[Synchrodyne]

Heathkit UK used a full-wave AM demodulator in its AFM-1 hi-fi tuner of 1962. In respect of that it claimed:

“A push-pull detector circuit maintains the symmetrical loading of the driver stage; it uses two germanium diodes connected in such a way that one conducts for each half of the R.F. waveform. This reduces both R.F. and audio distortion and, since the R.F. ripple frequency is doubled from 470 to 940 Kc/s, filtering of the I.F. signal is more effective.”

Although the AFM-1 was a UK-only model, this form of AM demodulator appears to have been carried over from established Heathkit US practice, having been used for example in the PT-1. Heathkit US also used two-diode voltage-doubling AM demodulators in some of its models.

On the other hand, Sturley had this to say about full-wave (double-wave) demodulation:

“It has been shown that only under special circumstances can double-wave detection prove superior to half-wave detection, and the latter is almost universally employed in receivers.”

No elaboration was provided, but there was a reference to an article by W.T. Cocking in “Wireless Engineer” for 1935 November.

Regarding the infinite impedance demodulator and loading effects, Langford-Smith observed:

“There is seldom any difficulty with a.c. / d.c. ratios as the grid resistance of the following a-f stage is generally about 0.5 megohm.”

This suggests that although it may have been less sensitive to the AC/DC load ratio than typical diode demodulators, it was not immune.

When all is said and done, I suspect that the devil was in the detail, and that circuit implementation had as much, perhaps more weighting in terms of performance outcomes than choice of basic circuit. Certainly, many of the hi-fi makers who put some effort into obtaining good AM performance chose conventional diode demodulators, amongst them Chapman, Dynatron, Jason and Quad. And wth the exception of the AM44, Armstrong used diode demodulators. As well as demodulator distortion, some attention was usually given to minimizing differential and modulation rise distortions in these designs.

The setmakers in general may have been less concerned about AM performance, particularly after the advent of FM-AM receivers. But the Murphy A188C was an example of an AM-only receiver in which better-than-typical performance was self-evidently an objective. It included variable selectivity with a wide audio bandwidth that I’d guess was around 10 kHz. It had a conventional diode demodulator. But the export TA160 model of the same era had a double-diode, full-wave demodulator. That was counter-intuitive. The TA160 had single selectivity, probably tailored for shortwave reception, and I doubt that the IF bandwidth allowed an audio response much beyond 5 kHz. So post-demodulator filtering would have been an easier proposition in the TA160 than in the A188C, with the latter perhaps more in need of the potential benefits of a full-wave demodulator. Different designers with different ideas, perhaps? Or maybe since the TA160 used two 6LD20 double-diode triodes, in order to provide two AF triodes, someone thought that they may as well make some constructive use of the surplus of diodes that were available as a result.


Cheers,