# QA403 measuring 76 SNR is loopback test? **URL:** \ **Category:** Uncategorized\ **Created:** [August 16, 2026, 6:07pm UTC](https://forum.quantasylum.com/t/qa403-measuring-76-snr-is-loopback-test/2376 "2026-08-16T18:07:41Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![turambar517](https://avatars.discourse-cdn.com/v4/letter/t/a88e57/32.png) [@turambar517](https://forum.quantasylum.com/u/turambar517)\ **Post date:** [August 16, 2026, 6:07pm UTC](https://forum.quantasylum.com/t/qa403-measuring-76-snr-is-loopback-test/2376/1 "2026-08-16T18:07:41Z")
Hi all! I got my QA403 finally out of the box and am trying to get a good baseline on audio measurements. I have a BNC to BNC looped from L+ Out to L- IN and all the other 3 inputs blocked with shorting caps. I am just running from the GUI at 192khz 12 dBV full scale, FFT to 256k, 1khz at 0dBV/1Vrms and getting back an SNR of 76.45dB and THD of -62.95dB. I feel like it should be better than that so I’m probably doing something wildly wrong. I tried switching cables and even tried an RCA to RCA with RCA to BNC adapters but no dice. I tried a couple differnt USB cables and added a powered USB hub for it as well. Can somebody give me a sanity check on this? Thanks! ![image (2)](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/a/a24826f6868fc0789caeccd962faa3833c250835.png) ---
**Author:** ![MarkZ](https://yyz2.discourse-cdn.com/flex032/user_avatar/forum.quantasylum.com/markz/32/4269_2.png) [@MarkZ](https://forum.quantasylum.com/u/MarkZ)\ **Post date:** [August 16, 2026, 7:11pm UTC](https://forum.quantasylum.com/t/qa403-measuring-76-snr-is-loopback-test/2376/2 "2026-08-16T19:11:30Z")
Yeah, that’s not right. I’d start at the beginning and see what’s not right. First, run at 48KHz and short all 4 inputs and see what you get. I’d set attenuation to 0. If that looks clean then I’d turn the generator up to at most 1/2V and check again. Here’s a loopback with balanced left and shorted right. The generator is set to 200mV and the FFT is 16K here which is plenty. ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/e/e8ecd6706235f1dd83e31ca872ebf4205e6fbb0c.png) ---
**Author:** ![Claudio](https://avatars.discourse-cdn.com/v4/letter/c/7cd45c/32.png) [@Claudio](https://forum.quantasylum.com/u/Claudio)\ **Post date:** [August 16, 2026, 7:50pm UTC](https://forum.quantasylum.com/t/qa403-measuring-76-snr-is-loopback-test/2376/3 "2026-08-16T19:50:01Z")
Hi @[turambar517](https://forum.quantasylum.com/u/turambar517). I see that the voltage you’re using to power the QA403 is too low. I’m reading a value of 3.87 V: this could also cause significant problems with the analyzer’s operation. ---
**Author:** ![turambar517](https://avatars.discourse-cdn.com/v4/letter/t/a88e57/32.png) [@turambar517](https://forum.quantasylum.com/u/turambar517)\ **Post date:** [August 16, 2026, 11:40pm UTC](https://forum.quantasylum.com/t/qa403-measuring-76-snr-is-loopback-test/2376/4 "2026-08-16T23:40:01Z")
Ah good call. Maybe my hub can’t supply enough power. I have ordered another one that should be in tomorrow. Thanks for input, both! ---
**Author:** ![Claudio](https://avatars.discourse-cdn.com/v4/letter/c/7cd45c/32.png) [@Claudio](https://forum.quantasylum.com/u/Claudio)\ **Post date:** [August 17, 2026, 2:16pm UTC](https://forum.quantasylum.com/t/qa403-measuring-76-snr-is-loopback-test/2376/5 "2026-08-17T14:16:05Z")
Also use a USB cable that isn’t too long and has an adequate gauge. ---
**Author:** ![turambar517](https://avatars.discourse-cdn.com/v4/letter/t/a88e57/32.png) [@turambar517](https://forum.quantasylum.com/u/turambar517)\ **Post date:** [August 18, 2026, 1:21am UTC](https://forum.quantasylum.com/t/qa403-measuring-76-snr-is-loopback-test/2376/6 "2026-08-18T01:21:02Z")
You all were entirely correct. New cable and supply and we are good to go! Thanks so much! I bought a USB C to USB B cable and that worked great. Didn’t have to use the power USB hub.