QA403 Test / Calibration question

QA40x
1

So I am really new to this and wanted to know if I am approaching this the wrong way. I have done a ton of reading and havn’t been able to find anything covering this or I am just dumb :). So I did the loop calibration on my Q403 which worked great and produced expected results. I then tried connecting the cables I made along with the dummy load (8ohm/4ohm) I created to test vintage audio equipment. So I am trying to do a base test on my env with out any amp connected to get an idea of what it looks like? So here is the newbie part not sure but I played around with the settings (Full Scale Input (dbV), sample rate, input signal voltage,Freq 1khz) to see what combination of settings gives me the lowest THD and SNR ratio? So the best THD % I could get was .010-.009% on both channels and my SNR dB was 65 to 66. with a input sig running about 1v RMS and the Full Scale Input was set to 0. ( reading up to understand the SNR db stuff) . So am I just testing how noisey the resistors and cables are and is it really a good test? I copied the setup from “Vintage Audio Review” on youtube ( note have noting to do with the channel at all) But I liked his approach and setup. here is what I used for my dummy load :

qa403 dummy
qa403 dummy2922×554 130 KB

2

Hi @fmajewsk, you can always compare your test setup to what you get in loopback. And once you know the limits of the system in loopback, you can start to apply that to your DUT and learn how your DUT influences things.

For example, connect in single ended loopback (L+ OUT to L+ IN, L- IN shorted)

  1. File->New Settings
  2. Turn off right channel
  3. Enable Gen1
  4. Increase FFT to 16k
  5. Increase averaging to 10
  6. Push SNR button
  7. Set 0 dBV full scal input

And then run. I get the following plot:

image
image800×958 17.9 KB

And that suggests that if you stick a DUT in the loop between L+ OUT and L- IN, you can readily detect degradation of SNR if the DUT has an SNR worse than 106 dB.

Now, I very much doubt you’ll be able to see the performance difference between a well made $1 cable a $300 cable in a 6" loopback test. Because there isn’t a measurable difference there.

Resistor noise below 1k is tough to measure because there’s not much noise there. A 1k resistor at room temp in a 20k bandwidth is about -124 dBV. The QA403 has a noise floor around -117 dBV. You need to be up to around 5k to start to measure the resistor noise with the QA403 naked inputs. But if you have a super low noise preamp like the QA472 JFET pre, you can measure resistor noise starting around 100 ohms or so.

3

A dummy load must be driven by a power amplifier - the QA403 is a line level device. This is you are seeing distortion and poor performance. Rather test it (with your cables if you want) into a 1k load.

4

@matt and @handsome Guys Thank you so much for the information! greatly appreciated!

5

@fmajewsk- Indeed you need to be measuring an amplifier to use the load you showed here. I am glad you found the material useful (it is my channel :grin: ) Since you will be doing a lot of measurements on vintage gear, I am more than happy to share what I have learned using the QA40x. Bob Cordell’s manual mentioned in there forums is a great resource as well.

6

@VAR HEY! love your content!!! very useful for getting setup on my bench! many thanks again!!

7

Thanks and I do have a video coming up at some point showing how I measure a preamp with the QA403.

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