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Old 12th Jun 2019, 2:42 am   #1
Synchrodyne
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Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,448
Default Dormer & Wadsworth (D&W)

I have some curiosity questions about the UK assembly maker Dormer & Wadsworth (D&W), of whom I don’t think that there has been much mention on this forum to date.

I know that D&W made solid-state FM front ends in the later 1960s and probably into the 1970s. Known users were Radford and Rogers, in hi-fi tuners.

Who else used D&W FM front ends? Were there any setmakers who used them as well as hi-fi equipment makers? Although I suspect that the Mullard modules were economically more attractive to the setmakers.

Did D&W start in the solid-state era, or did it have earlier activities with valve-based FM front ends?

What other radio/audio assemblies, if any, did D&W build?

And what eventually happened to D&W? I’d guess that the availability of FM front ends from Japanese sources such as Toko might have pushed it aside.

As to known users, Radford used the bipolar version of the D&W 341 4-gang model in its FMT2, and the 2 mosfet version of the 341 in its FMT3. (I think that the 2 mosfet 341 might have been an option late on in the export version of the FMT2.)

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Rogers used the 2 mosfet 341 in its Ravensbourne 2 FET tuner of 1968. It used a simpler D&W front end (model number unknown), 3-gang with mosfet RF amplifier and bipolar mixer, in its lower-priced Ravensbrook tuner of 1969.

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The 2 mosfet 341 was of the four-gang type with one RF amplifier, with a bandpass doubled-tuned input and a single-tuned interstage. This appears to have been the reverse of usual practice, which was to have a single-tuned input and a double-tuned interstage. For example, the Revox A76, which was an early benchmark supertuner, followed the usual practice, as did many Japanese products in the 1970s.

My understanding is that, at VHF at least (HF might be different), too much selectivity at the input compromises noise performance, even though it helps reduce large signal cross-modulation problems. Placing the bandpass at the interstage, where high selectivity is usable without worrying too much about noise effects, allows higher overall pre-mixer RF selectivity than would placing it at the input. Certainly VHF TV tuners, both valve and solid-state, usually had single-tuned inputs and bandpass interstages.

It is apparent that the D&W 341 was originally designed around bipolar devices, and later adapted to use dual-gate mosfets. Given that small-signal bipolar devices were known to have relatively poor RF performance, as compared with their valve predecessors when it came to cross-modulation and intermodulation effects, it could be that D&W thought that the better trade-off was to place as much selectivity as reasonably possible right at the input and accept the higher noise level. Then once then this was done, replacement of the bipolars by mosfets was done on a minimum change basis, meaning that the bandpass stayed at the input.

As to other users of D&W FM front ends, Sugden could be a candidate. This is the front end that it used in its R21/R51 FM tuner:

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It looks as if it could have been a varactor-tuned derivative of the D&W 341, retaining the bandpass input. Given that early implementations of varactor tuning were said to degrade cross-modulation performance, perhaps there was a case for the bandpass input where such were used.

The Sugden R21/R51 front end used different mosfets to those in the Radford and Rogers tuners, but this probably reflected the relatively quick progression in the early days. The RCA 40603 and 40604 were not gate-protected, and belonged to the 3N140 family. (Of which the variants and their nominal applications were the 40600 (VHF TV RF), 40601 (VHF TV MX), 40602 (TV 1st IF), 40603 (FM RF) and 40603 (FM MX)). The RCA 40822 was gate-protected, and belonged to the 3N187 family (40673 (all applications to 400 MHz), 40819 (to 200 MHz), 40820 (VHF TV RF), 40821 (VHF TV MX), 40822 (FM RF) and 40823 (FM MX).) I think that the gate-protected mosfets became available commercially late 1968 or 1969.

Other than that, any of the British hi-fi makers who were primarily audio specialists might well have been looking for third party help when it came to designing solid-state FM tuners. Leak’s difficulties in getting to the Stereofetic were well-documented in Stephen Spicer’s excellent history. I think it built its own front end as an integral part of the chassis, but based upon an intermediate RCA circuit. RCA went from all-bipolar to all-dual-gate mosfet FM front end circuits in four steps, namely single-gate mosfet RF + bipolar MX; single-gate mosfets RF & MX; single-gate mosfet RF & dial-gate mosfet MX; dual-gate mosfets RF & MX. It appears that somehow Leak got off at the third stop, not the final destination. Quad was quite late with its solid-state FM3 in 1971; whether that was a deliberate delay or circumstantial is unknown, but I think that its RF specialist, JD Collinson, had departed before then. Like Leak, it rolled its own FM front end as an integral part of the chassis, but it was probably based on the RCA two dual-gate mosfet circuit.

So Leak and Quad were evidently not D&W front end users. But there were other candidates, such as Lowther and Tripletone, and later on perhaps, Meridian. Armstrong I think had a history of doing its own RF work., so seems to have been unlikely to have used third-party front ends (although the 600 series might have been different.)

Possibly D&W would have been competing with not only German suppliers such as Görler, and later on Japanese suppliers, but also with those UK component and assembly makers who were active in the TV tuner market and who had also added FM front ends to their product lines. For example, it is known that Wharfedale used a German FM front end for its WFM-1 tuner of 1967.

Anyway, that was an interesting interlude in which D&W supplied high-performance solid-state FM front ends to the likes of Radford and Rogers.


Cheers,
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