Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
The amplifier block is a two-stage design incorporating an EF86 low-noise pentode in the first
stage and a paralleled ECC88 high mutual-conductance double-triode as the output stage. All the
landmark amplifier designs from Mullard's application laboratories specified the EF86. In fact,
Mullard described the EF86 as
a low noise pentode intended for use as a RC coupled AF voltage
amplifiers; particularly in the early stages of high gain audio amplifiers, microphone preamplifiers
and magnetic tape recorders
.
The output stage comprises a dual triode valve in which the two devices are wired in parallel. This
paralleling of the two triodes has the beneficial effects of doubling the mutual conductance and
halving the anode resistance. The latter characteristic is particularly valuable in ensuring a low
impedance drive to the output transformer.
The output stage directly feeds the output step-down transformer. The output is balanced and
earth free. A modest degree of negative feedback is taken from the anode of the output valve(s)
to the cathode circuit of the input stage to stabilise gain, extend frequency-response and lower
distortion. A change to this feedback network is employed to effect the gain switching (from
40dB to 46dB in the PHAB and from 24dB to >30dB in the PHAME), less feedback being applied to
achieve higher gain.
In the case of the Phædrus Audio PHAB microphone preamplifier, the grid of the input pentode is
driven from the input transformer. In the case of the PHAME, this transformer is not fitted.
Performance
At recording industry line-up (0VV=+4dBu) the magnitude of all distortion components on a 1kHz
sine wave input are less then -55dB below fundamental. This, and the A-weighted noise response
relative to maximum output, is illustrated below.
