Audio Processing for All Pantry TX's

[Argus25]

It is not really too complicated to do this, as all the work is done in the IC's. It is also much better to do this than feeding any pantry TX's modulator directly from the audio source.

I had this pcb manufactured some years ago so that it could simply be dropped into a Tek sinewave generator to turn it into an AM transmitter. (A modulated oscillator was already in there)

Everything to process the audio signal prior to an amplitude modulator is on this one pcb which is about 75 x 115 mm. (The two holes were to mount it to existing screws in the generator)

I have attached a graph which shows the time averaged volume compression.

It is a rough hand drawn graph I cleaned up a little. Graph A shows the level of compression with one stage (or channel) of the NE571 deployed and graph B shows two stages cascaded and graph C & D more theoretical stages added. There is little point going to more than two stages cascaded, as the compression is not a lot better and also the "no signal noise level" comes up too high.

So for example, looking at graph B, if the time averaged rms input voltage increases from 1v to 3v the output rms voltage only rises from about 0.26V to about 0.34v

Since there was only a +11.5V supply in the sinewave generator, I made this unit run from regulated 8V, but I found that this is about as low as you can go for good performance of the NE571, or its equivalent the SA571, it is better to run the unit from 12V and use a 10V LDO regulator to run the circuit.

I have the parts list as a 3 page pdf , attachment attempted. The pcb file is probably still at my pcb maker.

To set it up, all that is required is the external panel mounted 2k2 modulation level pot is set at about 50% rotation and the input drive from a test 1kHz sinewave generator on the input is increased until the peak clipping LED just lights.

(The peak LED circuit has a charge pump so a brief peak clip in the music content is extended in time to light the LED for a clearly visible period, also the LED is only lit when real peak clipping current flows in the B-E junction of T1 used as one of the clipping diodes)

Then with the LED just lit, the modulation level pot VR1 is adjusted so the amplitude modulator is modulated to 90%.

This way, in use, it is obvious when modulation level is such that the peaks are being soft clipped at 90% modulation. Also its not possible to modulate the carrier out. All of my pantry TX's have this modulation pot and clipping/peak modulation indicator LED on their front panels.

[Ed_Dinning]

Nice circuit Argus, it should be useable with most of the units that have been associated with this forum.

Ed