Today, I'm taking a short break from my usual recapping of Studer input modules, and am trying to find out whether a vintage BSS AR113 DI box that was noisy and weak last time I used it is faulty in a way I can correct.
To do this, I'm applying a tone from my Fostex TT15 generator on the unbalanced input, and monitoring the balanced output with my ScopeMeter. Easy as pie, until you think about the fact that it's phantom powered, and that it's not unlikely that the powering circuit is involved. Of course, we can power it with a mixer input channel, but I'd like to eliminate unknowns. Thus, the Phantominator
(no TM) was born:
(that's
http://vvv.besserwisser.org/Public/Bilder/phantominator.jpeg if the forum still is buggy)
It is a XLRF for the DUT, with pins 2 and 3 duplicated on binding posts and connected to a center-tapped transformer (from a modem, so allegedly shite audio-wise, but OK for 1KHz sine, and apparently, since that's what I'm running here, 15KHz sine.) with the center tap, also on a binding post so that I can inject +48VDC there. On the other side of the transformer, two binding posts are provided, giving a galvanically isolated output that has no trace of DC and also is safe to use with grounded scopes. Rounding out this is a ground binding post connected to pin 1 on the XLRF and thus providing the 0V injection point for phantom power.
Here, the Scopemeter measures output, the ABB-branded Gossen DC mA, and the tone generator, well, generates.
Now, the sine is a bit fuzzy, so I'll have to dig out my good specimen AR113 and compare.