The QuantAsylum QA403, the 4th generation audio analyzer from the company founded by Matt Taylor in 2012, has a lot to offer. In particular when a user gets tips and guidance from someone like Bob Cordell, no less. The QuantAsylum QA403 Analyzer was the result of a complete redesign imposed by the silicon shortages and supply chain disruptions, pre- and post-pandemic. It appears that the efforts made were well worth it. Jan Didden shares his impressions.
During the more than 50 years I have been fiddling with audio equipment, I have witnessed the incredible progress of having — if you were lucky — an audio frequency AC meter and a low-frequency RC oscillator on your test bench, to now where almost anyone can afford fully computerized analog and digital audio analyzers. In 1988, I built Bob Cordell’s analog THD analyzer [1], which was a great tool to analyze all those designs I dreamed up. I spent several months in 1992 writing software handbooks and from the proceeds bought an Audio Precision System 1 — at the time there weren’t many audio manufacturers that could afford one. That again extended my measurement possibilities, if not the absolute limit at -100dB.
Fast forward 30 years, and the price for such a system has dropped to a few hundred dollars. A $200 sound card and free software will do more and do it better than my beloved S1. But I am no longer satisfied with -100dB test capability; my current Audio Precision SYS 2722 is almost 20dB better.
Overlooking the field, on one side you have the Audio Precision APx line culminating in the 555B series and other professional packages such as the Listen, Inc. analyzers. On the other extreme you have solutions consisting of Sound Card DA and AD boxes running with often free and extremely capable software. What they have in common is flexible software where you can set and adjust anything you can think of, accompanied by the most esoteric signal generation and analysis methods ever devised.
Then QuantAsylum’s QA403 audio analyzer arrived on my desk.
The Hardware
All QuantAsylum (QA) products follow a similar form factor and color setting — a small box with connectors at the front and on the rear a USB (Type-B) connector, which is used for power as well as to communicate with the PC running the application (Photo 1). There are two input channels and two output channels and, interestingly, each channel has two BNC connectors. The in- and outputs are balanced as well as symmetrical and can be run single ended by just using one of the BNC connectors. However, I found that for best performance you should run at least the input side in balanced configuration, in single-ended mode there’s a bit more mains hum. To do that you’d probably have to fabricate a special cable or use some type of adapter. I used a stock Y-cable with an XLR on one side and two RCA connectors on the other side, with RCA to BNC adapters, and that worked flawlessly.
A set of green LEDs shows whether the unit is linked to the PC and running, and whether the built-in attenuator is engaged. There is a small female header connector on the front panel, and a corresponding I2S button on the software control panel that will allow testing of I2S devices. Input formats of 16- or 32-bit widths at 48kSPS are supported. Note that there is galvanic separation between the unit ground (connected to the BNC I/O connectors) and the ground of the USB connection/host PC. This significantly lowers hum and noise; you can connect the QA403 ground to the DUT ground without having to worry about the PC ground.
Figure 1 shows the block diagram of the output signal chain, and you can see that GEN1 and GEN2 are summed, and then a 1-of-6 multiplexer selects the signal source, which goes to one channel of the DAC. So GEN1 and GEN2 are not tied to left or right. The other DAC channel replicates this. Thus, what is on L is the same as R always, except, if needed, one channel can be muted (this is helpful if you are looking at crosstalk, for instance).
The QA403 has 32-bit ADCs and DACs and an in-built digitally controlled input attenuator that can be set to a maximum of 42dB attenuation. Maximum input level is +32dBV. The output supports +18dBV single ended and +24dBV balanced output signals (some earlier User Guides list this erroneously as +8dBV and +14dBV).
One thing to be aware of is that the unit consumes close to 1A from the USB power source. The actual USB supply voltage is not critical as it is up-converted internally, but it should not fall below 4.6V, so use the heaviest USB cable you have. The received USB voltage is displayed at the bottom of the screen and will turn orange or red if the voltage becomes too low.
A Sample Measurement
Let us, by way of introduction, see if we can make a representative measurement; you can follow this on the software panel if you wish (Figure 2). First connect all inputs to their respective outputs for a loopback connection. The User Interface panel is displayed at the left of the Graph panel (Photo 2). The software allows you to select some predefined measurements such as Gain, RMS, THD (+N), SNR (measurements area). There are two separate generators (GEN1 and GEN2) that can each be set to output a sine wave at a specific level and frequency, but also more complex waveforms like multitone, white noise, and frequency response. Let’s try the latter. In the Generators area you activate frquency response, this will be the test signal. To define the graph, you activate frequency and input in the display area along with the channel selection left. This means you will be graphing the left input channel with frequency as the X-axis.
In the axis field select dBV as the Y-axis, and xlog for a log X-axis.
Finally, I set up the parameters for the measurement fast Fourier transform (FFT): 192k sample rate in the run/stop and front panel area, flat top in the window area, and the FFT length and averages in the acquisition area. You can set the input attenuator setting in the full-scale input (dBV) area. The final setup is shown in Figure 2. Now press RUN in the top left corner and you get a nice frequency response similar to Figure 3. I have increased the generator level and selected the “1 to -1” Y-axis extend button to get a higher resolution of the response around the 0dBV line.
What I have glossed over is the fact that the frequency response test signal is an exponential chirp (“ExpoChirp”), and it is used to capture the impulse response of the output signal. You can configure the chirp by right clicking the FREQ RESPONSE button in the Generators area. By complex division of the input and output signal impulse responses followed by an inverse FFT, we get the frequency response. Sophisticated, fast, and accurate; and there is much more about this at the QA403 Wiki [2].
Because most of the measurements are burst-signal based, QA has thoughtfully added the IDLE option button allowing you to set the generators to single tones so you can do any manual measurements you like. Alternatively, you could measure power amplifiers with quite short bursts with a low duty cycle, to capture maximum power output in a safe way and without power supply sag from prolonged measurements. There is a CTRL-Space keyboard shortcut for that purpose that causes a single burst to be emitted. Its use is extensively described in Bob Cordell’s tutorial [3].
Now, this is all very logical, but I can imagine that for a quick response test you would rather not click all those buttons, although after some use it quickly becomes routine. The nice thing is that you can save all those settings in a settings file. Next time you want to do this, load the settings file and it’s all there again, and you can make the changes to some of the settings as needed. Bob Cordell has been creating many different settings files for a wide variety of situations, which can be downloaded [4].
Advanced Tests
But single-frequency sine waves are not the only signals the generators can produce. For several years designers have realized that single-frequency THD (+N) is just one way to look at a unit and in general does not correspond well to how a unit will sound. It is a great design tool to find the gremlins in a design, but intermodulation tests such as the CCIF and SMPTE IMD tests, although measuring the same amplifier nonlinearity, can give alternative presentations that are useful for a designer or tester.
A harsh test signal is the standardized Multitone signal. The Audio Engineering Society (AES) standard specifies 31 tones, covering the audio band, but with modern DSP computing power even a 1000-tone signal can be used. The QA403 also provides multitone capability, and a sample measurement in loopback is shown in Figure 4. The noise floor is low and there’s no sign of intermodulation products from the many different tones.
During the writing of this review, I had an opportunity to do some measurements on Audio Precision (AP) equipment and decided to look at the QA403 through the eyes of that test equipment. With an APx525 I first measured the AP’s own generator, with a 1V 1kHz signal, and the FFT is shown in Figure 5a. Next, I input this 1kHz AP source signal into the QA403 input (balanced) and looked at the result (Figure 5b). Both are very similar, very clean, with only the even harmonics a trifle higher for the QA403. This is very good!
b: APx525 1kHz 1V source measured on the QA403.
I also put the QA403 1V 1kHz generator output signal into a top-of-the-line APx555B, and the resulting FFT is shown in Figure 6. Again, excellent results!
Figure 5b illustrates another feature: by clicking each harmonic (or any other peak in the FFT) you obtain a tabular display of that peak’s magnitude relative to the fundamental. I needed some getting used to the fact that the peaks are numbered by their magnitude and not by their harmonic number. It does make sense if you remember that the markers can be placed on any peak, not necessarily a harmonic.
Pass/Fail
With an eye to production environments, it will be no surprise that the QA403 comes with pass/fail testing and reporting capability. It is easy to define some limit lines for say a frequency response and the system will display pass or fail depending on whether the graph stays within the limit lines or exceeds it.
Visualizers
Much effort has gone into the software to help interpret measurement results. Five so-called visualizers are available from a drop-down menu at the top of the menu panel: Filter Explorer, Oscilloscope, Residuals, THD Bargraph Display, and Wow and Flutter. Think of these functionalities as extensions of the basic graphing. You would typically do a measurement, stop it and select a visualizer from the Visualizer menu. A new graph will open and show the selected view.
Figure 7 and Figure 8 (these are from Bob Cordell’s tutorial [3]) show representative displays of the THD Bargraph Display visualizer and the Residual Display visualizer. The Bargraph Display is pretty much self-explanatory; the Residual Display shows the fundamental as well as the residual after notching out the fundamental. Signal voltage magnitude is shown on the left Y-axis and residual signal voltage magnitude is shown on the right axis. This is a helpful way to get an idea of the harmonic composition of the distortion, especially when you are doing things such as adjusting bias levels in a Class (A)B amplifier. The waveform shown in Figure 8 is not much different from a sinewave and suggests a dominance of the second harmonic.
Wrap-Up
The series of QuantAsylum products has initially been designed for use in manufacturing/test facilities. The QA403 is a small simple box, with very high performance. It can be set up quickly and repeated tests can be easily saved and reloaded. It doesn’t do anything an AP can’t do, but for the price of a single 555B you can get 50 of your employees their own QA403! (I probably should apologize to QuantAsylum’s Matt Taylor for referring to an APx, but he should consider it highest compliments!).
Naturally, when word spread about the QA403, audio amateurs and small design outfits got interested, and the product and its software have greatly benefitted from user feedback. For $600 it found itself a home on many an engineer’s design bench as well as with the serious amateur who wants an affordable and capable test set (Photo 3).
Do I have any gripes? Not really but there are a few user interface items I believe could be better. As an example, if you want to set the Y-axis, you have two buttons for a preset value. The default is +20dB to -20dB, and +1dB to -1 dB and these values can be set by the user—a nice feature. But to set the Y-axis extend to any other value, there’s two sets of up/down buttons where you can modify Y-min and Y-max values in 5dB steps. That could require a lot of clicking. I would prefer to have a drop-down box where I could type in arbitrary values for Y-min and Y-max, or a right-click on the axis in the graph to bring up a dialog box. But these are minor things, and you might disagree with me on that.
Anyway, a review like this can only scratch the surface of the capabilities of this powerful and flexible unit. I have not mentioned other functionalities such as the REST interface, which allows you to control the settings and read results over a Wi-Fi link via the PC running the software [5]. The REST protocol is a simple text-based interface appearing on more and more test equipment and it can be used to send commands to the equipment to set a state or output or perform a measurement, and subsequently request the result of the setting or measurement. This supports unattended and remote use of the QA403. The QuantAsylum support forum has many threads discussing its use.
Nor have I mentioned that most graphs can also display an on-screen table of values; a THD screen will automagically place markers on the fundamental and harmonics and show levels. You can also engage A or C weighting filters or a weighting of your own. For the FFT function, five different windows can be selected. The context menu for the generators offers a “Round to eliminate leakage” checkbox, which will “nudge” the oscillator frequency a bit to make it fall in the middle of an FFT bin for most accurate results. So, if you wonder why the test frequency is 998Hz instead of the 1000Hz you specified, this is why. There’s also a pair of cursors that can be used to measure values on a graph. You can select several sample rates at the top of the control panel, from 48kHz to 192kHz (there is a 384kHz button but that is grayed out in the current software version as it is in the process of being implemented). The 192kHz is of interest as it allows you to see up to the fourth harmonic of a 20kHz test signal!
The bottom line is that for $600, the QA403 is a very capable audio analyzer with a mercifully short learning curve that does pretty much all you’d want it to do, with excellent results. If you are in the market for a serious audio analyzer, review the QA403 User Manual and Bob Cordell’s tutorial. You’ll be pleasantly surprised!
Author Acknowledgement: While getting acquainted and doing measurements with this unit, Bob Cordell provided numerous tips and guidance, for which I am grateful. Bob also created an extensive tutorial [3] and a collection of time-saving settings files [4]. These files are available free of charge from the references. aX
References
[1] B. Cordell, “Build a high-performance THD analyzer,” Audio, July 1981,
https://www.cordellaudio.com/instrumentation/thd_analyzer.pdf
[2] “Frequency Response with Right Channel as Reference,” QuantAsylum,
https://github.com/QuantAsylum/QA40x/wiki/Frequency-Response-with-Right-Channel-as-Reference
[3] B. Cordell, “QA403 Tutorial,” https://cordellaudio.com
[4] B. Cordell, “Measurements Settings file,” https://cordellaudio.com
[5] The REST interface, https://www.redhat.com/en/topics/api/what-is-a-rest-api
www.quantasylum.com
This article was originally published in audioXpress, December 2024
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