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Posted by STEVE M on

'Technical advances have effected considerable increases in the quality of broadcast and recorded sound with television sound and FM transmissions including the full range of audio frequencies.  It is thus essential to have an amplifier which does full justice to a high quality audio signal.  Details of such a new amplifier have been released by Mullard in the booklet "Mullard 5 Valve 10 Watt  High Quality Amplifier Circuit (ref. no. MV8104).'

The above entry is from a press release by Mullard dated October 1954.  Today, the 5-10 is much revered  and not at all in the shadow of it's larger brother, the 5-20 - and so it should not be when it boasted an output of 10W showing a THD of < 0.4% with hum and noise being inaudiable being -74dB below maximal output.  All this and a flat frequency response to within +/- 0.5dB within the range 10 - 20000Hz to boot!

Of course, this amplifier was designed to showcase a range of Mullard valves, first, the first stage voltage amplifier used the low hum/low microphony EF86 pentode which was in turn coupled to a cathode coupled phase splitter, courtesy of an ECC83 dual triode.  An elegant design point was the g'' of the ECC83 being capacitively earthed with the bias for the second section being applied due to current flowing through a common cathode resistor - a nice touch that prevented motorboating or any form of instability at normal driving conditions.  

Interestingly, the output stage was configurable with an 8K or 6K secondary to adjust loading such that high power or lower power with a consequently better transient response could be enjoyed.

The powerhouse for all this was supplied by a GZ30 rectifier or if you wished to stay with an all B9A arrangement then alternatively, the EZ80.   THe PSU was of a resistance-capacitance design utilising a comparatively large reservoir capacitance of 50uF to reduce ripple current.

And here is Mullard Laboratories photograph of "one they built earlier." : -

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