Audio Processor for AM Transmitter

This audio circuit is used for AM transmitter audio procesing. It equipped with low pass filter circuit for AM spectrum. It's easy for use.

MK1 AM Audio Processor
My first attempt at an improved audio processor for GCR using a Plessey SL-6270 VOGAD (VOice controlled Gain Adjusting Device) which is designed more for PMR use than broadcast use but with the right parameters does a reasonable job. It deals with a massive dynamic range of inputs (about 60dB) which does tend to mean that if you don't set it up properly you can hear every tiny background noise in the studio. There's a bit of low and high-pass filtering on the input to avoid unwanted frequencies causing the compressor to 'pump'. There's also a lovely 7-pole Chebyshev low-pass filter on the output with a cut-off at about 6 kHz. All the op-amps are TL072 or TL074. Watch out for the difference between the 'earth' symbol and the 'ground' symbol as one relates to 0V and the other to mid-rail (typical supply voltage is 15V).

MK 2 AM Audio Processor
The Mark 2 version of the processor above. Now with an added limiter after the compressor (based around an MC3340 and a rather odd BSV71/BFR29 I.G.F.E.T.). A new 6-pole output low-pass filter to provide a tight fit to the transmitter specifications allowed at the time for closed-loop AM radio stations. There's a bit more HF boost (or pre-emphasis) before the clipper on this version to give the audio a more 'lively' feel. This design was in use at GCR for about 2 years until my super 3-band processor took over. Same precautions over the diagram as above apply and op-amps are also TL072 or TL072.


MK 3 AM Audio Processor (Part I)
The Mark 3 version got massively more sophisticated (and better sounding). This diagram is for the heart of the processor. It's a 3 band audio processor/limiter/compressor (call it what you will) with a few special features: (1) The decay on the bass compressor is tied in to the mid compressor which gives a much more balanced sound, and (2) there are cross-overs before and after each compressor, thereby reducing any distortion produced. The resulting sound is very loud and very impressive. The 3 audio bands compressed are 0-250 Hz, 250-1300 Hz and 1300-6500 Hz. The +/- 8V supply is quite critical as it sets the threshold for compression to the MC3340 chips. There's still a transmitter kicking around with this processor in it - if I can get my hands on it I'll record some output.


MK 3 AM Audio Processor (Part II)
This is the second half of the Mark 3 diagram and shows the input high and low-pass filtering; the clipper (with a snazzy LED to show when clipping is occuring); and a new and even better output 6.5 kHz low-pass filter together with a phase corrector to provide overshoot compensation (and therefore allow the transmitter to be driven harder).




AM Low Pass Filter Response
The frequency response of the output filter is flat to 6 kHz, -3dB @ 6.5 kHz, -23dB @ 7.6 kHz and -40dB @ 9 kHz, which fits exactly the allowed response for closed-loop AM stations for which it was designed. Without the overshoot compensator, the filter has an overshoot of around 2.6 dB; with it, overshoot is virtually nothing - thereby giving a 2.6 dB increase in loudness. All components (including the capacitors) need to be 1% tolerance for this circuit to work properly.

Source: ZFM