December 2016 9
the oscillating frequency. Triangle waves would take a bit
more work, but can be done with an additional op-amp. I’ll
leave that as an exercise for the reader, but think about last
Now, how do we make this frequency controlled?
There are a few different ways; for example, with a varactor
diode that acts as a voltage-dependent capacitor or by
creating a bidirectional current source to change the
threshold voltage. For this circuit, though, we will just do
something simple to show the principle.
We can use a resistor to yank on the voltage divider
node on the + input to the op-amp, and that will change
the threshold, making the duty cycle asymmetric in the
process but also changing the frequency. The change in
threshold will change where on the charge curve of the
capacitor that the op-amp output flips. The frequency
control input on the circuit shown in Figure 4 will definitely
need some current, so it should be buffered if it’s driven by
anything other than a low impedance source.
Also, the output frequency will be rather non-linear
with voltage. To fix that, we’d need to do something more
complicated that uses current sources to both linearize
the capacitor charging rate and to linearize the change in
threshold with voltage. A varactor diode won’t be linear
either, except in a certain region. In some future column,
we can explore how to make a better VCO.
Padding 70 Volt Audio
QI have been tasked with extending the sound system in our meeting hall for our local community center. One of the requirements is to be able to adjust the volume of a few new
speakers that are in some of the adjoining rooms. I had
planned to use L-Pads, but the twist here is that the system
uses 70 volt transformers. I am at a bit of a loss as to where
to place these L-Pads. Do they go between the 70 volt
line and the transformer, or between the secondary of the
transformer and the speaker?
Avis R. Ames
ASeventy volt audio systems are designed to work in a way similar to the power company’s high voltage lines. The voltage on the wires is increased so that the resistance of the wire
contributes a smaller overall voltage drop. The current is
lower for the same power: P = VI. So, if we increase V,
I is decreased for a given value of P. See Figure 5 for an
example that illustrates this point.
Since audio signals are AC, a transformer can be
used to transform the impedance — much like the power
company — to convert the high voltage/low current input
to a low voltage/high current signal that a four or eight
ohm speaker expects.
Now, padding down the audio means lowering the
voltage presented to the speaker system while roughly
maintaining the system’s expected impedance. It’s
called an L-Pad because the resistor network is in an
L configuration with one resistor in series and another
in parallel with the speaker (Figure 6). Those resistors
are typically variable (i.e., potentiometers) and the
potentiometers are ganged and matched such that the
impedance doesn’t vary much while the power delivered
to the speaker can be lowered.
Whether this is done on the high impedance side or
done on the low impedance side is not that important
electrically, except that the required resistances will be
different. What is probably more important is that the L-Pad
is able to be placed where you need it to be.
Chances are that the L-Pad would be easier to put
somewhere in the distribution line, far from the speaker;
perhaps mounted in a wall. That would argue for a high
QUESTIONS and ANSWERS
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n FIGURE 3. Simple relaxation
n FIGURE 4. Relaxation oscillator with
n FIGURE 5. Loss for high impedance
vs. low impedance.
n FIGURE 6. Speaker L-Pad example.