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Old 31st Jan 2017, 3:48 pm   #1
Leon Crampin
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Default Improving the Murphy A272C.

This set is the last of the Murphy floor standing baffle sets with a 10" loudspeaker and has FM. Therefore, a candidate for good sound.

Having had one for some years, I have been aware that the audio HF response is rather poor and that the AF gain is not really sufficient. Having to open up this set to fix an unrelated fault, I set about finding a way to improve the audio.

All circuit references are from the Murphy service data.

A 1nF capacitor (C72) is fitted between the anode of the EABC80 and earth. In view of the source impedance, this kills the audio HF - so I removed it. The Bush DAC90A suffers from exactly the same problem. Some improvement, but not enough.

The AF gain is set by the feedback network R42, R11. Increasing the value of R42 therefore increases the gain, so adding 8k2 in series with R42 (8k2 to the 'speaker) fixed the gain problem.

Murphy give alternative feedback networks for the table and console models, so they obviously went to some trouble to define the AF response - but with their component values, the table model especially, must have sounded like mud. For the A272C console, I added a 4n7 capacitor to the junction of the 8k2 and R42 to earth. This selectively shunts the feedback, reducing it as the frequency rises, giving top lift. Effective above about 4kHz, this modification shows that the single 10" Celestian 'speaker can give a good account of itself without a separate tweeter - which I dislike due to the likelihood of phase distortion.

The new components can be mounted on the output transformer using spare tags. The results are most encouraging, realising the excellent sound quality that the speaker and its baffle are capable of producing.


Footnote: The set had failed on FM due to C7 reducing in value from 5nF to < 100pF. It bootstraps the FM mixer and materially increases the IF gain, which becomes unusably low without it. Other Hunts devices in the set have to be evicted as usual.

Leon.
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Old 31st Jan 2017, 6:06 pm   #2
ukcol
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Default Re: Improving the Murphy A272C.

It has always surprised me that the consul baffle sets that were AM only had amplifiers with push-pull output stages but when Murphy produced this VHF/FM model they settled for a single ended output stage.
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Old 31st Jan 2017, 8:07 pm   #3
Nickthedentist
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Default Re: Improving the Murphy A272C.

That's worth knowing, Leon, thank you.

Quote:
Originally Posted by ukcol View Post
It has always surprised me that the consul baffle sets that were AM only had amplifiers with push-pull output stages but when Murphy produced this VHF/FM model they settled for a single ended output stage.
The Ekco C273 was the same, though doesn't seem to have suffered as a result: http://www.radiomuseum.org/r/ekco_c273c_27.html
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Old 4th Feb 2017, 2:02 pm   #4
Phil G4SPZ
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Default Re: Improving the Murphy A272C.

Nice work! Lovely sound quality, yes, but you've got to like the styling. A Murphy baffle set once arrived in our living room, but sadly my wife's screams unnerved me and it went into the shed, until I managed to swap it for a pre-war A30C which I donated to a museum, thus saving our marriage!
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Old 4th Feb 2017, 4:19 pm   #5
dave walsh
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Default Re: Improving the Murphy A272C.

Well I suppose it's all a matter of taste and not worth shattering domestic bliss for Phil but I notice that [in a 2008 thread] I mentioned that these Consoles seemed to be getting a bit trendy. I bought a 272 then because I'd never seen one with VHF but the AM 146 in particular [especially with a light finish] always looked quite 1930's to me [even though they are much later]. These seem to look good in a period or perhaps minimalist setting [eg bare brick and oak] so I remain surprise that they don't attract higher prices. As ukcol says the AM model has a pretty good amp. Some people have extracted these from a couple of sets making a good stereo units.

Even though I've got a 146 that is 98/% woodworm I'm not sure about gutting a set for this purpose but the output stages extracted and renovated are of a very high standard! As for the 272, Leon's suggested design improvements seem to be a very good option with minimum disruption to the radio itself. I must admit though that that something about the VHF Console [possibly the larger dial] makes it less stylish than it's older 146 brother.

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Old 5th Feb 2017, 4:09 am   #6
MurphyNut
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Default Re: Improving the Murphy A272C.

I have a A272C and a 146C but prefer the later despite being an earlier set without FM. It seems much better made, it sounds remarkably rich and pleasing to the ear.
They're always available on eBay at widely varying prices. I suspect they survive in large numbers possibly because they look so unusual and people have hung onto them because they think they're something rare.
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