Q
U
E
S
T
I
O
N
S
&
A
N
S
W
E
R
S
+5V
■ FIGURE 7
+5V
60Hz Clock
Generator
1 Second
Timebase
16
0.1
1
1 Hz Out
27pF
2
3
7 60Hz Out
10pF
11
3.58MHz
3
27pF
ELM440
S
JQ
CP _
KQ
R
32.768kHz
15M
10
74HC112
85
330K
4060
39pF
12 8
■ FIGURE 8
ELM440 sells for $8.00 CDN) and prices are the same no
matter where in the world you are. All sales are on-line
using major credit cards.
However, it’s been my experience that most circuits
reduce the 60 Hz down to 1 Hz before the clock is used.
If this is your intent, it can be accomplished using the
cheaper 4060 ripple counter and a 32.768 kHz watch crystal
(Figure 8). The 4060 divides the 32 kHz crystal frequency by
16,384, resulting in a 2-Hz output. This is further divided
by the 74HC112 J-K flip-flop to produce a pulse once every
second.
ELIMINATING PA FEEDBACK
QI just installed a new PA amplifier, Pyramid
#PA1000X, at the company where I work. The amp
works great, but since the installation, we have had
problems with feedback whenever anyone tries
to page using a cordless phone. I’ve tried reducing the
speaker volumes somewhat, and although it does help, it is
not a cure. Is there a preamp buffer circuit that I could build
that would allow me to tune out the feedback and also limit the input volume level (ALC), so some of the loudmouths
in the company don’t overdrive the amp?
Dan Elliott
AIf you have an equalizer laying around, it can be
used to filter out the feedback frequency. Of
course, you will have to experiment with different
settings. Start with everything set with no boost
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or attenuation, then move the controls — one at a time —
until you eliminate the unwanted frequency (be sure to
return them to neutral before going on to the next filter). I’d
start with the mid-frequencies first.
A guaranteed solution is to install a feedback
eliminator. These units have a microprocessor in them
that searches for the distinctive feedback fingerprint,
determines the offending frequency, and engages a notch
filter to eliminate it. The number of filters range anywhere
from 10 to 80, with the price increasing with the number of
filters. A good buy is the Behringer DSP 110 Shark — with
variable audio delay and 12 notch filters — which sells on
the street for $80. NV
January 2006 15
