NOTTINGHAM ANALOGUE STUDIO
128 CORDY LANE, UNDERWOOD, NOTTINGHAM NG16 5FD
Design and Manufacture of Quality Audio Equipment
Telephone: +(44) (0) 1773 762947
Fax: +(44) (0) 1773 533566
V.A.T. REGISTRATION NO. 667 3729 92
WAVE MECHANIC POWER SUPPLY
Why a power supply? But first, why an AC turntable motor?
Nottingham Analogue turntables excel by virtue of their almost elemental simplicity, relying as
they do on the fact that a rotating platter will continue to do so at absolutely constant speed
unless acted upon by outside forces. By coupling this steadily rotating mass to an arm base that
is stable around the axis of rotation of the platter, the result is a truly integrated unit.
By giving the platter adequate moment of inertia, the effects of any variability in outside forces are
minimized. However, the most important force, bearing friction, should if the bearing is correctly
made and lubricated remain essentially constant; hence, all that is required is to supply a small
amount of energy to the platter at a constant rate – just sufficient to counter the friction and
maintain a steady speed.
DC motors and their highly developed control systems may rightly dominate the small motor
market for “movement and placement” applications such as robotics, but are they what we need
in order to maintain this steady state? For this purpose, an AC synchronous motor backed by the
“mass” of the National Grid should be the answer. Unfortunately, although the laws of mechanics
show no sign of changing, electricity supplies are – and apparently always for the worse. There
are four main areas of concern.
1. Voltage variations
As a case in point, since “harmonizing” with the rest of the EC, the nominal UK domestic supply
voltage has fallen from 240V to 230V, but the permissible range of voltage has increased vastly,
now being from –6% (216.2V) to +10% (253V). This whole range will not be seen in any one
locality, but wide variations do occur between different areas.
For an AC synchronous motor, such voltage variations will not affect its overall speed, but will
affect its torque: not what we need for constant energy input (torque x speed = power). In
addition, the absolute voltage level has a very distinct effect on the dynamics and detail of the
music, with a clearly audible optimum voltage range. We need, then, to be able to optimize the
motor supply voltage and to hold it stable.
2. Frequency variations
These, by definition, will directly affect the speed of a synchronous motor. Supply frequency
variations are generally slow, but are easily demonstrated using an independent precision strobe
illuminator such as the “Zapper”.
Permissible variation in the UK is +/- 1% (6% equates to a semitone). Over 24 hours the
frequency must average exactly 50Hz to satisfy users of synchronous clocks, so that if the
frequency is slow at one point in the 24 hours, it must be fast
at some other point as night follows
day (sometimes literally!). For a power supply, some slight adjustment of frequency is desirable
for pitch purposes but once set, it must remain stable.
To Next Page
Loading...
