QA40x Release 0.997

Release 0.997 adds the following:


A settings menu has been added (Edit->Settings). For now, there are two settings, and both are related to acquisition.


Latency Compensation

Latency compensation allows you to work with DUTs that have path latency that exceeds 30 mS or so. While a normal analog amplifier has virtually zero latency (measured in microseconds), digital amps and especially bluetooth-connected gear can have latency in excess of 30 mS.

Setting the prebuffer to a larger value ensure the steady-state signal has time to propagate through the delay before being captured by the QA402 hardware.

First, let’s take a look at a DUT, which in this case is an audio delay implemented in a Digital Audio Workstation (DAW) called Cakewalk. The DAW is configured for loopback, meaning it will sample the input of the sound card, process it, and play it back. We’d expect the latency be fairly small. First, let’s look at the output of the QA402 in the time domain:

We see the burst is starting at t=0 as expected. Switching to the input (that is, what came out of the DUT) we see the burst comes back into the QA402 about 15 mS later. And because of the 15 mS delay, note the right end of the plot is chopped off–we won’t see the ramp down.

If we zoom in, we can see the delay is closer to 30 mS:

OK, so now let’s go into Cakewalk and set the delay for 250 mS using a delay plugin:


And now let’s take a look at the input time domain capture again:

We can clearly see the impact of the delay, and now more than half of the right side has been chopped off.

Now, go into the latency compensation and adjust the prebuffer to give 300 mS of compensation:


And now re-capture a burst. Note the capture time has been extended by the specified amount, allowing us to fully capture the burst in spite of the added delay. Notice the annunciator “Latency Compensation” in the display to alert you to the fact that this setting has been adjusted. Over-stating the latency isn’t a problem, but will impact the measurement time.

Pause Between Acquisitions

If the Pause Between Acquisitions is set to a non-zero value, then the system will pause for the specified time between measurements. This can be useful if you want to subject high-power amp to continuous testing BUT you want to keep the average power down due to load limitations. Here, we’ve set the pause time to 5 seconds:


And now by picking a small FFT (1K) we can deliver an ~80 mS burst every ~5 seconds to the DUT:

New Automated Test: AMP Gain and Distortion Versus Level

The AMP Gain and Distortion versus Level test is very versatile, allowing you to graph input level versus output level, or gain versus output level. This later mode is useful for looking at a compressor response curve. In Cakewalk we can specify a compressor as follows:


And the resulting curve from the test yields (note there is no calibration between dBFS used in the DAW and dBV used on the analyzer, but it could be done).

The plug-in also allows you to view distortion (all harmonics) and gives you the ability to graph 2H and 3H separately. The following setup was used to measure the Modern VST Virtual amp plug (a heavy, overdriven guitar effect).


The resulting plot is shown below. Note that we can see the compression kicking in around -25 dBV (this is gain! The plot above was output versus input, which is why the compression manifests differently). We can also see the total distortion is dominated by 2H until the level comes way up, and then 3H takes over.

New Automated Test: SPKR Impedance

The QA401 speaker impedance test was ported over, and is mostly unchanged, with some differences in filtering. Some of the more involved plug-ins with setup requirements will have a Help menu option as shown below. Clicking this will take you to the Github wiki.


A plot from the new plug-in is shown below, using the QA461 (not yet available):

The same setup using the QA401 software yields: