from 'The Gramophone' April 1962
This is an entirely new stereo amplifier and preamplifier
by Armstrong Ltd. The two main amplifiers are mounted on one chassis
designed for fixing to the floor of the cabinet, while the control
unit fits on the front panel. Push buttons are used for six functions
of the control unit, resulting in a neat and simplified appearance.
When the preamplifier was unpacked it was found to be what is known
in space parlance as 'in go condition'. The instruction book, however,
states that it is packed with front panel and control knobs removed.
On the other hand, the main amplifier should have come out of the
carton (according to the book) set for a mains voltage of 250 volts,
but in fact the voltage selector plug was packed separately at the
bottom of the box. I mention this because it might easily be possible
for someone, having read the instruction book, to decide that the
amplifier was in 'go' condition only to find that the count down
must be halted while a feverish search is made of the bonfire heap!
Each main amplifier consists of a high gain triode
stage coupled to a double triode phase splitter, followed by two
EL84s in push-pull, using the familiar ultra linear circuit. A
pre-set hum control is fitted for heater balance and power supply
is by valve rectifier with resistor capacitor smoothing. The control
unit has five knobs, which are, from left to right, input selector
switch, bass, treble, volume and balance. Below the knobs and to
the right are six push- buttons having the following functions:
on/off, high frequency filter, low frequency filter (rumble), stereo,
mono A-B, mono A. The last two mono functions are: mono A switches
channel A inputs to both amplifiers so that a monophonic source
such as radio will be reproduced through both loudspeakers. Mono
A-B switches channel A and B inputs in parallel to both amplifiers,
for use when a mono record is being played with a stereo pickup.
For each channel there are six inputs. Tape 1, for direct connection
to tape heads and compensated for CCIR characteristic. Tape 2, for
reproducing the signal from a tape recorder or tape deck having
its own electronics. Input 3, auxiliary, will accommodate a wide
range of crystal pickups. Input 4, equalized to RIAA characteristic,
is for low output magnetic pickups, Input 5, radio, and input 6,
microphone. The sensitivity of these inputs is quoted in the specification
but it is worth noting that the input voltage can be approximately
25dB above the figure quoted before overloading occurs. Thus, for
example, input 3, auxiliary, can be anything between 100mV and 2
V without causing distortion in the input circuit of the control
The treble control works in conjunction with the
treble filter so that the turnover point is progressively lowered
as treble cut is introduced. With treble at level, the filter produces
a slope of 12dB per octave starting at 9 kc/s. Treble at 11 o'clock
produces a slope of 9dB per octave starting at 7 kc/s. Treble at
10 o'clock gives a slope of 5dB per octave starting at 5 kc/s. This
system provides, in effect, a variable frequency filter giving a
wide range of control. The rumble filter comes into operation at
80 c/s with an initial slope of 6dB per octave, increasing at lower
On test, the amplifier nobly supported the manufacturers'
specification. The overall frequency response, taken by feeding
a signal into the radio input at 1 watt level was 20-20,000 c/s
±1dB. Power output measured 12 watts at 1 kc/s, 9.6 watts
at 50 c/s and 10.5 watts at 10 kc/s. The balance control gave a
change of 10dB in each channel. In the specification the manufacturers
give the range of the treble control as +15dB to -20dB at 20 kc/s.
There seemed little point in measuring this outside the audible
range and I therefore measured it at 10 kc/s where it gave an increase
of +12dB and a decrease of —11dB with the H.F. filter out.
The range of the bass control at 50 c/s was +13.5dB to -11dB. At
10 kc/s with treble control at level the filter produced a drop
in high frequency response of —5dB and with full treble cut
— 23dB. The low frequency rumble filter was checked at four
frequencies: 100 c/s, -3dB; 50 c/s, -7dB; 30 c/s, -12dB; 20 c/s,
—26dB. Both channels were checked with identical results.
Negative feedback measured 29dB and stability was excellent. A -
square wave at 1 kc/s was observed on the oscilloscope using a resistive
load and showed no trace of ringing until a 0.5 mfd condenser was
shunted across the loudspeaker terminals. This is remarkably good
with such a high order of negative feedback and pays tribute to
the design of the loudspeaker output transformers.
The construction of both units is rather different
from that of many other amplifiers. Large tag boards with components
neatly arranged are not used and my first impression was that the
general appearance was untidy. On closer examination, however, the
layout proved to be logical and efficient, and the components used
are of high quality throughout. It is to be expected that an amplifier
that per- forms so well on test will prove excellent under listening
conditions and this is no exception. Listening tests were therefore
conducted primarily with a view to finding out the effect of the
treble/filter control when worn and noisy LPs were being played.
The degree of control was entirely suitable for (these rather difficult
conditions for it was possible to remove a large amount of unwanted
noise whilst still retaining a good musical balance. Before I finish
I should like to make two small criticisms. There are four screws
securing the panel of the control unit to the chassis. On my unit
they were nylon screws, obviously designed to merge into the Perspex
panel. This they did not do, and since they were not symmetrical,
tended to be more noticeable than perhaps if they had been of contrasting
colour. The other criticism is concerned with the selector knob.
This is stiff in action, as one would expect from the number of
switch wafers and the type of index spring used. The knob is so
small that a very firm grip indeed is required to turn it, a situation
which, I am sure, could be easily remedied without spoiling the
appearance. Happily, neither of these two points detracts from the
performance of the amplifier, which is excellent.
PHILIP G. TANDY.