platform gain trick is done with
We replace the simple resistor-capacitor time constant circuitry in
a limiter with the network shown
in Figure 5. As you can see in the
figure, there are two capacitors;
one of which is driven with a
reduced voltage from a voltage
divider composed of resistors R1
and R2. When resistor R1 is equal
in value to resistor R2, the voltage
on capacitor Cr is twice that of Cp.
That means that on any peak in
the audio, Cr will always be at a
higher voltage than Cp.
Since the only connections to
Cp are to the diodes and the high
impedance input of the VCA, this
capacitor has no discharge path,
so it will hold the peak voltage.
Cp will discharge only when the
voltage on Cr goes lower than the
voltage on Cp. That's the only time
when both capacitors can
discharge through Rr. It's the
voltage difference between the two capacitors — set by the
ratio of R1 and R2 — that fixes the gain platform. With
equal values for R1 and R2, the platform is set for 50%, or
3 dB. Figure 6 and Figure 7 show the circuit diagram for
the platform gain controller.
■ FIGURE 8. Parts placement on the component side of the
platform gain controller circuit board.
Printed Circuit Board
A printed circuit board layout that uses just a single
copper plane is available with the article downloads; a parts
placement diagram on the component side is shown in
Figure 8. Figure 9 is a photograph of the populated circuit
board, and Figure 10 shows the external connections to
the circuit board, including one possible way to implement
the power supply.
The current requirements for the positive and
negative voltages are quite low — less than 25 mA
each. Probably the best way to power the
controller (as shown in the figure) is with a wall
wart power supply feeding a DC-DC converter.
One possible converter to use is a Murata
NMH0515SC two watt converter that converts a
five volt supply to ± 15 volt supplies of 67 mA
each. Use a regulated five volt wall wart, use the
recommended bypass capacitors, and keep the
converter away from the low level audio stages.
Such converters do generate a little EMI.
The input pots should be of the "audio taper"
variety, meaning you get more control at higher
attenuation than with a linear pot. A ganged pot is
preferred over two separate pots, but if this is in a
set-and-forget application, that's not too essential.
The meter (which is optional), is driven by the
control voltage signal, so it always has a little bias
voltage on it. I removed this with a zener diode (as shown
in Figure 7), but many analog meters have a mechanical
zero that will do just as well. My controller — which has an
LED bar graph display instead — is shown in Figure 11. This
was built into the box of a surplus oscilloscope plug-in
module. Waste not, want not!
■ FIGURE 9. Photograph of the populated circuit board.
January 2012 31