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Old 30th Oct 2017, 10:52 pm   #5
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Infinite Impedance detectors.

Yes, the undesirability of overly high Q in the final IFT could have been a factor mitigating against the use of the infinite impedance demodulator in high quality receivers. If one looks at British hi-fi AM tuners of the 1950s or so, then the diode demodulator looks to have been the majority choice.

Even so, a couple of examples with infinite impedance demodulators come to mind. One was the Rogers RD Junior of 1952. This had a 12AH8 mixer-oscillator, 9D6 IF amplifier and 6C4 demodulator. Amplified and delayed AGC was accomplished through a 6AM6 sidechain IF amplifier and a crystal diode. IF bandwidth was variable, with 7, 15 and 23 kHz positions being available. So, the final IFT must have been reasonably well damped in the widest position. One may wonder why not a diode-pentode for the AGC circuit, but perhaps it was because Brimar did not offer one at the time.

The other was the Armstrong AM44 of circa 1955. It was sometimes described as having both an infinite impedance demodulator and a cathode follower output, but I suspect that the apparent two stages were one and the same. There was three-position switched variable bandwidth, although I do not know the actual numbers. I suspect though that the widest position would have been 20 kHz or so.

The inherent low impedance output of the infinite impedance demodulator was a potential advantage in a tuner that might be used with a variety of amplifiers of differing input impedances. With diode demodulators, the load impedance they looked into had to be sufficient so as not to materially lower the AC-to-DC load ratio and so introduce distortion. A separate cathode follower stage after a diode was always a possibility, but this required a triode or a pentode with an “uncombined” cathode, precluding the use of standard European double diode-triodes or double diode-pentodes, which had “combined” cathodes. So an additional valve, such as a 6C4, would have been needed anyway.

Whatever was the reason for Armstrong’s choice of the infinite impedance demodulator for the AM44, it reverted to a diode demodulator – in this case driven by a class A triode – for its ST3 of 1958.


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