Silicon Chip – July 2019

siliconchip.com.au Australia’s electronics magazine July 2019 13

system consists of a NAD 3020A in-
tegrated amplifier, Bowers & Wilkins
DM10 speakers and a Wharfedale sub-
woofer. All vintage stuff – like me!
Given the above, it might be a good
idea to revisit the 2009 project and
come up with an up-to-date design,
because of the lack of players with
analog outputs available at a reason-
able cost.
Congratulations on a great maga-
zine. I have spent many happy hours
poring over the contents and building
projects over many years since its in-
ception and as far back as Electronics
Australia and Electronics Today In-
ternational.
Rodney Goodwin,
Tinana, Qld.
Nicholas responds: thanks for your
feedback. I agree that our DACs sound
a lot better than the analog outputs of
many disc players (if they still have
them) and pretty much all cheap
standalone DACs that you can buy. I
still use my prototypes regularly.
I revisited the 2009 design with the
Crystal DAC upgrade (February 2012;
siliconchip.com.au/Article/768) and
CLASSiC DAC (February-May 2013;
siliconchip.com.au/Series/63).
While these gave a small improve-
ment in audio quality, I think the
original 2009 DAC still sounds fine
in comparison. The advantages of
the CLASSiC DAC are mainly extra
features. But it still doesn’t have a
proper display as such. I agree that a
DAC incorporating a display would be
a great project. I hope I can find the
time to design one.

Can Class-D amp chips be used
to drive motors?
I’ve often thought of how a pair of
Class-D amplifiers, configured in bridge
mode with an isolating output trans-
former, could be used to form a vari-
able speed drive for small AC motors.
The input would be from a variable-
frequency sinewave oscillator.
Many of these small motors are
shaded pole types which may mean
that they can only be safely operated
over a limited input frequency range
before overheating (especially at low-
er speeds).
Perhaps the solution is to reduce
the voltage as the impedance falls, but
what consequences does this have on
the motor’s torque, especially starting
torque? What happens at higher fre-
quencies? Or is it just too complicated?

We seem to live in a world in which

there are many AC motors used for

common items. For example, RC cars

which use “brushless DC” motors that

are basically permanent magnet three-

phase AC motors.

Mark Schijf,

Doncaster East, Vic.

Comment: this is an intriguing idea,

but a Class-D amplifier has a lot of ex-

tra components and features which

aren’t needed for driving a motor. But

in some cases, a Class-D amplifier chip

may be the cheapest way of getting sev-

eral Mosfets wired up as half-bridges or

full-bridges, with integrated drivers in

a small package.

Much of the complexity of a Class-D

audio amplifier relates to the need for

the output waveform to closely follow

the input waveform, for low distortion

and noise, whereas this is not required

for driving a motor. Nor is a filter, real-

ly; the motor winding inductance does

a good job of converting a PWM wave-

form to a smoothly varying current.

Shaded-pole motors usually have

a low enough power rating that over-

heating is not likely.

Still, it’s good practice to avoid run-

ning any induction motor at speeds

much lower than it is designed to run

at for extended periods without addi-

tional cooling.

Suggestion for updated GPS Analog

Clock Driver

I am writing about the GPS-Syn-

chronised Analog Clock Driver from

February 2017 (siliconchip.com.au/

Article/10527) with a suggestion for

two optional enhancements that your

readers may be interested in.

I am currently with my elderly

parents who have two wall-mounted

analog clocks which they are unable

to reach. As such they need one of my

nephews or some other “younger” fam-

ily member to take down and put back

up when the batteries need changing

or the time needs to be reset for day-

light saving etc.

On thinking of these situations, I

was wondering if your Clock Driver

could be enhanced to include an op-

tional hand-held remote control unit to

perform the clock setup, adjustments

etc, as well as the option of using a

small 230V AC to 3V DC plugpack

which could be plugged into a power

point next to the clock.

Paul Myers,

via e-mail SC

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