Digital Interface - transformer seems to drastically weaken signal
Rob French (KC4UPR)
I've prototyped myself a variant of KK5JY's AnyRig digital interface ( http://kk5jy.net/AnyRig-v1/ ). The Teensy microcontroller piece of this is very straightforward to me, as I have previously embedded a Teensy in my uBITX as a DSP. In this case, I'm externally interfacing a uBITX to computer audio (trying to keep the uBITX relatively stock).
Anyway, the issue I have is with the 1:1 isolation transformer for the RX audio. It drops the RX signal level drastically! If I drive the RX line out straight to the Teensy audio input, I get a solid signal level of ~0.5V peak-to-peak, based on observing the signal level on my computer using Audacity (I have an op-amp preamp driving the line-out). If I put the isolation transformer inline, the signal level barely even registers! I have to amplify it significantly in software to hear the signal, when using the transformer.
I tried it with two different transformers. About the same on both. The transformers are cheap Amazon ones ( https://www.amazon.com/gp/product/B07HTBQ7FY/ ), but given that I really only needed them to cover ~300-3000 Hz, I figured they'd be sufficient... I wouldn't think they'd have that much loss! What am I missing? What should I look at?
(The RX line-out is driven by an op-amp pre-amp per this URL: http://rason.org/Projects/opamps/opamps.htm )