# QA-403 power readings off by 6db **URL:** \ **Category:** QA40x\ **Created:** [August 1, 2024, 7:33pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504 "2024-08-01T19:33:19Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 1, 2024, 7:33pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/1 "2024-08-01T19:33:19Z")
Hi I recently purchased QA403 to measure tube amps I am building. Initially, the measurements I was getting were correct, for example, my 3 stage 845 amp measured for 1Khz – 16W @ 3% THD, which I corroborated with the scope. The measurements were done at full scale input setting of 24dbV. Then recently, when I made some changes to the amp and ran thru the measurements, again at 24dbV input setting, all the output voltage and power measurements were off by a factor of 4 or 6db. I initially panicked and thought I messed up the amp, so measured the output with a scope which showed 16W output for the same input signal via a signal generator, and then I also used the quant as the signal generator and confirmed the same with the scope. Then I set the input to 18dbV and started with a low signal, I get correct readings for 1% THD, which was around 3W. As I slowly increased the input amplitude, the quant was overloading at 18dbV input attenuation as indicated with the clicking sound [i forgot the measure the input signal at which the overload happens]. The when I switch to 24dbV setting, the output measures 6db less!. I think I worked around the problem by setting the “input gain” at -6.00dbV in the dbV options tab, and now I am getting correct readings with 24dbV input setting - there was no logic to my trying this, apart from something to try :-), given my readings were 6db off! While I think I have found a work around, I wanted to post here to get input on - am I doing something wrong? - did I mess up something in the quant unit input module? Would appreciate any advice or guidance. I am total novice in this area, so I might not be doing something right. thank you Sridhar ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 2, 2024, 5:43pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/2 "2024-08-02T17:43:30Z")
I read and re-read another thread on input range violations with differential inputs with 402, mine is all single ended. Single ended output using RCA via the BNC adaptor and single ended input into the QA403 from the speakers with 8 ohms load using banana to bnc adaptor. Would appreciate if someone from Quant can give this post a read and respond thank you ---
**Author:** ![restorer-john](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@restorer-john](https://forum.quantasylum.com/u/restorer-john)\ **Post date:** [August 3, 2024, 10:37pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/3 "2024-08-03T22:37:34Z")
> [@sganti](#): > > single ended input into the QA403 from the speakers with 8 ohms load using banana to bnc adaptor. Are you using BNC to banana adaptors like these? I have had them turn up new with the negative and positive caps (red/black) on the wrong terminals. Make sure the GND is on the Black terminal side (test it with a DMM) ![IMG_3482](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/e/e7a172f73e2adef6c9c996db599d002a4bfd64fe.jpeg) ![IMG_3483](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/f/f0a5e96d858cef2052c3cdba5ffc80fd0cec0cbb.jpeg) Not saying that is your issue, but it would cause all sorts of problems if you had a transformer coupled output on a tube amp with repsect to ground/earth. ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 4, 2024, 1:40pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/4 "2024-08-04T13:40:25Z")
Thank you, I checked the banana to bnc connections, they are correctly oriented ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 4, 2024, 1:57pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/5 "2024-08-04T13:57:46Z")
After I checked the banana to bnc connection suggested by restorer-john, it suddenly dawned on me that I have never tried the left channel. My amps are monoblocks, so I can use either channel on the quant to test. So I changed the connections to left and and setup the readings for left channel on the screen, and viola! all the readings came out accurately corresponding to what I was seeing on the scope. Its a relief, 'cause as I am making changes to the amplifier, I don’t know if the changes are 'causing bad reading or is it the quant, so it was good get a good reading on the left channel of qa403. I am hoping someone from quant will get in touch with me regarding this unit… Correct 3% Distortion ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/b/bc7e0401b84b26837b535ab837954bd5d69cd894.jpeg) On the right channel, sometimes the reading would be 4.5W and sometimes 8W, so was hard to trust it ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 4, 2024, 1:58pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/6 "2024-08-04T13:58:38Z")
Frequency Response ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/6/6984cd179180a3873fd8718fceda95a24715e8f4.jpeg) ---
**Author:** ![Claudio](https://avatars.discourse-cdn.com/v4/letter/c/7cd45c/32.png) [@Claudio](https://forum.quantasylum.com/u/Claudio)\ **Post date:** [August 4, 2024, 3:21pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/7 "2024-08-04T15:21:47Z")
Hi. Try running the tests that are recommended by Quantasylum to check possible calibration problems with the QA403. You can find the procedure at this link: > **[Verifying Calibration](https://github.com/QuantAsylum/QA40x/wiki/Verifying-Calibration)** > > Releases (no source) for the QA402 and QA403 Audio Analyzer. - QuantAsylum/QA40x ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [August 5, 2024, 4:25pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/8 "2024-08-05T16:25:10Z")
Hi @sganti, agree with @Claudio. If you are ever in doubt regarding your measurements, fall back to loopback to verify you are seeing what you expect. And then use a DVM (and IDLE mode) mode to verify your DAC outputs (see the note at the link Claudio posted). And finally, there are a lot of settings, and sometimes you can be in a mode that isn’t working as you expect. In that case, do a File-\>New Settings to put the SW and HW back into a known state. ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 6, 2024, 3:15pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/9 "2024-08-06T15:15:12Z")
thank you all, will do the loop back test according to the link sent earlier and report back over the weekend. I am not using DAC for testing, I am using the internal signal generator and verifying with my external signal generator too. I did the new setting trick too and that did not change anything. thx ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 17, 2024, 6:34pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/10 "2024-08-17T18:34:00Z")
This is the output of the frequency response sweep, test 1 from the calibration testing. I did not know how to get the two specific plots for amplitude difference at 60Hz and 1Khz ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/d/d581789a3a7b57805788df22250d0dbbd56075d1.jpeg) ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 17, 2024, 6:35pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/11 "2024-08-17T18:35:11Z")
This is the output of the dvm performance test the right channel is different from the left ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/1/1bbf5a377870002949b5fb9a717e3f37f24b451f.jpeg) ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 17, 2024, 6:36pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/12 "2024-08-17T18:36:34Z")
I could not get a graph for the " Verify Relay gain Ranges" test, a blank page pops up, but the ending screen shot is give here ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/c/c1a54517153b3c0ef0b74376c9782c1192abf0a4.jpeg) ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 17, 2024, 6:37pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/13 "2024-08-17T18:37:22Z")
Quant Asylum Support I hope this can shed some light on what is going on with the quant asylum unit I have thank you ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [August 19, 2024, 1:16pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/14 "2024-08-19T13:16:19Z")
Hi @sganti, somethin is definitely up with the right channel. Do you see any sensitivity to the right channel measurement if you swap shorting blocks, wiggle the cables, wiggle the shorting blocks, etc? If you think the cables and shorting blocks are all solid, I think you’ll need to send this in. Can you send a mail to support and Jan will help. ---
**Author:** ![sganti](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@sganti](https://forum.quantasylum.com/u/sganti)\ **Post date:** [August 19, 2024, 3:39pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/15 "2024-08-19T15:39:15Z")
Thank you Matt, I had already captured the screen shots in a ppt and sent those to support when I posted here. I also followed up with an email to support with a copy of your message Thank you for helping out Sridhar ---
**Author:** ![emc.audioresearch](https://avatars.discourse-cdn.com/v4/letter/e/5fc32e/32.png) [@emc.audioresearch](https://forum.quantasylum.com/u/emc.audioresearch)\ **Post date:** [September 8, 2025, 2:57pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/16 "2025-09-08T14:57:53Z")
Im getting the same issue. My unit has a 6 dB offset inboth channels. Is there any calibration compensation we can do? This is the second time I power the unit. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [September 8, 2025, 3:27pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/17 "2025-09-08T15:27:14Z")
Hi @emc.audioresearch, can you share the plots? Thanks! ---
**Author:** ![emc.audioresearch](https://avatars.discourse-cdn.com/v4/letter/e/5fc32e/32.png) [@emc.audioresearch](https://forum.quantasylum.com/u/emc.audioresearch)\ **Post date:** [September 8, 2025, 4:40pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/18 "2025-09-08T16:40:34Z")
Hi matt, I sent it to the support email, but despite of the plots, the loopback in balanced connection reads +6 dB, when it should read 0 dB in both channels. ---
**Author:** ![emc.audioresearch](https://avatars.discourse-cdn.com/v4/letter/e/5fc32e/32.png) [@emc.audioresearch](https://forum.quantasylum.com/u/emc.audioresearch)\ **Post date:** [September 8, 2025, 6:21pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/19 "2025-09-08T18:21:32Z")
![Captura de ecrã 2025-09-08 192055](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/e/ee77bc37f1104000f4852c1b17211bcf4d28f270.png) ---
**Author:** ![emc.audioresearch](https://avatars.discourse-cdn.com/v4/letter/e/5fc32e/32.png) [@emc.audioresearch](https://forum.quantasylum.com/u/emc.audioresearch)\ **Post date:** [September 8, 2025, 7:41pm UTC](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504/20 "2025-09-08T19:41:09Z")
Hi Sganti, How did this situation turn out? I’m facing an uncalibrated unit here aswell. I hope they have a support to solve the calibration remotly. [Next page](https://forum.quantasylum.com/t/qa-403-power-readings-off-by-6db/1504.md?page=2)