Please see my "Understanding FFT Diagrams for Beginners" article if you need help in understanding the spectrums.
WinAudioMLS is a very good tool for high-precision measurements of an audio amplifier's key characteristics, such as THD at different frequencies and amplitudes, IMD, harmonic profile assessment - whatever can be assessed by looking at the FFT-based spectrum diagrams.
The first picture outlines the overall setup of the windows, used during the measurement process. Main window on the left, THD gauge on top of it, measurement pad at the top-right, two generators at the bottom right (I have them overlapped as I've got not too much space on the screen of the PC in my lab.
The picture above shows the File menu of the application.
The important thing - very useful - you can save the overall configuration and then "recall" it with a single click at the item in the list of the recently saved configurations - see the item "1 lenovo.wst" at the bottom of the menu - that's the one I normally use.
The first thing you need to do is to setup the ASIO interface (ASIO -> Configure) and instruct the application using it (ASIO -> select ASIO).
This is what the ASIO Settings dialog looks like.
Select the right Device, check / set the Channel Assignment, click "device control" button to see the ASIO driver's parameters screen.
OK for saving the settings, then make sure you have "select ASIO" ticked at the top of the ASIO menu.
The other menu, important for initial setup - Options. First of all, click on the top item "Settings".
Here is the Settings dialog. Set the FFT / display size - higher value increases accuracy but requires more processing power (runs slower). I use 131072.
Then set the Sample Rate. This one depends on your analyzer / sound card performance. The higher the sample rate - the higher the bandwidth you can measure within. My analyzer supports 192KHz sample rate, however, in this particular case, I use 176.4KHz to prevent some interference - I don't really remember now what issue it was.
You can leave all the other parameters as is for now. Click "OK".
Next important menu item is "Display properties".
Here you can set the frequency (x-axis, Hz) and amplitude (y-axis, db) ranges that will be visible in the main window. "y-max" (top) parameter will be corrected automatically as soon as you perform "set peak to 0db" later on. I set "y-min" to -140db. For measuring 1KHz spectrum, convenient values for x-min and x-max are 800 and 21000 (Hz) respectively. Click "OK" to save these settings.
Now you can test the analog loopback configuration of your measurement system. Connect the input to the output. Press the green "Run" button. Set the desired frequency on the generator. Unmute the generator - you should see the spectrum, including the base frequency component and possibly some harmonics.
The picture above outlines 1KHz spectrum (800Hz - 21KHz display window).
Set the output level close to 0db on the generator (making sure the input is not overloaded at the same time). Now you need to calibrate the amplitude scale.
Having the generator running, click the right mouse button somewhere on the main window. Click "set peak to 0db" at the bottom of the menu.
Now you see the base frequency peak aligned with 0db scale level.
Now let's test the loopback at, say, 20KHz.
Go to Options / Display properties. Change "x-min" to 18000, "x-max" to 81000 respectively. Click "OK". Mow you main display window's frequency response is set to 18KHz - 81KHz. Adjust the frequency at the generator to 20000 (Hz) and hit the "Enter" key. Unmute the generator.
You can see the 20KHz spectrum - the base frequency (20KHz) and harmonics 2 (40 KHz), 3 (60 KHz) and 4 (80 KHz), as well as the level of THD, calculated for you.
You can repeat the measurement at the different frequencies within the 20Hz - 20KHz audio frequency range.
Let's see how we can measure a 2-tone IMD (imtermodulation distortion) now.
Click the "IMD" button at the top right toolbar (main measurement window).
You will see the IMD parameters dialog, as presented above. Set the "Frequency #1" to 14000 (Hz), "Frequency #2" to 15000 (Hz). Marc the IMD Products Order #s you'd like to see (for example, all orders up to #5). Click "OK".
We will use two generators for IMD measurement. Set the frequency to 14000 and 15000 (Hz) respectively for those generators. Set the Volume (amplitude) at each generator to 5-6db lower value, than you had at the time of THD measurement - the overall amplitude of the combined signal will stay close to the same level. Adjust the "Display properties" (x-min, x-max) to 800 - 21000 (Hz) again.
Here is what you see while the generators are still muted. Unmute both generators.
Here is the IMD measurement of my analog loopback.
Note - I use RTX6001 audio analyzer as ADC / DAC hardware in my system. In case you use a computer's soundcard, its performance may be lower (higher harmonic components' values). However, knowing your soundcard's performance in the analog loopback configuration, you can see the difference when measuring the amplifier.
Finally, a few words on how to arrange the input attenuator in case you use a soundcard as a hardware. I don't recommend using a microphone input for measurements because of the higher noise and higher distortion, which is practically unavoidable at the higher gain, required for the mic input. Line level input is much more accurate and convenient. However, I will show the attenuator values calculation for it as well - if there are no other options, even a mic input is better than nothing.
Here is what the input attenuator for computer soundcard looks like. It basically consists of two resistors, forming a voltage divider. The table presents the values calculation, assuming the amplifier's gain is close to 29db, which is a de-facto standard for a power amplifier.
R_dummy is a dummy load resistor - normally 8 ohm. 50W rating is enough for the most of quick assessment measurements unless you want to measure some powerful amplifier at the full swing - then you need to arrange a powerful load on a heatsink, matching your output power or exceeding it for safer, less hot operation.
That's all for now, please let me know your question / comments - I will do my best to update this article accordingly.