Hey all..
Quick question,
I’ve thought of a potential issue when using the q403 generator outputs. I often connect to balances inputs that have 48 v Dc phantom power. In many cases I can’t turn that off. Will this fry the DAC section of the 403? Any thoughts would be greatly appreciated…
Cheers
Hi @skipburrows, this is a good question to wonder about. Most phantom supplies have a large pullup to the 48V. And then the device (microphone) will wiggle the pulled-up line. Typically, that pullup value is about 6.8K. And so, if you connected a phantom signal line to a dead short, it would pull 48V/6.8K=7.5mA. On the QA403 input or output, it will divert any input or output voltage in excess of about +/-13V to some internal zeners that can handle hundreds of milliamps. And the input and output opamps are OPA1612, and those have their own steering diodes that will shunt currents in excess of 10 mA to the supply rails.
Now, the worst-case is device unpowered. In that case, there’s no active supply rail to shunt the 7.5mA to and so it will backpower the internal rails. I’d not leave it connected like that for hours, but it’s fine for a few minutes.
The nominal case of the QA403 outputs connected to 48V via a 6.8k resistor isn’t a problem either for survivability. But your output range may be compromised because the output opamps are having to sink ~8mA more current to drive the output to 0V with no signal present.
If your phantom is higher than 48V, or if the pull-up is much smaller than 6.8K it might good to re-evaluate. But generally for 48V and 6.8K pullup, if the QA403 off the application of phantom to the DAC outputs for a few minutes shouldn’t be a problem OR with the QA403 ON the indefinite application of phantom shouldn’t be a problem.
Hey thanks Matt.
Being that I rarely have schematics of the tested device. Should I just measure pins 2&3 of the xlr and see what the resistance is between them? Just trying to come up with the best way not to fry my 403..
Cheers.
Two capacitors? For a mic input with 2k input impedance that means 22uF to 100uF sort of range, depending on how much flatteness you need at bass end.
Hi @skipburrows, maybe take your DVM and config the DUT so that you can read the expected voltage on pins 2 and then on pin 3. And then, switch your DVM to mA DC mode and measure again. Your DVM will put an effective short from pin2 to ground, and the current it reports will let you determine the pullup resistor value.