I’d like to be able to measure the maximum level and headroom of DUTs like EQ’s and compressors.
Obviously the QA403 isn’t able to do this as its max output is 20ish dB. So, I’m looking into solutions that can work in conjunction with the QA403 for balanced connections.
Any ideas? 
Hi @peppermint, the QA403 in balanced can do 16Vrms = 24 dBV = 26 dBu output. As you note, some pro audio can work up to 27.5 dBu = 18Vrms. I don’t know of an inexpensive way to get into that region. In a pinch, an OPA1612 powered by 4 9V batteries (+/-18V rails) at a gain of 2 could get you there. Run it inverting to avoid input common mode range issues, and you could get very close to the output rails which would give 12Vrms unbalanced or 24Vrms balanced = 30 dBu.
Thank you for chiming in MAtt.
I’m not sure if I’m doing something wrong but I can’t get past ≈+20dBu out of the QA403 in balanced.
Edit: I just attempted to use both GEN1 and GEN2 with the same settings and max amplitude (20.22dBu). Is this the right approach? Either way, I don’t seem to be able to set a value higher than +18dbu in the automated tests in order to achieve something along these lines (AP image)…
Hi @peppermint, can you confirm the following:
GEN1 button in GENERATORS control groupIDLE button in Run/Stop control group.Place DVM across L+ and L- output and confirm 16Vrms is measured.
Hi Matt,
Both L/R outputs read ≈15.856Vrms and ≈4kHz instead of 1kHz that the GEN1 is set to.
Hi @peppermint, ok so 15.856 Vrms = 24.003 dBV which is precisely as expected. The 4 kHz is unexpected. Are you sure about that?
It seems that I had the “Output Gain (dB) = +6dB” enabled in the “external gain” setting. It’s still a bit unclear to me what this does.
Also, I measured again today, with the same True RMS DMM and it indeed measures 996Hz. I really don;t know what happaned yesterday.
However, when I try to do some Automated tests like “AmpThd Vs OutLevel” there’s a higher limit and I get a warning that “value must be <=18”. This is regardless I choose dBV or dBu. This can['t be correct, right? Why the upper limit isn’t +26dBu when dBu unit is chosen?
Hi @peppermint. There is a clear explanation of how “Output Gain"” works at this link. Output Gain in "dbV Option" - #2 by matt
Thanks for the link. I will try to read it a bit more carefully but it’s still confusing to me.
I work exclusively in dBu and only with balanced devices so I use the QA403’s I/O as balanced.
When I set the generator at +4dBu, I measure +10dBu with my DVM across L+ and L-.
In order to measure +4dBu, I need to set the “Output Gain (dB) = +6.02dB” and I don’t touch the “Input Gain (dB)” (i.e. 0.00).
But now the maximum output I can get from the GEN is +20dBu and not +26dBu.
This is why I initially started this thread actually.
Also, all these measurement I mentioned above are valid with 'IDLE". With “RUN” activated instead of +4dBu, I measure ≈+2.78dBu
Hi @peppermint, OK, I see the bug now. When you have an output gain value set, the max output level is still limited to 18 dBV no matter what. What should happen is the max value should be limited to 18 + user specified output gain. This will be fixed for the next release. For now, to get the higher levels out, you will need to set your output gain to 0 and remember the output level is +6.02 higher if you are in balanced mode.
Thanks very much for reporting this, and sorry I didn’t understand it right away!
Thanks Matt, the confusion is most likely due to my broken English. Looking forward to the update!
I’m just thinking out loud, but could incorporating GEN2 into the automated test somehow increase the maximum output level beyond +26 dBu?
Hi @matt,
I’ve downloaded and installed 1.216 hoping to have this issued fixed as stated on the log.
However, unless I’m doing something horribly wrong, the behaviour seems to be exactly the same.
The DUT is transformer balanced I/O and I have set the "Output Gain(dB)=+6.02dB in the “External Gain” window. When I set the GEN1= +4dbu my DMM reads 1.227 Vrms across the + and - so, I assume that’s all good.
However, when I try to do an automated test (AmpThd Vs OutLevel) I can’t go past +18dB exactly like I described in the past.
Am I missing something?
Hi @peppermint, I think the issue is the use of dBu on the tests.
I have made a fix and released a special version where just the “Amp THD versus Input/Output Level” plug-in is active. This has fixes
https://cdn.shopify.com/s/files/1/1631/5609/files/setup_QA40x_1.217.exe?v=1749825335
If you set your output gain to 6.02 dB, then your max output should be settable to 18 + 6.02 = 24.02 dBV. And then, if you ask for 25 dBV, you’ll see:
Similarly, in dBu mode, the max output will be 18 + 6.02 + 2.22 = 26.24. So, a request for 26.0 dBu will succeed:
When time permits, will you see if this special version works as you expect? If it does, then the changes will be rolled into all the plugins.
Hi Matt,
Thanks for the prompt reply. This solution seems to do the trick.
I will try to test it a bit further sometime next week but so far it looks fine! Thanks!
Could you please elaborate on where the 2.22 dBu value comes from? Wouldn’t it make more sense for the upper limit of the ‘Stop Level [dBu]’ to correspond to the maximum output level the QA403 can handle without clipping, regardless of the user’s selected output gain setting (e.g., 0 dBu, +6.02 dBu, or any other value)?
I hope I make some sense
Hi @Pepptermint, yes, this question makes perfect sense. I’ve shared this picture before
It’s helpful to think about your vantage point for talking about measurements. When you specify an input gain, you are telling the analyzer “there’s some input gain…but I’d like to talk about everything from the vantage point of the DUT”. And then, when you add in a 30 dB attenuator for measuring a large amp, you are telling the analyzer “I’d like to talk about the things from the DUT view point.” And so, if the analyzer measures 0 dBV, the real measurement (at the amp) is is 30 dBV. And the analyzer will indicate a 30 dBV signal in spite of just 0 dBV at the analyzer input.
And it’s similar on the output side. When you tell the analyzer there is 6 dB of output gain, you are telling the anlyzer you want the levels you specify to be measured at the DUT input. The actual level at the analyzer will be different, depending on the output gain you have indicated.
