This is only a preview of the August 1997 issue of Silicon Chip. You can view 34 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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How about a “valvesound” amplifier?
I would like to suggest a singleended Mosfet transformer coupled
amplifier as a project, with a power
output of about 25 watts RMS. I’m
sure the output transformers could
be bifilar wound with a 1:1 ratio. In
my opinion, solid state amplifiers
that are capacitor coupled are a
little clinical. I’m sure the above
requested amplifiers will have a
smooth warm sound, sound
ing
sweet and open like that of a valve,
single-ended amplifier, but not
requiring a massive un-affordable
power supply as valve amplifiers
do. (R. L., Somerville, Vic).
• While such a project is feasible,
we do not think that it would result
in valve-like sound.
There are several reasons why
valve amplifiers sound the way
they do and the first is their moreor-less gradual overload characteristic. The second is that their
harmonic distortion is often more
or less second harmonic which
means that the distortion is musically related to the fundamental.
This often leads people to state
that such-and-such valve ampli-
Big brother is definitely
watching you
With all the current media interest at
present in hidden surveillance cameras, spy cameras, hidden audio devices,
thermal cameras, infrared cameras and
any other device that can be used for
surveillance, I am wondering just what
is out there and who are they keeping
an eye on. I am hoping you would do a
series of articles in your great magazine
on what there is at present, what the
future holds, how do we recognise it
and if possible some projects on their
detection.
Another possible project I would
like to see is a Digital Command Control for model railway locomotives that
is very small in size and suitable for
“N” scale. (S. F., Strathgordon, Tas).
• An article on surveillance cameras
would merely illustrate the fact that
these devices are now very widespread. In many cases, they are quite
92 Silicon Chip
fiers are “more musical”. Indeed,
they are more musical but this is
not the best high fidelity approach
since amplifiers that produce significant harmonic distortion which
may be masked also produce inter
modulation which is definitely not
pleasant.
The third reason why valve amplifiers sound “less clinical” is that
they usually have far less negative
feedback. The high negative feedback in solid state amplifiers has
two results. First, it greatly lowers
the inherent distortion of the circuit, leading to a much cleaner (or
clinical) sound. Second, it leads to
a much lower output impedance
and this results in more electrical
damping on the loudspeaker. This
leads to a tighter, less boomy bass.
Most solid state amplifiers these
days are direct coupled instead
of capacitor coupled and this improves the loudspeaker damping
factor even further.
In effect, while a solid state amplifier can be designed to drive a
transformer, it is still likely to have
a very clean, high quality sound.
The most recent example of this
is the 175W transformer coupled
design in the March 1997 issue.
apparent; you only have to look for
them.
Most shopping malls have cameras
for the main concourses, many large
buildings have cameras throughout
(including in the elevators), as do factories, petrol service stations, parking
stations, toll booths on expressways,
shops, banks and clubs.
In banks, hotels and clubs the
cameras are generally quite obvious
while in shops they are usually concealed but there are notices to say that
cameras are in use. Believe them. In
most cases, the cameras may not be
directly monitored but will feed video
recorders which run 24 hours a day.
In the event of a crime, the tapes are
examined by the police and often lead
to a conviction. There are also a number of TV programs which run along
this theme, with “Real TV” being the
main one.
In the major cities, video cameras
have been used to control traffic flow
for more than 20 years. Cameras are
also often used in apartment complexes and even homeowners are
now using them. We published a doit-yourself article on the subject in the
June 1995 issue.
The other point to recognise is that
all these cameras are watching you, not
just somebody else. If you are virtually
anywhere in a public place, you could
be on camera.
On a more cheerful note, one of
our contributors is working on a DCC
design for model trains.
Notes & Errata
Audio/RF Signal Tracer, June 1997:
users of this project will find that
the unit produces a lot of noise and
what may sound like “motor-boating”
when it is switched to high gain and
RF modes and with no connections to
the input probe and earth clip. This
is normal and is a function of its high
gain. As soon as the unit is connected
to a circuit the noise drops and the
wanted signal will be heard.
There is a discrepancy between the
circuit on page 40 and the wiring diagram on page 43. The circuit shows the
signal from switch S2b coupled to pin
3 of IC2 via a 0.1µF capacitor and pin
2 grounded. The PC board has this reversed, with pin 3 grounded and signal
going to pin 2 via the 0.1µF capacitor.
The PC board is correct.
If the unit is to be used on valve
amplifiers, there is the possibility that
connecting the unit to a voltage above
100V may blow the LM318 IC’s input
protection diodes. To prevent this, we
suggest soldering two 1N914 diodes
in inverse parallel across the 100kΩ
bias resistor to pin 3. These diodes
can be installed on the copper side of
the PC board.
12V/24V Motor Speed Controller, June
1997: there is a mistake in the text on
page 30, third paragraph down. The
text states “Make sure that they (the
diodes) are connected in the right direction across the motor; ie, anodes to
the positive supply line.” The diode(s)
should be connected with cathode to
the positive supply line, as shown in
the circuit and wiring diagrams.
Flexible Interface Card for PCs, July
1997: the circuit on page 25 shows
4.7kΩ resistors to the LEDs of the 4N28
optocouplers but 1.5kΩ resistors on
the wiring diagram on page 27. Either
SC
value will work.
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