This is only a preview of the January 1994 issue of Silicon Chip. You can view 29 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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Near field
studio monitors
Could you please explain to me
what is meant by the term “near
field studio monitors” when applied to speakers suitable for use
in a recording studio. How do
these speakers differ from good
quality hifi speakers? Has SILICON
CHIP ever described a kit for near
field studio monitors or a speaker
kit that would be suitable for this
application and if not, could this
be done?
From what I have read, these
monitors may be suitable for use
with a PC fitted with a sound card
and amplifier, where high-quality,
high-level sound is required. As
these sorts of systems are becoming
popular, it may be a good opportunity to produce or revisit a kit
after which the transistors again died,
taking the fuses with them.
With a circuit diagram in front of me
and limited access to an oscilloscope,
I’m stuck – not willing to sacrifice another two transistors for three minutes
of music. I have included a copy of
the circuit diagram and indicated the
offending components. If you have
any suggestions, I’m listening. (J. D.,
Blackburn South, Vic).
• There are a number of possible reasons as to why your amplifier’s output
stage is not working as it should. The
first approach should be to measure
the voltages marked on the circuit with
no load attached. It is quite possible
that the degree of heating in the 5W
(marked 2W on the circuit) resistors
is normal. If the associated voltages
(+15.4V and -15.7V) are correct, then
it should be OK.
However, the most important thing
to do when you have replaced the
output transistors in a power amplifier is to set the quiescent current
using, in this case, trimpot R130. The
relevant voltages for this are shown
at the emitters of Q109 and Q110 (ie,
+0.618V and -0.589V). Therefore, you
should connect a digital multimeter
across the 220Ω resistor between the
emitters of Q109 and Q110 and adjust trimpot R130 until the voltage is
1.207V. Leave the amplifier operating
for at least half an hour after doing this
94 Silicon Chip
approach to this type of speaker
system. I appreci
ate your comments and look forward to hearing
from you. (R. C., Stockport, SA).
• In audio parlance, “near field”
refers to the response of transducers at very close proximity, in the
pressure region. Hence “near field”
microphones are used by vocalists
(where they almost swallow the
microphone). We would assume
that “near field studio monitors”
are intended for use in the confined spaces of recording studios,
at listening distances of less than
one metre.
If this is so, such speakers are
unlikely to be suitable for use in
domestic living rooms. We have
contacted a number of audio equipment distributors on this question
but none of them have been able to
give any information.
adjustment and redo it if the reading
has increased.
The equivalent quiescent current
through the output tran
sistors can
be calculated by dividing the total
voltage between the output transistors
(+0.012V, -0.007V) by the total resistance (0.66 ohms). This gives a quiescent current setting of 28 milliamps.
Wireless microphones
need muting
Over the years, the magazines have
designed bucket loads of FM wireless
microphones but they are not much
good for singers because they usually
don’t have enough dynamic range.
They aren’t very good for announcements on PA systems either because
they must be on all the time, even
when you aren’t speaking. If you turn
them off, you get random radio noise
coming out the speakers (unless you
are using an expensive tuner).
So why have we all built one of
them? Probably because they are really “neat” gadgets. How many of us
are still using them? Not very many,
I suspect.
I’ve heard that FM stereo is transmitted with a signal frequency to signify
that it is stereo (around 16kHz). My
suggestion is to add this frequency to
one of your FM microphone circuits
and then get a tuner with a stereo LED
and use the LED output to switch a
relay which disconnects the speakers
when the mic is off. This stops noise
from coming out the speaker when
the mic is off or there is no audio. The
relay could also be used to switch on
music when the mic is off.
Can you make a project out of it? It
just seems like a logical improvement
for the old FM mic! (C. P., Coromandel
Valley, SA).
• It is true that quite a few such circuits have been published over the
years and that some are much better
than others. The design featured in the
October issue of SILICON CHIP is quite
good although it does not incorporate
your idea of muting.
FM stereo signals are transmitted
with a pilot signal of 19kHz at ±7.5kHz
deviation of the FM carrier (ie, 10%
modulation). We would be reluctant to
use this pilot signal as a mute control
as you suggest because it would automatically switch the tuner into stereo
whenever it was present. This is not
a good idea for wireless microphone
applications since tuners always have
an inferior performance in stereo mode
compared to mono mode.
What you really need is a VOX
circuit to switch off the microphone
when it isn’t being used but which
leaves the transmitter section operating to keep the tuner “quieted”. We
shall see what we can do.
Note & errata
Solar-Powered Electric Fence, April
1993: C4 should be increased from
10µF to 470µF to improve the supply
decoupling and prevent erratic operation of the inverter circuitry.
UHF Remote Switch, December 1989
and August 1990: in some cases, the
MC145028 decoder (IC2) may not
operate correctly since the specified
oscillator components cause it to
operate at 770Hz which is outside
its recommended frequency range of
1kHz to 400kHz. The solution is to
change the timing components so that
the oscillators operate at about 2kHz.
For IC1 in the transmitter, replace
the resistors at pins 11 & 13 with 220kΩ
and 100kΩ resistors respectively and
change the .01µF ceramic capacitor
at pin 12 to a .0022µF polyester type.
For IC2 in the receiver, change the
resistors at pins 7 & 10 to 39kΩ and
180kΩ respectively. The capacitors at
pins 7 & 10 are unchanged.
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