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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th Jul 2019, 9:50 am   #21
John Caswell
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Default Re: Dormer & Wadsworth (D&W)

A very minor correction to David's comments above. VITS = Vertical Interval Test Signal.
The manufacturer was Gorler in W. Germany (at the time) and their units, both valve and semiconductor were a staple diet in many West German radios as well as B&O, Heathkit etc
Larsen and Hoedholt (L&H) the Danish manufacturer produced both "front ends" based on dual gate MOSFETS and a complete FM tuner module using that front end, plus the ubiquitous CA3198E for the IF and a KA2265 stereo decoder, you added varicap voltage controls, plus meters etc. and a frequency counter.
Using this module, the L&H 7255, is was possible to build, which I did, a state of the art FM tuner which knocked spots off off most things available at the time.
I also built, a bit earlier, a mono valve FM tuner from a circuit in the American "Radio Electronics " magazine that had 3 RF stages, plus bandpass coupled IF stages. It caused me a few headaches in the beginning, but goodness me how sensitive and selective it was, if signal was passing the aerial it was sucked in. Useless for stereo of course.
On another of David's points about small worlds, I would never have an affair as I know I could walk out of a hotel on the dark side of the Moon, and the next door would open and comment like "****** me John what are you doing here" would come hurtling out
Sorry for the digressions.

John
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Old 13th Jul 2019, 8:35 am   #22
Synchrodyne
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Default Re: Dormer & Wadsworth (D&W)

I think that Görler FM front ends were also used by some of the UK manufacturers, such as in the Wharfedale WFM-1 tuner. So D&W would have been competing with Görler as well as other UK manufacturers.

Given that D&W was a relatively obscure organization, with no apparent electronics history prior to its entering the FM front end field, I wonder if it was say an engineering company that for one reason or another elected to enter a specialized electronics area. FM (and TV) front ends were as much about mechanical engineering as as electronic engineering. That was the case with Cyldon, originally an engineering company that stepped into the TV front end market c.1953 with a turret tuner based upon the Amercian Standard Coil prototype, ahead of any similar move on the part of the established setmakers.


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Old Yesterday, 4:36 am   #23
Synchrodyne
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Default Re: Dormer & Wadsworth (D&W)

Looking at the chronology of the D&W solid-state FM front ends, it is known that the silicon bipolar version of the D&W 341/AFC FM front end was used in the Radford FMT1 FM tuner. This became available early in 1966, as shown in this excerpt from the 1966 Audio Fair catalogue.

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This timing would have made it a reasonably early application of silicon planar transistors.

The two dual-gate mosfet version of the same front end, the 341/AFC/FET/2 was used in the Rogers Ravensbourne 2 FET FM tuner, which was shown for the first time at the 1968 Audio Fair.

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The same 341/AFC/FET/2 front end was used in the Radford FMT3, which was listed (at that time alongside the FMT2) in HFYB 1970.

A FET-based front end, presumed to be the 341/AFC/FET/2, had been offered as an option on the Radford FMT2 from early 1967, as shown in this excerpt from the 1967 Audio Fair Catalogue.

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If the presumption is correct, then this would have been a very early application of dual-gate mosfets in an FM front end. What appears to have been RCA’s initial commercial release of a dual-gate mosfet was the TA2644, announced in Electronics World 1966 October. TA2644 was in fact the development type number for what became the 3N140, although when the latter name was applied I do not know. RCA had used the TA-series development type numbers quite often when discussing mosfets in its publications, and they were shown in early edition datasheets under the definitive type numbers. The RCA 4060n series of dual-gate mosfets (3 for TV applications, 2 for FM) were in the 3N140 group, although the three TV types had their own TA-numbers as well. Anyway, it does appear that the basic type of dual-gate mosfet used in the 341/AFC/FET/2 was commercially available by early 1967. Of course, it is possible that D&W had used jfets in 1967, such having been used in American FM tuner front ends since late 1965.

On the basis of circumstantial evidence, yet to be corroborated, the varicap derivative of two-mosfet 341 front end was used in the Sudgen R21/R51 tuner. This was first shown at Sonex 1971 (as reported in WW 1971 April), with the first HFYB listing being 1972. This was not a very early use of varicap tuning in a domestic product. For example, AREF (Denmark) had offered varicap-tuned FM front ends from late 1967 (as reported in WW 1967 December). However, as early varicap applications were reputed to adversely affect signal-handling and spurious response performance, D&W (and its customers) may have chosen to wait for some progression along the learning curve. On the other hand, from the start, dual-gate mosfets offered benefits as compared with small-signal bipolar devices.

Re D&W’s use of bandpass input tuning with a single-tuned interstage in its 341 series, support for this approach may be found in a 1966 RCA paper “A Comparison of Solid-State and Electron-Tube Devices for TV-Receiver RF and IF Stages”. Therein it was said, in respect of the poorer cross-modulation characteristics of bipolar transistors as compared with valves:

“Further improvement in cross-modulation can be achieved, at the cost of additional sacrifice in noise figure, by use of additional pre-selection prior to the transistor input. For this reason, it is good design practice to use a double-tuned input circuit in transistor rf stages. As shown in Table IV, a noise figure of 3 dB is typical for bipolar transistors. Therefore, a degradation of as much as 1.5 dB will still provide a device comparable to tubes.”

Although RCA was discussing VHF TV front ends, I think that as Band II is between Bands I and III, interpolation to the FM case is reasonable. Normal practice for valved TV front ends was a single-tuned input with a double-tuned bandpass interstage, and that was retained when dual-gate mosfet RF amplifiers were introduced (initially by RCA in 1968).


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Old Yesterday, 6:00 am   #24
Radio Wrangler
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Default Re: Dormer & Wadsworth (D&W)

This is where we get into geographic differences. In some parts of the world with a congested FM band full of commercially competitive stations in a power race, tuners with all the front-end selectivity you can get are needed. In other places with sparsely-populated and well-ordered bands, noise figure is to the fore to give people in fringe areas better S/N ratio.

I find the 'Tuner Information' website amusing with all their shoot-outs trying to find 'The Best' tuner. It's a hopeless pursuit without splitting it up into categories, however they have lots of interesting information on there.

David
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Old Yesterday, 9:13 am   #25
Ted Kendall
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Default Re: Dormer & Wadsworth (D&W)

Quote:
Originally Posted by Radio Wrangler View Post
I find the 'Tuner Information' website amusing with all their shoot-outs trying to find 'The Best' tuner. It's a hopeless pursuit without splitting it up into categories, however they have lots of interesting information on there.
Curious site, that. Lots of useful stuff, agreed, but some of the assertions about subjective audio quality are risible, given the lack of control over programme material. Mind you, they aren't alone - I saw a site somewhere which largely consisted of one man's pugnacious assertions about the sound quality of practically every receiver brought to market between the 1960s and the 1980s...
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Old Yesterday, 2:08 pm   #26
Radio Wrangler
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Default Re: Dormer & Wadsworth (D&W)

I find it very difficult to keep a straight face when confronted with assertions about subjective audio quality, so I stopped even trying.

When someone next praises a sound for 'air', cock your head on one side, listen intently for a while and then question whether it's normal air... is the oxygen/nitrogen ratio correct? it does sound a little stale. Do this totally deadpan (if you can!) and see how they react. If you act like a pundit, their training kicks in and they have to accept what you just said.

I have some quite decent speakers and an amplifier which can handle them. No brand marks anywhere but very definitely well made. It's quite amusing when you come across people who don't know what to say about what they've been hearing because they would have to see the make and model names before they would know what they are supposed to say they've heard. It's either very very funny or very very sad. And I can't decide which.

David
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Old Today, 3:21 am   #27
Synchrodyne
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Default Re: Dormer & Wadsworth (D&W)

The simpler, three-gang D&W FM front end used in the Rogers Ravensbrook FM tuner had a single-tuned input, a dual-gate mosfet RF amplifier and a bipolar mixer. It seems likely that it had an all-bipolar predecessor of otherwise similar configuration. If so, this might have had the lowest noise, but the poorest strong signal performance of the group of 3- and 4-gang models.

Conceivably D&W might also have offered a two-gang all-bipolar FM front end with an aperiodic input, although that type would have been more for setmaker than for hi-fi applications.

D&W was not alone in choosing a bandpass input and a single-tuned interstage for its four-gang, mosfet FM front ends. Armstrong did the same for its 600 series, I think using an in-house design and not a third-party unit.

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