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of getting reasonable power from car
battery voltage into 4Ω loads?
Still on audio topics, I would like
to express my annoyance at the PMPO
marketing terms used by reputable
audio equipment manufacturers to
sell hifi equipment. Words like 400W
PMPO are a common sight on cheap
midi systems which don’t even draw
more than 50W or so from the mains.
In fact, looking at the amplifiers and
drivers in these systems, they don’t
look like they can deliver more than
10-15W RMS per channel. I am quite
sure there is no mathematical basis for
these figures and they simply make it
hard to choose an amplifier with the
power you require.
I am pleased to say, however, that
quality Japanese brands such as Sony
and JVC do use correct RMS terminology and believable figures. Surely the
use of such meaningless figures should
be prohibited by Australian standards.
Similar situations apply with car
speakers; the $45 speakers I mentioned
above are marked “80 watts maximum
music power” though they are sold
as 40W speakers and in practice they
don’t like much volume through a true
20W RMS good quality amplifier. (S.
Longer fade for
diesel horn
I always enjoy the magazine and
the model railway projects. The
whistle and horn described in the
July 1994 issue have a very good
tone but the diesel horn needs to
have a longer fade as the air bleeds
off and the horn trumpet continues
to “ring” in the prototype. If you
could change that, it would be
perfect. Any suggestions?
A very impressive digital controlled throttle was demonstrated
at the October 1994 Liverpool
model railway exhibition with
AC on the track permanently and
locos addressed by the controller.
This seems the way of the future.
Do you see it as a viable project?
Finally, I must say how much I
like your circuit diagrams, compared with those tiny computer
generated symbols of some other
magazines. (T. B., Kogarah, NSW).
• It is possible to increase the fade
time by using a bigger capacitor
for the 22µF capacitor at the base
J., Surrey Downs, SA).
• You are right about the input coupling capacitor’s polarity and, ideally,
the input capacitor for the balanced
supply version should be a bipolar
type.
The LM383 can deliver around 1112W in bridge mode since its output
swing is doubled and it “sees” a 2Ω
load. The figure of 16W is only possible at the higher supply voltage of 20V.
We don’t have any simple answers to
your need for power amplifiers run
from 13.8V DC; a big inverter is the
only way.
We agree with you about the absurdity of amplifiers with PMPO figures. To
our knowledge, an Australian stand
ard for hifi equipment has never been
promulgated although our publisher
spent a good deal of time in the 1970s
sitting on a standards committee for
that purpose. Since so little hifi equipment is manufactured in this country,
there seems little point in having a
local standard anyway.
Car speakers are rated with nonsensical figures and even those that
do have high ratings are often quite
inefficient so that the overall loudness
and dynamic range is not marvellous.
of Q2. Try 100µF or larger. Alternatively, try connecting a 100kΩ
or larger resistor in series with
diode D1.
We are familiar with the train
control you mention which was
featured at the October model
railway exhibition. Generally
called “Command Control”, these
systems enable realistic control of
a large number of locomotives on a
layout, without the need for block
switching and so on. The drawback
is that they are quite expensive
and require a control receiver to
be installed in every locomotive,
together with a changeover switch
if it is desired to run the loco on
other layouts with conventional
controllers.
Thanks for the compliment on
our circuit diagrams. They are actually generated on a CAD system
but our draughtsman, Bob Flynn,
has spent a considerable amount
of time to produce a library of
symbols which look very similar
to those used when our diagrams
were hand-drawn.
Notes & Errata
25W Amplifier Module, Dec. 1993: the
wiring diagram for the single supply
version (Fig.3) shows the 1µF input
capacitor installed the wrong way
around; the circuit diagram is correct.
Also the 1µF input capacitor for the
dual supply version should be a bipolar electrolytic or other non-polarised
capacitor such as an MKT polyester.
Multi-Channel Remote Control, May
1994: the Vcc (supply line) to a number
of ICs (IC4, IC6, IC8, IC9, IC10 & IC11)
is open circuit on the PC board, as
supplied by the author. This presents
a problem when using the outputs for
latched operation.
To correct the error, wire a link from
pin 7 of IC2 to the pin of C7 (.0047µF)
which is closest to the outside edge of
the PC board. A corrected PC pattern
is available if necessary.
Some codes may not operate correctly due to the thresholds being quite
critical on IC12a. This causes the rate
pin (RB) of IC1 to pulse erratically and
therefore IC1 is not able to receive a
valid code. To correct this problem,
replace R5 with a trimpot and adjust
it so that the voltage on pin 6 of IC12b
is halfway between the voltage at pin
5 when transmitting a valid code with
link SW13 (on the transmitter board)
in and when transmitting a valid code
with link SW13 out.
The author has experimented with
this and, with a supply rail of 6.39V,
the value for R5 worked out at 6.8kΩ.
The voltages on pin 5 varied between
2V and 3V and so the threshold was
set for 2.5V.
50-watt Stereo Amplifier Module,
Feb. 1995: the parts list should show
2 x 22µF 16VW electrolytic capacitors
(not 4). In addition, 2 x 47µF 16VW
electrolytic capacitors should be added to the list.
Digital Effects Unit, Feb. 1995: The
parts list should show 19 330Ω resistors rather than one. On the circuit
(Fig.2), the 3.3kΩ resistor shown at the
input to the modulation filter should
be 22kΩ. An extra 3.3kΩ resistor
should be included between the positive side of the 100µF capacitor and
the junction of the 1.8kΩ and 22kΩ
resistors.
Finally, the two 56pF capacitors
shown on the PC board overlay (Fig.3)
should each be 560pF, while the unmarked electrolytic capacitor at top
right should be labelled 10µF.
SC
March 1995 93
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