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RFI-Resistant USB Audio Interface

Posted: Sat Jun 17, 2017 6:51 am
by Dale H. Cook
An occasional client switched from using a Blue Box in STL mode to a stream feed after installing a cable modem at their 5 kW AM transmitter site. The installation was done by their IT guy who did not understand that an unbalanced output from the computer's sound card might be susceptible to RFI. Rather than spend what could be a great deal of (possibly unsuccessful) time trying to get the RFI out of the current installation, and instead of spending money (which they can't really afford) on a low profile pro audio PCI card, I would like to solicit suggestions for a reasonably priced balanced output USB audio interface that you have successfully used in a high RF AM environment (the tower fence backs up to the transmitter shack).

Re: RFI-Resistant USB Audio Interface

Posted: Sat Jun 17, 2017 7:40 am
by kkiddkkidd
Dale,

Have you tried an RDL STA 1 or 2 line driver to convert to balanced? I have used those with good success in similar hi RF situations. Build a short, good quality unbalanced cable and put the RDL on the back of the computer.
Dale H. Cook wrote:
Sat Jun 17, 2017 6:51 am
An occasional client switched from using a Blue Box in STL mode to a stream feed after installing a cable modem at their 5 kW AM transmitter site. The installation was done by their IT guy who did not understand that an unbalanced output from the computer's sound card might be susceptible to RFI. Rather than spend what could be a great deal of (possibly unsuccessful) time trying to get the RFI out of the current installation, and instead of spending money (which they can't really afford) on a low profile pro audio PCI card, I would like to solicit suggestions for a reasonably priced balanced output USB audio interface that you have successfully used in a high RF AM environment (the tower fence backs up to the transmitter shack).

Re: RFI-Resistant USB Audio Interface

Posted: Sat Jun 17, 2017 10:33 am
by Deep Thought
Have you tried a 600:600 ohm isolation transformer? If placed at the computer that should take care of the problem (unless the RFI is in the computer itself). It'll also help protect the computer from the inevitable lightning surge which the stick-on will not.

Re: RFI-Resistant USB Audio Interface

Posted: Sat Jun 17, 2017 11:21 am
by RodeoJack
This doesn't address the budget issue, but if all else fails, get a "USB Matchbox II" from Henry. It replaces the computer audio circuitry, is balanced in and out and can produce +10 to drive a transmitter directly, if you needed to.

I have 3 of these in a 5kW AM station, sitting less than 100 feet from the tower. RF is always an concern there, because they also have 2 FMs in the building.

I went through the same issues with unbalanced computer outputs. At this location, the Henry box fixed the problem.

Re: RFI-Resistant USB Audio Interface

Posted: Sat Jun 17, 2017 4:58 pm
by Shane
If you go the 600:600 transformer route, get two for the spare you'll need after the first one sacrifices itself on the altar of lightning. :D

Re: RFI-Resistant USB Audio Interface

Posted: Sun Jun 18, 2017 9:57 pm
by ChuckG
Shane wrote:
Sat Jun 17, 2017 4:58 pm
If you go the 600:600 transformer route, get two for the spare you'll need after the first one sacrificed it self on the altar of lightning. :D
I started adding MOV's across the primaries and secondaries and the carnage stopped.

Re: RFI-Resistant USB Audio Interface

Posted: Mon Jun 19, 2017 10:34 am
by grich
Good place for a 111C repeat coil. Ground the electrostatic shield terminal. Lots of terminal space to solder in the MOV's.