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Leon Crampin · 31st Jan 2017, 3:48 pm

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.

31st Jan 2017, 3:48 pm	#1
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	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.