Split band audio signal processor 2007 for SSB and AM

I build my first split band speech processor already in 1976. Since then I have used it.

For some reason that external processor has given better results than the integral one's in my

Icom rigs.

The idea in split band processor concept is to avoid harmonic and intermodulation products, minimize distortion.

Smaller distortion is achieved by clipping the signal in narrow frequency bands and having efficient band-pass

filters after each such clipper. Threfore filters are needed before and after the clippers.

The challenge in making a split band processor is in getting the 200mH level coils. Those are not easily available.

Some time ago my favorite outlet had Vogt 2x39mH common mode filter coils at bargain price, 1 Euro each.

So I bought 50 of them and begun to build another split band processor. My idea was to make the filter bands

more narrow than I had in the vintage version, this time exactly one octave each.

The block diagram and schematics are enclosed. As only one inductance value was available

different impedance levels had to be used for different frequency bands. On lowest band two inductances

were connected in series to avoid 200 ohm impedance ( which might work as well). So 24 coils, 156mH each, are used

in this processor.

Each inductance was measured. As there was variation +/- 10% in inductance, the capacitors had to be

matched individually in each filter in order to reach the targeted responses. The component values shown

in the schematics are for ideal case.

The same settings as with the 30 year older version seem to be the best: Clipping amount on the

lowest band can be up to 13 dB, on the mid band a bit more, up to 15 dB and on the highest band between

13 and 21 dB, which is the practical maximum.

In summing up the bands best results are achieved by attenuating the lowest band 3-4 dB and

the mid band 1-2 dB. The highest band is not attenuated. The full 10 dB tuning range is not quite needed.

The microphone gain is set to give 4-5 Vpp during speech peaks in the output of LM386.

That is about the point were overdrive indicator LED starts blinking. With my Heil headset the setting

is -6dB from maximum.

Pekka, OH1TV

28.12.2007