This was the first stereo MPX unit to come around as shown here...
http://fmamradios.com/BTS-1A.html
I have been interested in designing an all transistor version of this unit. Replace the transformers in the circuit with transistor stages to flip phase and so on.
The thing I see with this original stereo unit should be obvious. That is we are talking pure transistors, no opamps like todays MPX units.
I'd imagine using all transistors, and phased transistor circuits to replace the audio transformers in the original circuit would only lead to increased fidelity.
The issue I have with todays MPX encoders is that they are mostly based on noisy opamp circuits. There is unnecessary circuity inherent in opamps that cause noise.
I honestly believe a trip back to the past to see how things were done would make for a better future in stereo separation and noise, lack of pilot interference, and characteristics that aren't effected by audio 1harmonics because of inter-stage circuits to reduce that.
Almost all the MPX circuits I see today are based on noise producing circuits. Ways to cut corners to simplify circuits, when in reality the transistor equivalent wouldn't probably just work as good, but rival said IC circuits, and even be simpler?
A great example is the Part 15 unlicensed transmitters using BA1404 cloned chips. They all operate on the same principals that I can tell. Switch the 38kHz to produce DSB and mix it with halved 38kHz frequency to achieve 19kHz pilot, then take both and mix to a MPX signal to run into a mono transmitter.
Keep in mind the first MPX unit was shown above, using nothing more than two tubes. Today we have MPX circuits using a dozen chips at least.
Ah who am I kidding, no one cares about audio quality anymore anyways.
http://fmamradios.com/BTS-1A.html
I have been interested in designing an all transistor version of this unit. Replace the transformers in the circuit with transistor stages to flip phase and so on.
The thing I see with this original stereo unit should be obvious. That is we are talking pure transistors, no opamps like todays MPX units.
I'd imagine using all transistors, and phased transistor circuits to replace the audio transformers in the original circuit would only lead to increased fidelity.
The issue I have with todays MPX encoders is that they are mostly based on noisy opamp circuits. There is unnecessary circuity inherent in opamps that cause noise.
I honestly believe a trip back to the past to see how things were done would make for a better future in stereo separation and noise, lack of pilot interference, and characteristics that aren't effected by audio 1harmonics because of inter-stage circuits to reduce that.
Almost all the MPX circuits I see today are based on noise producing circuits. Ways to cut corners to simplify circuits, when in reality the transistor equivalent wouldn't probably just work as good, but rival said IC circuits, and even be simpler?
A great example is the Part 15 unlicensed transmitters using BA1404 cloned chips. They all operate on the same principals that I can tell. Switch the 38kHz to produce DSB and mix it with halved 38kHz frequency to achieve 19kHz pilot, then take both and mix to a MPX signal to run into a mono transmitter.
Keep in mind the first MPX unit was shown above, using nothing more than two tubes. Today we have MPX circuits using a dozen chips at least.
Ah who am I kidding, no one cares about audio quality anymore anyways.
