an 8/4/2ohm resistor in series, and achieve a useful amount of attenuation.
Doing it properly, you'd have 1/2, 1/4 or 1/8th Ohm resistors across the speaker for 8/4/2ohm in series. In all cases, the speaker will see 1/16th of the driving voltage, giving 24dB of attenuation.

Fact.  Today's amps, at the mid to high end are all similar within the price range they reside.  The Ford -vs- Chevy -vs- MOPAR days are long since gone.  Now it's as much about the business model and proper power matching to application.

What the testing essentially shows is simply where the manufacturers make their compromises in the development and execution of design relative to performance.

I don't doubt that. IMO there's still some room to discover 'amp uses' (sub duty, mains, etc)

Everything thus far has been a meaningful response to your questions.  Attitude is as important as content.  :)

I know a few have and would venture to guess most that have replied have the working knowledge and understanding in real world application that you are seeking - your objective is to elicit them sharing it.

 :D Indeed brother!

The best testing strategy:  Test test test, then operate the amps in systems - that's where the most "testing" should take place.  Playback, live sound, basic PA, etc.  Gain real world race track experience to apply what you see on the dyno.  Once you have gone through a pile of broken components, both amps and drivers - you will know how to setup and operate for optimum results from what gets hauled around in the truck - which will translate into being both competitive and surviving in the real world.

Rock on brother - don't be deterred.

Good point, thanks!
Goal is to have a nice 'listening' portion.

Kick/Bass through subs, + load bank; running amps full tilt, see which ones get squishy. Repeat for the 'sagging' voltage line.
Same for mains, see how they do at stupid levels (and low levels, to).

Perhaps I misunderstood his suggestion, but I'm pretty sure he meant using a heating element instead of a speaker, since a heating element is basically just a very-high-power resistor (which, importantly in my mind, won't exhibit much frequency-specific impedance fluctuation).

As an example, a heating element rated at 1800W at 120V is 8 ohms, two in parallel would be 4 ohms, etc.

I'm building a load bank for 2/4/8 ohms, 27/13/13 kw respectively. 2nd channel will be loaded down with a generic 5ohm 5500w load as I don't care about asymmetric loads that much.

I think the idea is to dump most of the current into the load bank but pad the speaker down 20-30 dB so its a 'fun' listening experience.

The specific resistor network might need to be experimented with. I have some ideas with pi-pad and o-pad balanced networks. Chris's idea is good too. Not sure how to solve the 5kw .2 ohm resistor at this point.
Which is fine as I plan on spending a weekend in March doing this again.
Tomorrow is mostly getting feet wet.

What are you hoping to learn from this testing?  While there are a million specs you can test against, there are only a few that matter.

And in terms of live sound, there are 3 things that matter.

1. Does it make sound?
2. Does it sound good enough? (Both in quality and total output)
3. Is it still making sound?

Yes, you can go on and on about subtle differences in the quality of each amp, but if it fails any of those 3 tests, it doesn't matter.  Now, if you're actually engineering the amp, then more attention to detail is necessary.  As the end user though, it's mostly a waste of time.

Reality today is it would be pretty tough to find ANY amp from a reputable manufacturer that doesn't produce decent quality sound.  You may be able to see some differences with detailed measurements, but they would be pretty tough to pick out in a blind side by side test.  Note, this is comparing just the amplifier section, not DSPs that are often baked into amplifiers these days.

So, for testing purposes, the things that really matter to test are how the amp might behave in real world situations.  One of the biggest that you'll find differences is how the amps behave with crappy power.   Low voltage and voltage fluctuation are probably the most common scenario you'll run into.  Constant low voltage can be fairly easily tested with a simple variac. 

However, constant low voltage is a fairly rare scenario.  Most of the time, your meter will read an acceptable voltage, but it can swing wildly once load is applied. This can get tricky to measure reliably, as the swings can be fractions of a second.  To get an accurate graph, you need a way to map voltage over time.  Additionally, you need a way to vary that voltage both by draw of the amp and by draw of something other than the amp.  You may find different behavior depending on where voltage drop happens.

On less reputable manufacturers, you may find that they exaggerate their specs in one way or another.  Back in the day, amps used to be rated on how much power they could deliver with a full sine wave 24X7.  Reality is, in most cases amps don't ever need to do that.  They need to be able to deliver bursts of sound.  The shorter said burst needs to be, the bigger your rated power can look.  Some 'cheaper' brands take this to an extreme and the burst isn't even enough to make a single sinewave at low frequencies.

Being able to measure that would be useful.  You rarely will find that detail on a spec sheet.

The last thing that's important is that the amp keep making sound after a life of abuse on the road.  This can be much harder to assess from a brand new amp.  You can look inside and kind of guess based on general construction methods on how it might hold up, but at the same token, some high end designs have shown to develop problems young in life.  This isn't necessarily something you'd be able to test for, but would need to collect historical data to determine the life expectancy of the product.

Good point(s). It'd probably be helpful to state my goals. At this point to drive conversation. I was hoping for what others did, but I've found plenty of 'tests' in my own journey by now.

----

Since I'm doing many more installs these days I need to know how an amp behaves for various programme material in different amplifier 'brackets'.
I need a solid *cheap,* *moderate,* and *high-end* amp option.

The abundance of LF content in today's music means that the 8ms 20ms and other 'short' burst power tests aren't saying what an amp is actually capable of.

I'm impressed with the Behringer NU4-6000 amps I got from Art, but I'm curious how much different they are from a 'real' amp (now that I have a few *real* amps) to test.

----

I'm still narrowing down my 'test' options. Both what I can accomplish and what I want to accomplish. That and getting methodologies written down so the tests are repeatable (so many tiny details).

I just realized I don't have a good way to graph power vs something so I might just give voltage into known load values.

To learn.
Title: Re: How to test amps properly?
Post by: David Sturzenbecher on February 05, 2021, 08:43:33 pm
No offense to anyone here, but I'd prefer answers to my questions.

I will be doing the tests with or without help.
Might as well help me use the best testing strategy.

Have you looked at what tests are performed to get CAF (Common Amplifier Format) data? I would start there in determining which tests are relevant.
Title: Re: How to test amps properly?
Post by: Nathan Riddle on February 05, 2021, 09:06:22 pm
Have you looked at what tests are performed to get CAF (Common Amplifier Format) data? I would start there in determining which tests are relevant.

I have in the past and for whatever reason didn't think about it... Thanks!

These tests look great, and are simple to do.

I think I'd wanna go a bit lower in freq.

But they are definitely good for repeatable and objective measurements!

Thanks!
Title: Re: How to test amps properly?
Post by: John Schalk on February 07, 2021, 09:48:09 am
I had no idea that the amp charts showing plots of the outputs for a Lab, Crown, and PSoft were produced by Bink.  I've studied them several times as I feel that provide some very useful information with regard to how one would want to utilize class D amps for different duty cycles.  One of the things I have trouble with these days is deciphering what the actual output power is for new amps.  I realize that manufacturers have to use the "burst power" rating to compete with everyone else, but some have stopped listing the FTC ratings altogether making it hard to figure out just what you're buying.  My request would be for you to produce a chart like Bink's 8/4/2 ohm plot from time 0 to 100 seconds for any newer amps you test.

Note: I'm almost certainly done buying power amps so any data you produce will be for reference only, at least for me.  But don't let that stop you!
Title: Re: How to test amps properly?
Post by: John Roberts {JR} on February 07, 2021, 10:29:30 am
I had no idea that the amp charts showing plots of the outputs for a Lab, Crown, and PSoft were produced by Bink.  I've studied them several times as I feel that provide some very useful information with regard to how one would want to utilize class D amps for different duty cycles.  One of the things I have trouble with these days is deciphering what the actual output power is for new amps.  I realize that manufacturers have to use the "burst power" rating to compete with everyone else,
and because burst or short term power better resembles real world music signals, and similarly loudspeaker power handling characteristics.
Quote

but some have stopped listing the FTC ratings altogether making it hard to figure out just what you're buying.
perhaps because the FTC power rating is an archaic leftover from older amplifier technology. Class A/B amplifiers suffered worst case power dissipation at 1/3rd power, FTC instituted a 1/3rd power pre-conditioning stage to reign in inflated power claims (from the 1970s). Modern amplifier technology (like class D) exhibits completely different power dissipation behavior and different limiting constraints.
Quote
My request would be for you to produce a chart like Bink's 8/4/2 ohm plot from time 0 to 100 seconds for any newer amps you test.
This is a mature topic, and I wrote about it at length during the original power amp test rodeo performed by Bink (Michael Knowles). Short answer, there is no single power/duration metric that can provide all things to all people.
Quote
Note: I'm almost certainly done buying power amps so any data you produce will be for reference only, at least for me.  But don't let that stop you!
I am mildly irritated by the implication that amp manufacturers cheat or lie about amplifier performance specifications, while over the years some probably have. It was so bad back in the 70s that the FTC imposed regulations, since then not so much. A better use of this community (IMO) is to ask members about their experiences with new amp models to identify the few snakes in the woodpile (if any). Modern technology has made amplifier design easier, I wish we had these class D chip sets back when I was in the trenches.

JR
Title: Re: How to test amps properly?
Post by: Helge A Bentsen on February 07, 2021, 11:30:00 am
FWIW burst ratings seem to correlate pretty well with how a modern amp performs when listening to a system. Replaced an old iron amp recently with a new class D for a subwoofer application.
The old iron was specced at 225W/channel, the new one is 80W/channel/200W burst.
The new amp sounds better and goes as loud as the old amp, it's definitely putting out more than 80W when it's pushed with music.
I matched gains on new VS old relative to the mains.
Title: Re: How to test amps properly?
Post by: John Roberts {JR} on February 08, 2021, 10:04:40 am
FWIW burst ratings seem to correlate pretty well with how a modern amp performs when listening to a system. Replaced an old iron amp recently with a new class D for a subwoofer application.
The old iron was specced at 225W/channel, the new one is 80W/channel/200W burst.
The new amp sounds better and goes as loud as the old amp, it's definitely putting out more than 80W when it's pushed with music.
I matched gains on new VS old relative to the mains.
+1...

This is TMI but back in the late 80s at Peavey I designed a small audio amplifier with a trick power supply (PMA70+), without getting into too many details the amplifier could put out 1x continuous voltage (35W) 24x7, 2x voltage or 4x power (>100W) transiently for tens of mSec, and 50-60W for tens of seconds. As I expected it sounded more like a 100W amp than 35W, while being sized and costing like 35W.

 I had grand plans to design an entire series delivering progressively more transient power but sadly the technology didn't scale up cost effectively. The expensive high power components were still needed, and the savings in transformer/heat sink components didn't create enough of a value proposition.
======

Of course modern class D technology is a different ballgame with dramatically improved thermal performance. Of course class D has been around a long time (Peavey was making class D amps before I started there in the mid 80s). So while class D is mature the semiconductor industry has stepped up in the decades since then to deliver high voltage at high current switching devices that were only a wet dream back then. I suspect there are also canned class D controller chip sets available now, but I have been out of those trenches for years.   

Of course junior engineers can still screw up canned designs but it appears that modern technology is quite good.

JR
Title: Re: How to test amps properly?
Post by: Tim McCulloch on February 08, 2021, 02:44:04 pm
So, Nathan-

What are you measuring to find?  What parameters affecting the electrical or thermal characteristics are you trying prove or disprove as having a causal affect on the output signal?

This is like Smaart - you can measure lots of things but determining where and how one is taking the measure is determined by the "why" of the measurement -  what is being done with the results of the measurement.

We can measure a lot of factors in a power amplifier in use or under test, but what does any or all of them have to do with the electricity delivered to the loudspeaker?