Comparing the QuantAsylum QA403 to the Gold Standard

The American company QuantAsylum offers a range of affordable, semi-professional audio analyzer tools and instruments. In this article, we test the QuantAsylum QA403. The quality of the generated signals and the measuring unit was checked with an Audio Precision APx555. The measurement capabilities of the unit are described and where there are differences with APx555 and/or audioTester, they are mentioned.

Low-Budget Audio Analyzers

The professional audio meter market is dominated by Audio Precision and its products. They are excellent, but also costly. Even used devices without support still cost several thousand euros. For the hobbyist, there is usually only cheap or free software (audioTester V3.0 or REW, etc.) and built-in or external sound cards to work with. This has some disadvantages because the sound cards are designed to record or play music, not as a measuring device. The software usually has no access to the amplitude and range settings of the sound card, which are also quite small compared to an audio meter. The input and output levels are not calibrated, either.

Disclaimer

This article is not intended as and cannot replace reading the ‘Quick Start’ manual and documentation. If you have the opportunity to try a QA403, first familiarize yourself with the unit. The software also works in ‘Offline Mode’ without needing the instrument.

Housing and Connectors

The QuantAsylum QA403 is housed in a 177 mm × 44 mm × 97 mm case with an attractive color scheme. On the front are eight isolated BNC connectors for the two differential inputs and outputs, the expansion connector and three LEDs. If you want to use cinch (RCA) cables, you can use appropriate adapters from BNC to cinch. The manual describes how to make cables for XLR connectors.
 
The expansion connector provides digital signals for direct testing of DACs and ADCs, but this feature was not tried in this review.
 
The LEDs signal the link, the activity of the device and the status of the attenuator.
 
On the rear panel, there is a USB Type-B connector for connection to a PC.

Power Supply

Power is supplied through the USB connection; the voltage and current are displayed in the status bar at the bottom of the software interface. Currents over one ampere are not uncommon. Therefore, the connection should be made using a low impedance cable with a USB 3 or USB Type-C connector. A single USB 2 port may work, but it is better to use a cable with two USB 2 connectors or a powered hub. Unfortunately, a cable is not included with the device.
 
My RME Babyface Pro 2 FS audio interface only consumes about 500 mA, but it has a socket for an external 12 V power supply. When it is used, the current on the USB connection drops to zero. This would be a nice feature for the successor of the QuantAsylum QA403.

The QuantAsylum QA403 Features Galvanic Isolation

But there is also an important advantage: the power supply is galvanically isolated from the PC! This certainly reduces interference from the PC and prevents ground loops. Even the APx555 doesn’t have galvanic isolation.

Software Interface

The next picture shows the surface of the software. Version 1.173 was used in this review. The language is English. The control panel is on the left, the graphs are on the right. Above the graphs are buttons showing the selected measurements. Underlined controls (e.g. ‘Stop’ or ‘dBV’) have a submenu which opens with a right mouse click. I’ll refer to the manual where needed.


 

Sample Rates

The QA403 can be operated with sample rates of 48, 96 and 192 kS/s. It is the same for the generator and digitizer. As the manual points out, measurement quality is best at 48 kS/s. This has been confirmed during the review. If you do not need higher frequencies, you can stay with 48 kS/s.
 
The freeware tool audioTester also allows using 44.1 kS/s and multiples, with independent rates for the generator and digitizer. But in most cases, you probably don’t need this.

QuantAsylum QA403 Generator Signals

Test signals available are sine wave, dual sine (for Intermodulation Distortion aka IMD measurements), multitone, white noise and chirp for frequency response measurement. There are no square, triangle, pink noise or pulse signals.
 
With audioTester, pulses can be used to measure the frequency response of loudspeakers. Echoes can be suppressed with a time window. This does not seem to be possible with the QA403 software.
 
Amplitude and frequency can be set with up/down keys or entered directly in the submenu. The step size is defined there too. In the manual, you can find a block diagram of the generator.

 

 

 

 

 

Digitizer Settings

With ‘Full-Scale Input’ the level of the input signal is set to the range of the ADC. For safety reasons, the software always starts at the highest input value. If you know the level, then you can choose a better range. The optimal setting for THD measurements can be different from SNR measurements; therefore there is no auto-range option as features the Audio Precision device.
 
The ranges 24, 30, 36 and 42 use an additional attenuator, which has a significant influence on the noise floor.

Display Options

In the ‘Display’ field, you can choose between frequency and time display (oscilloscope), between ‘Input’ and ‘Output’ (digital input signal of the DAC) and one can select the left and/or right channel. The colors of the graphs are fixed, and one cannot freeze curves and superimpose new measurements for comparison. Audio Precision and audioTester let you do this. But it is only software, and so it can be extended later.

More Options of the QuantAsylum QA403

The option ‘Axis’ lets you choose between linear and logarithmic X-axis, and you can define the Y-range. The size of the FFT and averaging is selected under ‘Acquisition’. Both settings change the measurement time.
 
Various window functions can be selected under ‘Window’.
 
Above the graphs, additional measurements can be shown, which you can select under ‘Measurements’ or activate by clicking there. For example, THD, THD+N, SNR, etc.
 

 

Automated Tests with the QuantAsylum QA403

Here you can define automatic tests and display them as a graph, for example THD over frequency and/or amplitude.
 
Contrary to audioTester, the QA403 software allows controlling the amplitude (audioTester can only change the digital level of the DAC). The tests are explained in a wiki on GitHub.

Quality in Loopback

First, the generator was connected to the digitizer in single-ended (loopback) mode. That is, one output of the generator is connected to one input of the digitizer. The second input must be short-circuited with a BNC terminating resistor of 0 Ω, 50 Ω or 75 Ω. Sinusoidal signals with levels between −20 dBV and +15 dBV were generated and measured. The full-scale input was optimized for THD. For THD+N (SNR) occasionally a smaller range may be better.
 

A 1 kHz test signal was measured at all three possible sample rates. According to the manual, 48 kS/s is best, but the differences are small. Therefore, the frequencies of 100 Hz, 200 Hz, 500 Hz and 2 kHz were measured at 48 kS/s, whereas 5 kHz was sampled at 96 kS/s and 10 kHz at 192 kS/s. Table 1 shows the values. The corresponding measurement functions were used.
 

As a second test, the loopback was done with differential signals. Both outputs of a generator are connected to the two inputs of the digitizer. Only a 1 kHz sine wave was measured. Table 2 shows the results. Overall, the values are very decent.

 

Quality of the Generator Signals

Since it is not possible to attribute the unwanted signals such as harmonics and other spurs to either the generator or the digitizers, we must measure them separately with a much better instrument. The ultimate audio measuring device is the APx555, which I was allowed to use for a weekend.
 

For simplicity, I used the analog THD+N function of the APx555 and compared it to the same function of the QA403. Table 3 shows the results for single-ended, Table 4 for differential measurements. For single-ended several frequencies were measured, for differential only 1 kHz because the differences were small. On average, the APx555 performed about 10 dB better.
 

 
Notice that there is only one even harmonic at 2 kHz. Besides the small, odd harmonics, there are no other spurs. For a device in this price range, this is excellent. You can further improve the generator signal with external low-pass or band-pass filters, as long as you use the best available op-amps there.

Digitizer Quality

Of course, the APx555 analog generator was also measured with the digitizer of the QA403 at 1 kHz. Tables 5 and 6 show the results for single-ended and differential measurements.

 

 

Level Accuracy

The levels of a 1 kHz sine wave of the QA403 generator were measured with the APx555 digitizer. As a reference, the levels of the APx555 generator were also measured. The accuracy is very high. The values can be found in table 7.

 

Missing Features on the QuantAsylum QA403

Missing from the QA403 are the analog generator and the variable notch filter in the digitizer, which are essential for ultra-low distortion measurements. They can be bought as an extra device QA480 from QuantAsylum. Unfortunately, I can’t say anything about its quality because the device was not available. Maybe another time.
 
The digitizer has no auto-ranging, so you must select the appropriate range yourself and thus keep an eye on the THD and SNR values.
 
Some articles on the net mention the absence of XLR sockets as a major inconvenience. I think the BNC sockets are the right choice considering the small size of the QA403. You can easily make your own adapter cables by cutting a BNC cable in two and attaching XLR connectors. How to do this is described in the documentation.

Big Advantage of the QuantAsylum QA403

A significant advantage of the QuantAsylum QA403 is the galvanic isolation of its power supply. Neither the APx555 nor USB audio interfaces have this.

Author’s Recommendation

If you need and can afford the best audiometer currently available, there is no way around the Audio Precision APx555B. If, on the other hand, you are just starting out in measuring audio, I would recommend downloading freeware audio measurement software and using your desktop PC’s built-in sound card. Sound cards of laptops are mostly unsuitable, as they often have only one microphone input.
 
If you want to increase the quality, an external USB interface is recommended. Such devices are abundant.
 
If you want a semi-professional device, the QA403 is the right choice. It avoids the inconveniences of the sound card solution, like missing level calibration and insufficient level adjustment possibilities. Again, I would like to emphasize the galvanic isolation of the power supply. The QuantAsylum QA403 can be improved further with, for example, external filters. Considering the price, the quality is superb. I even bought the test unit.