Hi first post here.
Got the Q403 and happy with it. I am trying to fit it to my lab environment witch is audio repair lab and need to make the unit balanced xlr connectors and since I am mesuring mic pre amps there is allways the fear that connecting the Q403 output to MIC in with +48V on culd dammege the d/a’s outputs…
Do you think I shuld build it a breakout box with dc blocking caps on the outputs?
Has anybody done it before?
Those caps need to be very big in value (4700uf) in order to match the -3 db point planned on the 403’s inputs…
I will be happy to hear your thoats on that.
Thanks.
Oded
Hi @Deadcap,
The danger for the QA40x inputs and outputs is when 10 mA or more is flowing. A +48V phantom supply is usually provided through a 6.8K. So, the max current that can flow is around (48-14.5)/6800 = 4.92mA.
Is your phantom power provided through a 6.8K or so?
Thanks, Matt
Thanks for the fast and accurate answers Matt 
Ok I guess you have calculated the +48v - rail voltage threw the protection diodes to +vcc on the outputs?
Anyway its good to know they exist there…
I encounter 99% of the time 6.8k resistors for Fantom power which I believe to be a standard.
Is 10ma the max current threw the out’s protection diodes?
Thanks
Oded
Hi @Deadcap, yes, the 10mA is what the protection diodes can sustain. These diodes are in the OPA1612 opamps, so you can read more about them in the OPA1612 spec. Parallel to the opamp’s steering diodes, there are other discrete steering diodes on the board that steer the current into TVS devices that are bias “on” around 12-13V. Those can handle a 100mA continuously, and 1A surge. The discrete circuits, due to inductance, are more of a second line defense rather than front-line. So, the 10 mA should drive your analysis.
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Dear Matt thanks for those details it is useful to know when I am operating safely with the 403 as I am connecting this instrument to all sorts of other instruments at the lab all day long…
On an other subject, Today I have connected the 403 in my lab for the first time and saw that the oscillator is not operating continuously I have connected it to an oscilloscope and saw that the sine wave has a short interrupt at about each second and this interrupt is synced with capturing speed of the analyzer…
I need the oscillators to be a steady sine wave during fft work.
It is a critical matter to me especially when measuring compressors that have dynamic behavior highly depended on input signal changes…
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Use the Idle button. That will make it continuous.
Hi @Deadcap, if you need a constant sine, you can use the IDLE button in the Run/Stop group (below the RUN button). When pressed, if you are not making measurements, then a constant sine will be generated at the GEN1 settings (freq and amplitude).
If amplitudes are changing during FFT operations, the FFT result is really hard to interpret. For this reason, there are a few plug-ins and methods that can help. Please see the links below: