[stv014]

The impedance utility is now included in the package. It may still need some improvements, though.

 

[aos]

Digging an old thread but that's exactly what I just found for myself and googling led me to this thread. I mean, RMAA seems to subtract 6dB from channel crosstalk results (making it look much better than it is). I confirmed this on 2 different devices (one amp and the other dac+amp) and using different true RMS measurement instruments. That's a pretty strange mistake to make, ins't crosstalk in dB just one channel's RMS voltage divided by other, then you take a decade logarithm and multiply by 20? It's the simplest measurement of them all and since you need ratio, not absolute values, it can be calculated precisely with a soundcard (assuming both voltages are not too high and too low).

 

[stv014]

  I mean, RMAA seems to subtract 6dB from channel crosstalk results (making it look much better than it is).

 
This problem has already been discovered a while ago, and is mentioned for example in this older post. Apparently, they still did not fix the bug after 2.5 years then. There are (or were, if already fixed, I have not used RMAA recently) also issues with how the A-weighted noise level is calculated, again making the measured device look a couple dB better than it really is.

 

[aos]

Yes, that is the post I was referring to. I wasn't able to find anything else on the internet.
 
THD is much harder to measure, never mind at the performance level that a sound card or a DAC is capable of. But there is a possibility that your mismatch was caused by something else. RMAA will give measurements that are, just like you say, 2-2.5dB different from each other, by simply setting your level differently when "calibrating" at the beginning. It will accept down to -3.5dB or so but it asks of you to set it to -1dB. If you set it to -3dB instead, which I like doing to reduce distortion, then you will notice your results are better by about the amount of the difference between your level and -1dB. So they don't compensate for that.

 

[stv014]

THD is much harder to measure, never mind at the performance level that a sound card or a DAC is capable of. But there is a possibility that your mismatch was caused by something else. RMAA will give measurements that are, just like you say, 2-2.5dB different from each other, by simply setting your level differently when "calibrating" at the beginning. It will accept down to -3.5dB or so but it asks of you to set it to -1dB. If you set it to -3dB instead, which I like doing to reduce distortion, then you will notice your results are better by about the amount of the difference between your level and -1dB. So they don't compensate for that.

 
I was not referring to THD measurements, but rather noise and dynamic range. It can be tested easily by using the "Generate WAV" function of RMAA in 44.1 kHz/24-bit format, then adding noise to the WAV file (for example by reducing its effective resolution to 16 bits with TPDF dither using this utility), and analyzing the modified WAV with RMAA. Other measurements, such as crosstalk, can be tested with the same generate->process->analyze method.
 
In the case of the dithered test signal, RMAA 6.4.1 prints these values:
 
Noise RMS:      -96.7    -96.8  dB
Noise RMS (A):  -97.8    -97.8  dBA
Peak value:     -90.3    -90.3  dBFS
 
The peak level is correct, but the noise level should be -93.3 dB unweighted and -95.7 dB A-weighted, or -93.7 and -95.8 with a 20 kHz bandwidth. These are referenced to a 0 dBFS sine wave, which is what one would expect RMAA to use as the reference as well, especially since 0 dB on the graphs corresponds to 0 dBFS in the WAV file. In any case, a lower reference level would make the measured noise levels worse, not better. I think what RMAA really prints as "Noise RMS" is relative to a 0 dBFS square wave, that is why it is lower by 3 dB. That is not necessarily a problem, but this fact could have been made clear by the program. On the other hand, "Noise RMS (A)" was probably meant to be referenced to a full scale sine wave, but the A-weighting filter in RMAA is not correctly normalized to have 0 dB gain at 1 kHz; that explains the 2 dB difference.

 

[Greenears]

@estreeter  I don't really have a dog in this thread hunt, so please forgive me for going back to the OP.  Just FYI I was involved in Audio (and Video) testing for about a year of my career in the late 90s so I do understand and appreciate what you are trying to do, and I think it's all way cool.  And of course, you have gotten @stv014 going so I was just reading the posts and the elegance of his solutions as an admirer of good engineering as always even though I don't plan to implement (we have a small @stv014 fan club going on some other threads). 
 
Be that as it may .... my question for @estreeter is what are you trying to accomplish by measuring the delivered power and impedance of various amps? And for what purpose & application?  Just curious.  I'm wondering if there may be a more elegant or simpler way of accomplishing your goals if we have an idea of how you are going to use this.  Or if you're just tinkering then that is just fine too. 
 
By the way completely off-threadI just visited Thailand (home of @estreeter last month and I wish I could be back there and spend a lot more time it is a really nice place to visit and I recommend it to all.