Matt
I have been playing with Mark Z’s QA40xPlot and doing most of that in loopback.
This is when I found that my QA402 does not meet its THD spec as per the Product
Brief: -110 dB (loopback, single-ended, L- shorted, -2 dBV input, 128K FFT, Hann window)
I am getting in the area of -92dB THD with these settings.
I started to look at “everything”…USB Hub, Cables, Appliance electrical interference,
Neon lighting, Processor Power, RAM. I Ran the tests on 3 different PC’s, 2 with plenty
of horsepower and less than a year old and one with “enough” horsepower. One of the PC’s
was even in someone elses house (Who runs a 401) 10 blocks away so all different cables,
power supplies etc. All had basically the same results (within a dB).
Then I ran the QA40x Verifying Calibration found in the QA40x Wiki. Steps 1 & 2 ran fine.
Step 3 gave me exactly 1.000V RMS at 60Hz and 1KHz on all 4 outputs (+ & -) with a Fluke 87V.
Step 4 (Verify Relay Gain Ranges) is where I saw some odd results. Traces attached.
Any ideas what is causing this? Is this normal. Is this repairable? Mine is an early 402
(July 2021 Order# 217921A) I CAN get -110dB THD but only at -18dBV. I almost always run my
testing at Line level (-10dBV). I have tested a few power amplifiers but nothing over maybe
70W. Very conscious about DC.
Duh, I just figured out. I didn’t do the zoom in on the graphs. When zoomed in they’re flat like the examples in the wiki document. What clued me in is I’m actually running a test on a power amplifier today, and I did the same amp gain and Distortion versus amplitude test. The result was fairly flat running through all the relay stages. I went back and ran the verifying calibration again and saw what I had done. Thanks
HI @BarryLambert, something does look up with your THD. If you look at the Product Brief for the QA402 on page 5, you’ll a measurement at -10 dBV, and you can see the spectrum there along with the settings. Can you replicate and show that plot?
Hi @BarryLambert, yes, the THD figures on the QA402 seem very degraded. The 60 Hz could be environmental. If you make the noise measurements shown on page 4 of the product brief, does the 60 Hz go away? If you rotate the device around in free space, does that change your 60 Hz levels? There can be a lot of ways the 60 Hz is creeping in, and it’s probably something on your bench that is coupling in.
Does the right channel exhibt the same THD? What is shown on the screen is L+ input with L- input shorted. If you flip that and loopback into L- and short L+ does it show the same? And then use L- and R+ and R- OUT and see fi the levels change?
it’s not clear yet if the excess THD is coming from the generator or ADC. But swapping the inputs and outputs should help you determine. Please report back when time permits!
