Hi @skipburrows, thanks for the procedure! I think you can readily test this. I’ll be a bit verbose in places below, but let me know if there are steps for which you need more detail.
Analyzer Setup
- File->New to put the QA40x software in a known state
- Set to dBu mode (right click dBV button in AXIS control group, and select dBu in Y Axis). And also click the +6.02 dB output gain button because we’re going to run in balanced mode
- Set to 16k FFT
- Right click on RMS in MEASUREMENTS control group and change Measurement Stop Frequency to 22 kHz
- Disable right channel (click on RIGHT button)
- Add RMS measurement (click on RMS button)
- Add THD measurement (click on THD button)
- Set to + 18 dBV full scale input
- Set to +4 dBu 1 kHz output (right click on GEN1)
- Enable GEN1
- Press IDLE button
At this point, take a DVM and measure across L+ and L- and confirm it is +4 dBu/1.227Vrms. The QA403 is now generating a fixed 1 kHz balanced signal at +4 dBu.
Basic Setup
Check meter at “GR” to be “0”. If not, adjust “Meter Zero”
There is no connection to the compressor at this point. So, just confirm 0 reading with meter switch at GR setting
Apply 1kHz @ +4dBu at the input. Check that the meter reads “0”
at “Input” Setting. If not, adjust “V gain”.
Apply balanced QA403 output signal to compressor input and verify 0 reading on meter with compressor meter switch at INPUT setting.. Adjust if needed “VGAIN” as needed. There is no bursting from the QA403 at this point.
Check “Output” for 0 on meter. If not, adjust “Pinch off”. The actual
measured output will be about +4.3dBu.
While still in continuous mode, switch meter select switch to OUTPUT and again confirm 0 reading on meter.
Set compressor attack to minimum
Now, turn off IDLE mode. Run a single cycle via on the QA403 (ctrl+space) and confirm RMS measurement is reported as ~4.3 dBu.
Switch to time domain and inspect the plot. I’ve posted a plot of the time domain measurement in loopback below so you can get the flavor of things.. The goal here is to ensure any attack artifacts are limited to the first 40 mS. We can increases this 40 mS limit via Latency Compensation, but I suspect with Attack at minimum all artifacts will be finished by 40 mS.
Check distortion. Should be .08% or better using noise+thd, 20–
22kHz filter method. If not, adjust “Distortion Null”. If you are not
getting these results, your measurement setup may be incorrect.
The FET’s really go bad.
Press space bar to run continuously. Display will update at 16K/48K = 340 mS = 3X per second.
If distortion is not better than -62 dB (0.08%), then adjust Distortion Null on the Compressor to minimize. Remember the display updates 3X per second, so adjust, and then wait for the display to catch up and stabilize.
Compression Adjustment
Set “Threshold” to 0, Ratio to 2:1, “Attack” fastest. Release =
fastest, “Make-up Gain” = 0
We’ve already set Attack to minimum. But set the other settings as required
Step A:** Apply 1kHz @ +4dBu at the input. Adjust “Trim Threshold”
until the measured output is -1.5dBu
We’re still running in bursted mode and displaying measured compressor output level. Adjust Trim Threshold to achieve -1.5 dBu reported.
Share the time domain plot at this point
Step B: Increase the generator level by exactly 10dB, to +14dBu.
Adjust the “Ratio Trim” so the measured output is -6.5dBu
Still running in bursted mode, increase generator output to +14 dB and adjust Ratio Trim to achieve -6.5 dBu
Share the time domain plot at this point
Repeat Steps A & B. If you do this several times, you can achieve
an accuracy of 0.01 dB. All units are calibrated to this standard,
as checked on an Audio Precision.
Repeat as needed. Only the first time domain plots are needed.
We might need to tweak. But let’s try this for a start. It’s very important to share the 3 time domain plots. This is where we’ll need to determine if additional latency compensation will be needed (or not).
Hopefully we can get this sorted so you don’t need to take the AP box on the road with you!