more pre-emphasis stuff

[Opcom]

Just some LTspice stuff using RC and LC circuits and op-amps to see if something can be made to give a decent LPF action around 3.4KHz as well as pre-emphasis up to that point.

Looks OK on tyhe PC. Maybe more pre-emphasis than 10dB @ 3.3KHz would be useful but it's what was had. How odd for me to use solid state op amps. Not sure what came over me. Maybe this century or something.

[KK4YY]

I modified the active LPF (Q9 on the AF unit) in a Kenwood TS-711A all-mode 2M xcvr to open up the FM transmit audio response. Changing a few components on the PCB and using a good mic was all it took. Instead of cutting off at 3khz it now has a rising characteristic from 1khz out to 6khz then rolls-off. A few fat coupling caps brought up the bass response. It sounds much better now... at least on simplex, which I prefer.

Not having LTspice, I used an online active filter calculator and iterated values until I got what I was looking for.
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm


Don

[Opcom]

I have done versions of the low pass part that go out to around 6K and one that is more resrictive than 3400Hz. Not sure what's really necessary but it was a good exercise to figure them out.

The active filters look pretty good. One has to try different values in LTspice just the same as on that page. Or calculate them from formulas in books.,