from 'The Gramophone', December 1956
Pamphonic Amplifier 1003. Pamphonic Reproducers
Ltd. Price 27 gns.
Output Power: 10 watts. Output Matching Impedances: 3.5 and 15 ohms.
Frequency Response: Substantially flat from 20 c/s to 50 kc/s. Distortion
: 1% at 1kc/s at 8 watts.
Hum and Noise: Mic., -53db, Pickup, -54db, Radio/Tape, -65db.
Negative Feedback: 20db. Sensitivity: 2.5 mV to 100 mV, according
to input selected.
Selector switch: Mic., Tape, Radio, Pickup (Decca LP), Pickup (N.A.R.T.B.),
Pickup (78 r.p.m.)—all velocity characteristic.
Bass control: -16db to + l5db at 50 c/s. Treble control: -16db to
+16db at 10 kc/s.
Volume control: graded, with on/off switch. Auxiliary supplies:
300V, 30 m.a. D.C. 6.3v, 2a, A.C.
Valves: 2 x 6AQ5 Brimar ; 2 x ECC83 Mullard ; 1 x EF86 Mullard ;
1 x GZ32 Mullard.
This is a most interesting amplifier of the "Junior",
10-watt type. Both Control Unit and Main Amplifier are in one rugged
metal case of quite handsome appearance, finished in mottled green.
It needs no separate housing but can be put on a shelf or in a bookcase.
It is thus of ideal shape, size and appearance for the flat-dweller
or indeed for any location where space is limited or where a "contemporary"
approach is called for.
Although only 6 valves, including rectifier, are
used, the circuit is actually of 5 stages with twin valve, phase
inverter stage and push-pull output. Moreover, the various controls
are effectively distributed so as to avoid undesirable interaction,
and in this way a high degree of stability is assured. Thus only
the input selector comes before the first valve (EF86); then between
that valve and the first half of the ECC83 come the fixed equalisers
for the different recording characteristics; between the two halves
of the ECC83 come the bass and treble controls on the now famous
Baxendall circuit; then come the volume control feeding the ECC83
in the phase inverter stage; and finally come the two 6AQ5's in
the ultra linear push-pull output stage. The output for coupling
to the input of a tape recorder is taken from the upper end of the
volume control but after the coupling capacitor of that stage. The
main feedback loop goes from the secondary of the output transformer
to the first cathode of the phase inverter stage.
The more I examine the circuit of this amplifier
the more I become impressed with the skill - no, cunning, of its
designer. He obviously set himself certain limited objectives as
to output, frequency range, size and cost, and he has achieved those
objectives with remarkably little distortion at high output and
with no surrender of stability. I was particularly glad to see how
he had avoided the risk of inadvertent positive feedback to the
input stage causing the first valve to become unduly sensitive and
therefore prone to self-oscillation.
The carrying out of the design commands respect.
The choice of components, output transformer included, has been
quite generous and the layout and wiring are beyond reproach. I
like the edge-illuminated Perspex panel which show up the graduations
of the various controls even in the most adverse conditions of lighting.
The performance, as one could only expect, is
first class, and entirely adequate to get the best for modern records
and FM transmissions in ordinary home listening conditions. The
fact that there is no steep-cut treble filter perhaps makes it less
suitable for some of the older recordings, but the value of such
a control is often exaggerated: it is not an adequate answer to
excessive surface noise, for example; that, I believe, lies in the
pickup and the stylus.
Some readers may wonder whether the record equalisation
is the best compromise amongst the many possibilities. For example,
I myself would have chosen to have American Columbia pre-standard
in place of Decca LP, since the former was one of the extremes.
Fortunately, however, it matters little since the Baxendall treble
and bass circuit is versatile enough to make up for differences
of that sort.