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Analog Disovery as Audio Analyzer
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jaxbird:

--- Quote from: mtdoc on December 26, 2015, 04:33:17 am ---Excellent Jaxbird. 

I have no immediate use for this but it will be fun to play and learn with and it's great to see someone making use of the ADs full potential.  I think it's a remarkable little piece of hardware.

Merry Christmas to you!

--- End quote ---

Many thanks, Merry Christmas to you and your loved ones.

You can use this with any kind of amplifier, from simple opamps to high power amplifiers and tubes etc. It's really just something I felt was missing from the official software provided with the analog discovery. Just to make it easy for everyone to obtain common amplifier performance parameters.

jaxbird:
I hope everyone got some understanding that working on this project has been delaying my promised release of the Analog Discovery Impedance Analyzer (arguably a much more generally valuable product) I know I promised a swift release, apologies about the delays, I will release very soon. It is just a lot more complex than the audio analyzer, so I think the audio analyzer is a good way to test the deployment.

https://www.eevblog.com/forum/projects/impedance-analyzer-build-and-experiments/135/

Mark:
Hi jaxbird, I just downloaded your software and look forward to using it, I have found the Analog Discovery to be a most useful box of tricks since getting one for Christmas last year.  I am currently working on an amplifier so I will have a chance to try out your software next week (when I repair the amp... I released the magic smoke!).  It's a 50W amp running up to 500kHz. 

Thank you, Merry Christmas and a Happy New Year!
Zero999:

--- Quote from: jaxbird on December 18, 2015, 07:48:12 am ---
--- Quote from: bson on December 15, 2015, 09:36:58 am ---
--- Quote from: jaxbird on December 15, 2015, 06:26:25 am ---In essence what you are saying is that there is no point in standardized testing of amplifiers as using resistive loads are not equal to speaker loads.

--- End quote ---
Yes.  Did you follow the suggestion to run a simulation with different kinds of loads?

--- End quote ---

You seem persistent, so let's do a little experiment, instead of a simulation, to find out how much difference there really is and what it looks like.

Here is a comparison between using a typical small 2 way speaker with an impedance between 5 Ohm and 20 Ohm, compared to a 6 Ohm resistive load.

2 Way speaker's impedance for reference:



6 Ohm load resistor's impedance: (not very exciting)



Distortion measurement with Speaker load:



Distortion measurement with Resistive load:





The only difference here is that the amplifiers distortion is actually overall lower with the speaker load ;) Especially at the frequencies where the speakers impedance is higher.

--- End quote ---
Just a couple of questions.

Presumably those plots were taken from the voltage on the amplifier output, not from the sound from the speaker.

If the speaker's impedance varies at different frequencies then doesn't the power dissipation and thus output vary significantly?

If you plotted the real power dissipated in the speaker, you'll probably find it varies hugely as the frequency is changed. Does that mean the output will be distorted by the same factor?
nfmax:

--- Quote from: Hero999 on December 30, 2015, 06:32:20 pm ---Just a couple of questions.

Presumably those plots were taken from the voltage on the amplifier output, not from the sound from the speaker.

If the speaker's impedance varies at different frequencies then doesn't the power dissipation and thus output vary significantly?

If you plotted the real power dissipated in the speaker, you'll probably find it varies hugely as the frequency is changed. Does that mean the output will be distorted by the same factor?

--- End quote ---
A1: Yes, the power supplied by the amplifier to the speaker does vary with frequency, a lot. Importantly, it can be reactive power: a capacitive or inductive speaker impedance will simply sling the power right back into the amplifier (which must dissipate it, as heat).

A2: The sound output from the speaker does not vary nearly so much. Speakers are designed so that the sound output (usually measured as sound pressure level, on-axis and 1m from the speaker) is proportional to the voltage on its input terminals. The transduction efficiency, and hence the current drawn from the amplifier, varies with frequency, but this should ideally not affect the output SPL. Usually the variation with frequency depends on the resonant frequency of the various drive units and enclosures within the speaker. Transduction becomes more efficient close to a resonance - though resonances bring problems of their own. The directivity of the speaker also varies with frequency: this may show up in the impedance curve, but even if it does not, varying directivity with frequency means the off-axis frequency response may be anything but flat!

All these effects are, in principle, linear and so do not constitute harmonic distortion. Speaker distortion is usually much worse than that of the amplifier, maybe around 1%, though the very best electrostatic speakers (like the Quad ESL63s I'm listening to now) are down around 0.1%.

Max
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