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Author Topic: How to test amps properly?  (Read 2555 times)

John Roberts {JR}

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Re: How to test amps properly?
« Reply #10 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
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Tim McCulloch

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Re: How to test amps properly?
« Reply #11 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.
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Chris Grimshaw

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Re: How to test amps properly?
« Reply #12 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
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Steve-White

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Re: How to test amps properly?
« Reply #13 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.
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Steve-White

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Re: How to test amps properly?
« Reply #14 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.
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Russell Ault

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Re: How to test amps properly?
« Reply #15 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
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Brian Jojade

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Re: How to test amps properly?
« Reply #16 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.
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Brian Jojade

Nathan Riddle

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Re: How to test amps properly?
« Reply #17 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.
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David Sturzenbecher

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Re: How to test amps properly?
« Reply #18 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.
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Nathan Riddle

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Re: How to test amps properly?
« Reply #19 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!
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ProSoundWeb Community

Re: How to test amps properly?
« Reply #19 on: February 05, 2021, 09:06:22 pm »


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