■ FIGURE 14. The input-output transfer function of
the platform gain controller
when the input signal is
always increasing. A very
high output level is
maintained over a very
large change in input level.
more complicated than
what we discussed earlier
in the article. There are a
few more diodes and a
transistor. The diode
connection — four diodes
(D3-D6) in series — may
seem strange. The reason
for this is that we're taking
advantage of the fact that diodes are not ideal rectifiers.
Silicon diodes have a threshold voltage of about 0.7 volts,
below which they don't conduct well. Four diodes in series
will bump that threshold up to nearly three volts. The
reason for the diodes and the transistor that they drive is to
add a "gate" function to the platform gain controller. As we
know from our discussion of compressors, when the audio
input signal is removed (as for the natural pauses in speech)
the noise whoosh kicks in. The gate prevents the release
capacitor from discharging through R31 during such
pauses. The controller holds its last gain state.
■ FIGURE 12. Meter calibration for the unit
when the series zener diode is used.
than is already present in the laboratory-quality audio
oscillator, or added by the PC sound card used as an input
device. The second harmonic of 440 Hz is 880 Hz. Some
of the extraneous peaks are from power line harmonics on
the signal leads — the bane of any audio engineer.
Well, that wraps things up. I hope you’ve ‘gained’ some
insight into stereo gain controllers. NV
Tuning and Performance
Once the circuit is assembled and passes the smoke
test, it's time to adjust the trim pots R9 and R10. These pots
balance the DC input currents of the operational
transconductance amplifier. Since the outputs are DC
coupled, this is done by putting a voltmeter at the left
channel output, adjusting R9 for very close to zero volts,
and then doing the same for the right channel and R10. If
this isn't done correctly, you might force the processor into
a very low gain state when you first apply an audio input.
That's because the DC "thump" will be processed as a very
large change in the input signal.
Figure 14 shows the input-output transfer function of
the processor when the input signal is always increasing.
You can see how a very high output level is maintained
over a very large change in input level. If we decrease the
input voltage, the signal would need to drop by 3 dB
before leveling off, because of the gain platform.
Figure 15 gives you an idea of the audio quality of the
platform gain controller. The spectra shown in the figure
are taken using Audacity — an open source audio editing
and processing program that works on both Linux and
Windows. I use the Linux version. Voltage-controlled
amplifiers are notorious for introducing distortion and
noise, but the LM13600 seems well behaved.
The dark trace is the oscillator input to the controller,
set at 440 Hz and a voltage to reach the controller
operating midpoint. The light trace is the controller output.
The controller adds just a little more noise and distortion
■ FIGURE 13. Artwork for an
analog meter scale.
■ FIGURE 15. Audio
quality of the platform gain
recorded a 440 Hz
mid-range input signal (dark
trace) and the resultant
output signal (light trace).
The controller adds just a
little more noise and
distortion than is already
present in the input.
January 2012 33