Interesting how simple things were, or rather made to be as simple as possible, and yet from everything I read about this unit it sounded better than some of the more modern digital MPX circuits which makes sense to me since they are based on noisy OP amplifiers and other stuff to make something simple more complex than necessary.
The circuit for the RCA is on the bottom of the page. Look at the simplicity! Compare that to modern MPX stereo generators which use a dozen IC chips.
I wonder how hard it would be today to implement a similar design using only transistors (no ICs at all)? I think the hardest part would be getting hold of quality audio transformers but if someone had the know how it could be done using very few parts I bet!
With all the stereo discussion going on I figured this thread may capture some interest amongst you guys ;D
Funny, I was working with a buddy on the weekend on a CSR-5 receiver whose performance sensitivity wise blew a modern digital digital receiver 70 years newer out of the water.
Looking at a lot of SDR schematics, it's like the sloppiest receiver in the world, just all fixed up in software and DSP.
I think if someone really knew what they were doing, they could use transistors and avoid the need of transformers entirely. As you know, transformers are really bad for good audio fidelity.
I agree with you Kage, I dislike over-complication for no good purpose. Usually, simpler with modern components, makes for superior results.
Sounds like xfmrs have improved since I was playing with them back in the 60's, cmradio!
I still say that I would use semiconductors in circuits that would eliminate the need for transformers entirely. If the circuit configuration is properly designed, this could be accomplished.
I have some (ok, a 150# box) of those Hammonds that were good for only voice, so I know the ones you mean
The best way I can think of is a nice instrumentation opamp, say one from Burr-Brown or Analog Devices. Should be good for 120dB of CMRR and are under $10 and most often ~$3 nowadays.
The more I look into this the more easy it looks like it would be to design one from scratch using transistors. All of those transformers could be replaced with transistors.
The two output transformers could be further removed using a simple combiner circuit albeit using a transistor amplifier.
So that circuit would take care of the 38kHz subcarrier. Then all would need to be done is making a 38kHz oscillator, a divider to get a 19kHz pilot, and some basic transistor circuits to get the L-R (input into the DSBSC circuit) and the L+R, and then finally mix it all together using another transistor amplifier to get the final stereo MPX.
Probably sounds way easier in my head than in reality but I am sure with enough coffee, ingenuity, and time I could figure out a really simple way to get MPX using nothing more than transistors and very basic building blocks. The part that frightens me is getting the phase of all audio stages to be identical.
My guess is that it would sound spectacular considering there would be no noisy opamps in the design, no cheap and dirty digital switching like used in the BA1404 clone chips, and any unwanted artifacts can be cleaned up in individual stages, unlike when using an all in one chip where one is stuck with it's permanent issues.
Since I have a computer spectrum and waterfall analyzer, a basic oscilloscope and most of the other tools I need I don't see any reason why this can't be designed.
If any future plans for this idea come up or I get something working in the lab I will make a new thread and let you all know.
I think it may be possible to get a design so simplified that other hobbyist could build it themselves.
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