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Investigating amplifier sound quality


Kvalsvoll

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This is something I have though about a long time now - finding a method to reliably verify sound quality differences in power amplifiers.

Finally I have set up the necessary instrumentation to try out the method in practical experiments.

 

Today good sound is easy to achive, most of the audio playback chain can be made sonically transparent with relatively little effort and money spent. 

However, through building and designing amplifiers myself, and measuring and listening to many different amplifiers, I have got this controversial idea that power amplifiers can actually sound different.

Obviously they sound different if you push beyond power limits, and also amplfiers can be made so good that they are transparent for any source material.

But where is the limit, how good does it have to be, and what measurements can reveal the transparent ones from the only-good ones.

 

The problem with listening tests for verification is that it is time consuming, unreliable and difficult to do proper blind testing of amplifiers.

If you could test using a software abx-tester, like the one in foobar, the task would become easier.

 

By recording the output signal from the amplifier while playing at decent volume, you get a sample file that can be compared to the original in an abx software player, and you can do the comparison on any playback system - headphones, speakers.

The playback will of course be affected by the amplifier in use for playback, but considering descriptions of the seemingly huge sonic differences described in reviews, it should not be a problem to hear at least some of the sound characteristics from the test object.  

 

After rigging the recording system, the first task is to verify that the recorder is sonically transparent, or at least much better than the amplifiers I want to test.

The first recorder system is made easy and practical, thought I could test this first, if it fails I have some options to improve it if necessary.

 

I recorded some samples from the output going to the amplifier, so that I can use abx to verify if there is an audible difference from the original sample to the recorded sample - with no amplifier in the loop.

I also took one of the samples and sent it through this loop 5 times, as I suspected the differences would be too small to be detected.

If the 5x loop sample still sounds good, then the recorder can be assumed to be good enough for the purpose.

 

After several attempts trying to get an abx with positive outcome, I conclude I am not able to hear any difference from the original and the 5x looped sample.

I used two different set-ups with different speakers and amplifiers, and I used headphones.

 

It might be interesting to observe that the recording loop includes the DAC and pre in a commercial avr.

 

To test that the method actually can detect differences in sound quality I encoded 128K mp3-samples and compared those to the original.

All mp3-files testet positive.

 

I recorded 2 amplifiers, they are quite different in topology and also measures different, though both measure very good on parameters assumed to have significance for sound quality.

The problem now is that I have a very hard time trying to hear any difference between any of the amplifiers and the original sample.

Tomorrow I may do a recording of an amplifier that measures worse, perhaps that one is bad enough to be detectable. 

 

If there is interest in this, I can put up the samples on my web site and provide links, so that you can try it yourself.

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The system used for playback and verification through abx is not a part of the chain creating the recorded samples, and therefore does not need any verification, other than simply being good enough to reveal differences.

 

A good starting point would be to test original vs mp3, if the playback does not reveal differences here, it will certainly not be realistic to hear differences in amplifiers.

Also remember that the listener is part of the playback verification system, and tests like this will require some training with abx-testing before you are able to get useful results, I always find the mp3 tests to be more difficult than expected, it requires focus and attention to be able to get a full score.

 

Frequency response is not a problem in any part of this, all pre and dacs are flat.

In the recording part, noise is a problem, and nonlinear distortion can also be a problem.

The instrumentation I used now has slightly better noise floor than a 16-bit signal, worse than a 24-bit, all nonlinearities are sufficiently low.

I verify all this when I make the recording, by playing test signals, so that I know I have all settings and levels right.

 

The point of this method is that it enables you to listen to different amplifiers on your own system, or any system, that can play a digital sound file.

If you use a computer not set up for high quality audio playback, such as using the on-board analog sound output, it may not be good enough.

But the only thing required for making a computer into a top class digital source is a dac.

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So far, I can not say I have a good success rate in telling the amplifiers apart from the original.

 

I can not identify the 5x loop-test from the original, I thought, today, yes, now I have it, there is a small difference.. then the result shows 2 correct out of 10.

 

But I was able to get a positive result for 2 amplifiers, one good commercial and the other is the C15, I can also distinguish between the amplifiers.

But that is because there is a small amount of noise from the amps, and the C15 has less noise.

To detect this, I had to listen to a sample before the music actually starts, and raise the mv to 0 dB on the commercial and +3db on the C15.

The recording is done at mv -6dB, so this will raise the noise up to barely noticeable.

The C15 was difficult even at +3dB.

 

I need to find a really bad amplifier.

 

I think I will make an article on this, and prepare the samples so that you can download them and try.

Doing abx testing requires attention and patience, I may not have the patience required for this.

 

Maybe I should post this on one of the audiophile forums, so that someone with golden ears can prove there is a difference.

Or maybe not, as I suspect this could cause quite a lot of anger in those communities..

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Well, I think we already knew that differences in amplifiers are very, very small, and if the amplifier measures good enough, it will be completely transparent - like wire with gain.

(Don't try to discuss this with the a-f crowd, because in that world even wires have huge sonic differences, like "adding an octave exetension to the bass" just by changing some copper leads..)

 

My quest here is to determine where this limit is, and what measurements can quickly reveal potential problems.

 

Some 30 years ago I had finished my first all-my-own amplifier design, with cascoded voltage gain stage, exotic tranisistors and all the feedback done according to the audiophile standard.

To my surprise, to me it sounded exactly like the amp boards they replaced.

I concluded I don't have those golden ears, after all, those experts in the magasines describing in detail how the different amplifiers sounded were much older and more experienced.

 

Now I am not so sure, I think there may be a difference, on the right source material.

I expect this difference to be relevant only for acoustic instruments and voices, if you play InfectedcMushroom any amplifier with sufficient power will do.

To be able to tell for sure, it is necessary to do a double-blind test, and that is what this method does - you can compare original sample to the sound sampled from the amplifier output, driving a loudspeaker at decent volume.

 

So far the results for the test equipment with the 5-times loop-through should be interesting for many of us, as it actually confirms that a decent avr dac+pre is completely transparent in sound - it is not possible to detect any difference compared to the original sample.

 

This will of course not change anything in the audiophile world, those who already think amplifiers are not important will agree (at least until I actually manage to find a difference), and those who think "the sound stage is much deeper" will never follow or believe my logical and scientific approach.

 

Loudspeakers and room acoustics are of course where the real battle is, and where the resources must be put if getting the best sound is the goal.

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I uploaded sample files to the web site, they should be accessible by typing this link:

www.kvalsvoll.com/Articles/abx/

 

They are tagged with a very brief descriptive title.

Start by looking at the tuttabella_test files,

Do an abx of the mp3 - it is harder than you think..

 

I have not yet managed to get a positive result for the music samples with amplfiers.

I made the im_* test signals to see if some very nasty test signals works better.

And they do - I can verify a difference between amplfiers and original for those signals, which is interesting, because that indicates a possibility for audible differences in amplifiers. 

 

I will eventually do an article, which decribes the different file samples, the test method and how this is done.

Until then you will have to figure it out from the sample files titels, and do a little creative guesswork.

 

Basically, the sample files are music and signal samples, in original version and sampled from the loudspeaker output of the amplifiers.

Amp 1 is a very good commercial amplifier,

Amp 2 is a low-budget avr,

C15 is a 15W design by me.

 

 

WARNING:

Some of the files contain signals that may or may not be audible - DO NOT TURN UP THE VOLUME BEYOND NORMAL LOUD LISTENING LEVEL.

You may end up destroying tweeters or amplifiers, be careful.

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So far I have not been able to confirm an audible difference between the original (music) sample and any of the tested amplifiers.

 

And so far I have none confirmed from others that may have tried to listen to the samples, though I suspect very few have actually tried, since I have not published this other than posted here and at the local nisseforum.

 

Which means it is safe to assume a reasonably good power amplfier is sonically transparent provided it has sufficient power.

And this is good, because then resources can be used where it really matters and makes a true difference.

This is really nothing new or unexpected, other studies, though done differently, have all come down to the same conclusion.

 

But amplifiers and processors/avrs can have other far more important issues, such as what maxmercy found when investigating how processors handle bass management:

http://data-bass.ipbhost.com/index.php?/topic/352-measuring-equipment-for-71-ulf-potential/#entry5849

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