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continued from page 95
from stores such as Jaycar, Dick
Smith Electronics and Altronics.
The above really answers your
question in full but your letter does
raise a few points of interest. First,
as noted in these pages last month,
the 100W module described in
December 1987 will not run safely
with supply voltages much above its
nominal ± 40V DC.
Second, it is not really intended
for driving an output transformer.
To do so, it would require flyback
diodes (such as 1N5404) across
each output transistor (ie, each
diode reverse biased). The diodes
safely damp any spikes which
would otherwise be generated by
the transformer's inductance when
the amplifier is driven into clipping
(or if the protection circuits
operate). The amplifier should also
incorporate a trimpot adjustment to
minimise any residual DC output
96
SILICON CHIP
voltage which would cause direct
current to flow in the transformer.
By the way, we suspect that the
output transformers in your
Millbank amplifier are autotransformers which can be configured
for 50V or 100V lines. This means
that your amplifier is mainly intended for public address applications.
The 100V (or 50V) output is intended for driving lots of small speakers
in parallel with its own step down
transformer.
The idea is to keep on adding
speakers, of say 5-watt rating, until
the amplifier is fully loaded. Thus, a
200W amplifier with 100V lines
could drive 40 5 watt speakers.
If readers are interested in a
100W public address amplifier
with suitable microphone and mixing facilities, we would like to know
about it. If there is enough interest,
we'll do it.
~
Printed circuit boards for SILICON
CHIP projects are made by:
• RCS Radio Pty Ltd, 651
Forest Rd, Bexley, NSW 2207 .
Phone (02) 587 3491 .
• Jemal Products, 5 Forge St,
Kewdale, WA 6105. Phone (09)
451 8726 .
Notes & Corrections
Capacitance Adapter for DMMs,
Nov. 1987: to give more range of
calibration adjustment, trimpot
VRl should be changed to 200k0
and the associated 120k0 resistor
reduced to 47k0.
24V to 12V Converter, Dec. 1987:
the wiring diagram on page 31
shows the lOOJLF capacitor
(associated with D9) incorrectly
polarised. The circuit diagram on
page 30 is correct.
To provide crowbar overvoltage
protection in the event of a circuit
mishap, connect a 15V 5W or 20W
zener diode across the 13.6V output. The zener's anode should connect to the positive output terminal.
If the output voltage exceeds 15V
the zener will conduct heavily and
blow the fuse. The zener may also
fuse and become short-circuit.
Note: Jaycar Electronics can supply a 15V 5W zener diode, type
1N5352B, which would be suitable
for this application.
