Interfacing test equipment with audio gear: 50 ohm BNC to balanced XLR - Page 1
Forum section: electronics / projects

author: DaJMasta
author_rank: Super Contributor
timestamp: September 05, 2016, 12:09:43 am
content: I want to mess around with my mixer and I want to do it by feeding in something from a function generator and then reading out onto a scope (as well as recording using the inbuilt digitizer and taking a look with some spectrum analysis software), but I'm uncertain how to interface them with minimal distortion. I want this to be accurate enough to characterize the preamps and line level handling of the mixer, something that's as transparent to the signal as possible.The trouble is the format, your normal 50 ohm or hiZ mode on the test equipment and the balanced pair on the XLR and going back and fourth between them (I am looking for both a dedicated output and a separate input, two adapters). The closest commercial product I've found has been an adapter made for SPDIF audio that goes between 110 ohm balanced XLR and 75 ohm BNC coupled through a transformer, and while I think that's theoretically fine, I don't know whether it will be viable at audio level bandwidths - the AES3 protocol it's designed for seems to operate at a few MHz. I think the transformer isolation is great - no need to worry about ground loops between the test equipment and the mixer - but I don't know if these are likely to be good enough to be usable or if I should be building my own thing.So does anyone have experience interfacing these kinds of gear? Do you think an SPDIF transformer could do good job in the audio band? If not, do you have a preferred audio transformer or other solution to recommend?Thanks!

author: flynwill
author_rank: Regular Contributor
timestamp: September 05, 2016, 03:57:52 am
content: 99% of the time there's no problem feeding a balanced input from an unbalance source. There are inexpensive phono to XLR adapters that simply connect the center conductor of the phone plug to one side of the balanced pair and the outer connector to the other two.If you want to feed the microphone inputs from your signal generator you will likely want to add a 20-30 dB attenuator in series.If you really want get to balance line levels from unbalance signals then the most commons solution is the Henry "Matchbox".Also several companies make audio frequency balanced line to unbalanced transformers, Jensen being one of the more popular.The balanced/unbalanced AES transformers are optimized for AES digital data will not work very well at audio frequencies.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 05, 2016, 06:12:46 am
content: Thanks for the tip! Looked into it a bit and apparently a direct box - typically used for hooking basses and such directly to a PA system - seems to be exactly what I'm looking for - an audio frequency tuned isolation transformer with a selector to share/float the ground of the balanced line and some switch-in attenuator options. Pyle's PDC21 (or 22 for two channels) looks like it will be very inexpensive, and they're advertising .013% THD at 1KHz, a -104dBu noise floor, and a max -3dB frequency response extending beyond the audio band (5-35KHz).... for less than $20 US.Will probably pick one up and give it a shot! I'd certainly hear other suggestions, though.EDIT: Just occurred to me, if you've got a dual channel function gen, you could just set channel two to 180 degree phase and direct drive the balanced input. I think you'd loose out on some modulation or other features on some generators, but it should do the job.