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.
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
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)
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.
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…
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:
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.
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.
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
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.
