Rob French (KC4UPR)
I'm looking to potentially add an audio DSP filter to my uBITX... I've got a Teensy 3.2 lying around that hasn't been used for anything, and I figured that 'd be the perfect toy to try out for this application. Some thoughts and questions in no good order...
(1) I'm thinking I'd like to tap off the VOL-H location... so the DSP filter would come before the volume pot. What are the pitfalls to this, if any?
(2) I would also use the Teensy as the interface to the computer for digimodes. So by having the Teensy prior to the volume pot, I would be able to have separate control over the level into the computer, and the level through the speaker/headphones. I could adjust audio output with the volume control for my ears, but meanwhile have a separate, independent control over the level seen by the computer (more so for e.g. CW w/ fldigi rather than PSK or FT8).
(3) Buffer? Transformer? Both? I see that the JackAl has a similar arrangement, though it taps the audio off of VOL-M. Why is the 1:1 isolation transformer prior to the preamp? Would it be viable to have a high-impedance buffer first (to decouple the two outputs), and then the 1:1 transformer prior to the Teensy?
(4) Is there a spec or expected audio voltage level at the output of the uBITX audio preamp? (prior to the volume pot and LM386) The Teensy audio adaptor has adjustable sensitivity, so I would think that I would want to set the input sensitivity (which goes up to ~3V, I believe) based on the design output peak voltage of the preamp, plus a small pad.