<<TAKE 3>>
Thanks for the well reasoned replies. The ART device looks interesting, about $105 here in Oz.
We really are lucky to have such knowledgeable folk who are willing to share and help. (Without the "GOD MODE" attitude found on most specialist forums).
Marcel, as for the RFI, I can't really blame the little Behringer for having the occasional conniption. The environment isn't *exactly* what the designers had in mind.
The station is my amateur radio (ham) station. Hi everyone I am VK2CSW and I am a sad-sack radio geek.
The interface is for digital modes such as FT-8, JT65, WSPR, etc. The radio is a FT-857D (@ ~100 watts) into a 20m wire dipole above the roof.
Most of the RF getting into the mixer was via the headphones that were being used to assess the 'cleanness' of the digital tones - ironically this was being done to avoid causing RFI.A toriod ferrite on the headphone lead sorted a lot of it. The set up is not really conducive to avoiding RF in the (presumably) un-screened mixer (see attached).
The mixer is right next to the transmitter and directly below the antenna. Once I cleaned up the input signal (was being overdriven) there is no obvious RFI in other devices around the house (TV's, AM radios and the like).
As I have said I have been playing with a simple 1:1 transformer interface which I have been struggling with due to the lack of test tools. Well lack of test tools and issues with the super deluxe quality of $5 USB sound cards - thus the idea of bypassing the sound card by using a USB Audio device. It also has the advantage of not accidentally transmitting any windows noises, on-board mic or the thrash metal MP3 I happen to be listening to. I have ordered a $30 TFT oscilloscope kit - cheap and nasty but as I am losing sleep trying to work out why a simple circuit isn't working, I think is a good investment.
I have ordered some USB extension cables and will knock up some interface cables to move the mixer further away - originally, in good ham radio fashion, I made the cables as short as practical (to avoid RF issues, yes irony again).
There isn't any appreciable ground or 50Hz hum being picked up, so the ground isolation isn't quite as important right now. If it does raise its head I might experiment by bodging up some cables and chucking a DI into the circuit.
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