Speech processor for amateur radio AM-transmissions

The aim was to modify the flat frequency response of an electret microphone and limit the modulation bandwidth of AM-transmitter.

The transmitter in mind was an old Collins 32V-2 from 1950.

In practice +/- 7.5 dB pre-emphasis ( 15 dB more gain on 2000Hz compared to the gain on 200Hz) has shown the best results

for me in on-the-air-tests.

The phase rotator gives some more loudness to the signal, without introducing any distortion.

More about the concept see www.w3am.com.

The frequency response is flat between 200 and 3000Hz.

The phase delay between 2000Hz and 200Hz is 800 degrees.

The 90 degree point of each phase rotating stage is on 340Hz.

You may wonder the 10k resistors from output of each op-amp to ground. LM 324 has some cross-over distortion and

this resistor adds bias current to the output transistor and eliminates that distortion.

With better op-amp's those resistors are not needed.

OH1TV

19.2.2007