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post #1 of 1370 Old 11-28-2014, 05:35 PM - Thread Starter
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Measuring Amplifiers

I'm setting up to do some amplifier measurements. I think it would be great to have measurements taken like Langston Holland has performed in the past. Seems like there hasn't been any consistent measurements being taken in some time since Chasw98 stopped.

I have a scope to measure peak voltage, and in turn, power. I have parts coming to setup a resistive load bank.

Unfortunately, my scope is not the greatest and doesn't provide THD numbers. I'll be using it to measure max output, but I'm thinking of using ARTA or REW to measure distortion with stepped sine waves. If anyone has experience with this, please share. I may get a nicer scope before starting this. Update: looking at Rigol DS1074Z, upgradable to DS1104Z via software.

I'll be setting up a webpage sort of like data-bass to compare and graph various results. I'll also be outlining the test procedure, so if anyone else with comparable equipment wants to take measurements, there will be a standardized procedure.

The first data collected will be CEA2010 bursts at various frequencies, I'm thinking: 5,10,20,30,40,80,100,200,500,1000,2000,5000,10000, 15000,20000 hz

I'll capture the input and output waveform until distortion becomes significant and use this as peak voltage. From there, wattage can be calculated with with Vrms^2/R.

The next tests would be sine sweeps at various levels to measure FR and THD. This is where I think REW or ARTA will be needed with a voltage divider. Any suggestions for a good low noise audio interface to use for this? Not looking to break the bank here. Update: Looking at the Scarlett 2i2 for now. Seems to measure well.

Ideally I'd like to perform these tests at 2, 4, and 8 ohms, with one channel, both channels, and bridged if possible. Once I get my heating elements in I'll see how I can wire them up to attain this.

I'll also post hi res pictures of the amps internals. It should be a nice repository if I get it going.

Also going to measure idle current draw, noise at 1 foot, and possibly RF noise.

I have a few amps to start testing with to get the procedures ironed out and the website built, then I'll start testing amps for anyone who wants.

If anyone has any feedback, I'd like to hear.

UPDATE 2014-12-02
Got my heating elements. About 13.4 ohms each. Impedance in parallel should be about 4.45 ohms.


UPDATE 2014-12-03
Got my audio interface. Scarlett 2i2. Measures well. I should be able to sufficiently test amps with this.


Sweep, nice and flat..


0.0004% THD



Intermodular distortion nice and low


Waveform from IMD test


Square waves


CEA 2010 burst


Took some pics...












Last edited by notnyt; 12-18-2014 at 04:28 AM.
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post #2 of 1370 Old 11-28-2014, 06:23 PM
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Excellent! I always love reading how actual measurements compare to a manufacturer's rated specs.

Somebody on PE TT had measured the output of the Crown XLS DriveCore amps, which I use now. I can't remember specifics, but I seem to recall that the rated specs were very close to the measured specs. I also think they stated that below 20 Hz the signal dropped very sharply so that one wouldn't need a subsonic filter when used with subs. I may be way off, but that's what I seem to remember.


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post #3 of 1370 Old 11-28-2014, 06:41 PM
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Start a kick starter for one of these; http://www.wccaraudio.com/smd-amp-dyno-ad-1.html I am sure you could get enough people diy and pro audio to pitch in.
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post #4 of 1370 Old 11-28-2014, 07:30 PM
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Not, simply use a pot or switched resistive divider and take the output of that to your soundcard. You'll need the level reduction as the S/C will only accept a volt or two and use REW or ARTA etc to measure the THD. THD is a relative measurement so absolute levels are not that important. Use a good SC; the M Audio Audiophile 192 has excellent measured specs, but I'm unsure of much newer gear as I haven't look into it that closely in recent times.

Use a good wideband meter like a Fluke 287 if you can borrow or steal one to measure the voltage as accurately as possible. There is also no such thing as peak power, so publishing number based upon the Vpeak won't give anything useful. Use Vpeak to calculate the Vrms and use that to calculate the power.

Pete Millett also sells a neat SC interface kit that is well designed which you may be interested in. I will most likely spring for one in the NY as I'll be doing another major round of testing next year as I have a ton of gear under construction which I'll need to test thoroughly

I've done this before many times, so PM me if you need any help.
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post #5 of 1370 Old 11-28-2014, 09:19 PM - Thread Starter
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Quote:
Originally Posted by Manic1! View Post
Start a kick starter for one of these; http://www.wccaraudio.com/smd-amp-dyno-ad-1.html I am sure you could get enough people diy and pro audio to pitch in.
The point is to not have magic box spitting out measurements. I want to actually capture waveforms. I don't think I'll need any more equipment, my scope should do fine. For THD I'll just need to get a decent audio interface.
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post #6 of 1370 Old 11-28-2014, 09:35 PM - Thread Starter
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Quote:
Originally Posted by A9X-308 View Post
Not, simply use a pot or switched resistive divider and take the output of that to your soundcard. You'll need the level reduction as the S/C will only accept a volt or two and use REW or ARTA etc to measure the THD. THD is a relative measurement so absolute levels are not that important. Use a good SC; the M Audio Audiophile 192 has excellent measured specs, but I'm unsure of much newer gear as I haven't look into it that closely in recent times.

Use a good wideband meter like a Fluke 287 if you can borrow or steal one to measure the voltage as accurately as possible. There is also no such thing as peak power, so publishing number based upon the Vpeak won't give anything useful. Use Vpeak to calculate the Vrms and use that to calculate the power.

Pete Millett also sells a neat SC interface kit that is well designed which you may be interested in. I will most likely spring for one in the NY as I'll be doing another major round of testing next year as I have a ton of gear under construction which I'll need to test thoroughly

I've done this before many times, so PM me if you need any help.
M Audio Audiophile 192 sounds good, I'll look into it. I have a scope to do voltage measurements so I can trigger at the burst and capture multiple channels for comparison.

The only part I'm not sure on now is how to calculate THD for the burst tests.
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post #7 of 1370 Old 11-28-2014, 09:42 PM
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Fantastic!!!!!!!

Is your quantity of measurement frequency points too many? Is there value in having the frequency points so close?

An octave is a doubling of frequency right? So what if you did intervals of 4x instead of just 2x? IE:

5Hz
20Hz
80Hz
320Hz
1280Hz
5120Hz
20480Hz

Even that maybe too many datapoints --- not sure how linear the data will be? I suppose if you measure a few amps and it seems very linear between capture points you can decrease the capture points.

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post #8 of 1370 Old 11-28-2014, 10:05 PM
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I'd cut down on the number of test frequencies in the middle and focus on the extremes, (5, 10, 20, 40, 1k, 10k, 20k) I believe that is where you are going to see any major drop-offs in power. I believe most amps will be fairly linear between 100hz-10k.


This sounds like it could be pretty neat if everything works out.
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post #9 of 1370 Old 11-28-2014, 10:52 PM - Thread Starter
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Quote:
Originally Posted by mtg90 View Post
I'd cut down on the number of test frequencies in the middle and focus on the extremes, (5, 10, 20, 40, 1k, 10k, 20k) I believe that is where you are going to see any major drop-offs in power. I believe most amps will be fairly linear between 100hz-10k.


This sounds like it could be pretty neat if everything works out.
I was thinking that, but with proper workflow setup, it shouldn't be that difficult to take measurements at different frequencies.
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post #10 of 1370 Old 11-28-2014, 11:20 PM - Thread Starter
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I'll probably use the REW CEA 2010 burst sample, which looks like this at 100hz



And at 10hz..
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post #11 of 1370 Old 11-28-2014, 11:41 PM - Thread Starter
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This is the crappy onboard soundcard from my PC I'm using for testing.. Interesting how bad it is...

5000hz square wave


1000hz square wave


100hz square wave


10hz square wave
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post #12 of 1370 Old 11-29-2014, 02:09 AM
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post #13 of 1370 Old 11-29-2014, 02:16 AM - Thread Starter
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So I think I want to use a USB audio interface so I can test with whatever laptop or small form factor computers I have in my shop. Any suggestions?
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post #14 of 1370 Old 11-29-2014, 02:47 AM - Thread Starter
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Looking at the scarlett 2i2 for audio interface. Would pref something cheaper, but don't see anything that measures better for the price. The balanced io is necessary here.
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post #15 of 1370 Old 11-29-2014, 04:15 AM - Thread Starter
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DS1074Z and 2i2 ordered... now I just need to assemble a voltage divider and get some TRS and TS cables, as I think everything I have is XLR.
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post #16 of 1370 Old 11-29-2014, 07:00 PM
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What O-scope do you have?

I was just getting ready to buy a used one in the next month before I build a D-class amp. Saw a few 1Mhz O-scopes I could build but lots of things to buy so on the back burner for now.
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post #17 of 1370 Old 11-29-2014, 07:07 PM
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What happened to the old thread of the same name?

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post #18 of 1370 Old 11-29-2014, 07:29 PM
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Hey Nyt, good to see you stepping up for this. The website is a good idea, I'll go there for sure. I saw one of those 100X probes in action recently for an O-scope. This thing is goofy looking, right?

One thing that I am curious about even more than THD is how long it takes each amp to recover from going into clipping. I've read some stuff on ESP's website and it mentioned that every amp recovers from clipping a bit differently. Maybe drive the amp into clipping with a sine wave and drop the volume down suddenly to it's max point and capture the waveform? Maybe this test would expose insufficient power supplies.

The DS1074Z looks nice man. Will you take any video footage of the scope part? Amplifier porn.

What amp will be your first victim?
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post #19 of 1370 Old 11-29-2014, 08:04 PM
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This should be good! Looking forward to it!

It seems amp testers last about 3 years and then burn out....

My cheap arse is even willing to throw a few bucks into the testing to see the inukes and clones tested....

It would help me a great deal if there were a few summaries of what all of this means and how it relates to real world HT sub-woofer usage and the rest of the frequency range. How long a voltage needs to be maintained for 99% of music and HT explosions, etc. would help.

There seems to be a great deal of confusion as to what 'Peak', 'sustained', and 'average' power really means. Modern amps seem to have several levels of cutting back draw from the mains to keep from tripping the main power supply. How does that impact voltage and performance in real world usage?
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post #20 of 1370 Old 11-29-2014, 08:19 PM - Thread Starter
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Quote:
Originally Posted by chrapladm View Post
What O-scope do you have?

I was just getting ready to buy a used one in the next month before I build a D-class amp. Saw a few 1Mhz O-scopes I could build but lots of things to buy so on the back burner for now.
I have a USB hanntek 6022be. It's not so good. Got a DS1074Z (upgrading to DS1104Z) coming.

Quote:
Originally Posted by Shreds View Post
Hey Nyt, good to see you stepping up for this. The website is a good idea, I'll go there for sure. I saw one of those 100X probes in action recently for an O-scope. This thing is goofy looking, right?

One thing that I am curious about even more than THD is how long it takes each amp to recover from going into clipping. I've read some stuff on ESP's website and it mentioned that every amp recovers from clipping a bit differently. Maybe drive the amp into clipping with a sine wave and drop the volume down suddenly to it's max point and capture the waveform? Maybe this test would expose insufficient power supplies.

The DS1074Z looks nice man. Will you take any video footage of the scope part? Amplifier porn.

What amp will be your first victim?
Sure, will take some vid. NU1000DSP first on the block since its already in my shop.
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post #21 of 1370 Old 11-29-2014, 08:19 PM - Thread Starter
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Originally Posted by jpmst3 View Post
What happened to the old thread of the same name?
Crap, didn't realize it was the same name :/
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post #22 of 1370 Old 11-30-2014, 12:51 AM
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Measuring Amplifiers 2: Electric Boogaloo
Lonely Raven and Thatsnasty like this.
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post #23 of 1370 Old 11-30-2014, 02:24 AM
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What amps do you have on hand to test and which ones are you planning to test?

Are you going to test "long term" output, sustained for 4-6 seconds?
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post #24 of 1370 Old 11-30-2014, 02:30 AM - Thread Starter
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Quote:
Originally Posted by Decadent_Spectre View Post
What amps do you have on hand to test and which ones are you planning to test?

Are you going to test "long term" output, sustained for 4-6 seconds?
Maybe, but this isn't as interesting, after 4 seconds of sine wave testing all amps will be producing max current draw from the wall * the amps efficiency.
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post #25 of 1370 Old 11-30-2014, 02:40 AM
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After a certain time, but SP claims 4 seconds of 6000W output as I recall. I would be interested to see if their claims are true and also see what other amps can do long term. Many times the amps do limit the current as the time increases.
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post #26 of 1370 Old 11-30-2014, 05:02 PM
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Quote:
Originally Posted by notnyt View Post
Crap, didn't realize it was the same name :/

It used to be a sticky too. I am not sure what the deal is there.

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post #27 of 1370 Old 11-30-2014, 05:56 PM
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-40db is 1% THD.

If you can figure out the difference in db's, you can calculate each odd/even harmonic's THD with this:
http://www.sengpielaudio.com/calculator-thd.htm

The 2nd and 3rd harmonic calculated manually should be enough because
every amp I've measured and every amp measurement I've ever seen
shows that once 1-5% THD is reached then the amp is basically at it's limits.

I too would agree to skip 500, 2khz and 5khz.

1khz is sufficient.
Most amplifiers don't rolloff -3db until >10khz and <60hz.

-3db is when the voltage or current (or both) drops by 70.7% (50% is -6db FYI)
So 50V @ 4ohms is 625watts, and 312.5watts is 35.35V @ 4ohms, 156.25watts is 25V @ 4ohms.



I've measured all of my amplifiers under no-load conditions, idle mains draw, and inrush spike. (Still don't have a load bank to test ultimate-watts.)

The Chas's original thread can be found here:
Measuring Amplifiers

The light-hearted amplifier tests I did can be found here:
BassThatHz, Fluke 43B O-Scope Adventures, Thread

The EP4000 is a well known work-horse, and can be used to gage the accuracy of your voltmeter and ammeter.
Its output rails clip at about 80v RMS.


No idea how many amperes it outputs.

My Fluke43B is pretty cool. It's not as accurate as AudioPrecission (seems to have noisefloor of 0.5%THD or a few microvolts at idle.) but it has a Sag/Swell function that I've used to calculate Max/Min and Average power.
This was one LMS @ 4ohms outputing about 100db of musical-power.


This was about 18watts average, 90watts peak.
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post #28 of 1370 Old 11-30-2014, 05:58 PM
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post #29 of 1370 Old 11-30-2014, 06:43 PM
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Quote:
Originally Posted by notnyt View Post
This is the crappy onboard soundcard from my PC I'm using for testing.. Interesting how bad it is...
That doesn't necessarily mean that it is bad. Square waves are a very harsh thing to produce or measure.

Go download Rightmark Audio Analyzer and post your loopback results.

Here is my Oppo 105 looping back into my Realtek ALC892 (Asus P9X79 motherboard), which is my loopback.


THD:


Noisefloor

(Seems to match the 60hz buzz and NF from my Emo Mic-1 rew graphs. Maybe it isn't the mic. hrmm....)

IMD:


FR:


Crosstalk:


You only need about -50db to calculate THD.

-96db is CD quality. I'm getting just over 100db like that, which is <0.001% THD.
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post #30 of 1370 Old 11-30-2014, 06:45 PM - Thread Starter
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I'll take more measurements when I get the 2i2. Don't care about that sound card, was just testing.
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