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I wrote a research paper on the then-new MTS system in college (circa 1983-84). The main component of the dbx NR (the one most noticeable on undecoded dbx) is 50% dynamic range compression.
Re-tuning, adding a 4.5 MHz SIF Amp (6AU6 or equiv.) and Ratio Detector (6DT6 or equiv.) ahead of the MPX input, and adding an expansion stage (6AT6, etc.) after the L-R Det to the FM-MPX circuit in the RCA RC23 and/or RC30 tube manuals plus building a +180V power supply (built around AES #P-T269EX and 6X4 or equiv. tube) to power it seems quite feasible. The FM-MPX circuit calls for +180V @ 15mA plus heater power for the 12AX7 and 6CL8. Add to that the B+ current demands and heater supply requirements of the other tubes ... 4.5 MHz Amp & Det, Expander, perhaps a 12AU7 as a stereo output buffer (twin cathode follower stages) if desired, and it's all set. The expander input circuit should be driven by the signal across C5 in Circuit 23-10 of the RC23 manual and the expander's output should drive the phase splitter (pin 7 of 12AX7).
Last edited by jshorva65; 04-17-2004 at 10:29 AM.
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