Compressor Applications for
Resistive Optocouplers
An audio compressor serves to reduce (or compress)
the dynamic range of the input signal. This keeps the
level more constant, so that it will be clearly heard Figure 1
above background sounds, whether they be noise or
accompanying musical sounds. An ideal compressor
introduces little distortion and noise when it is inactive,
and controls the audio level in a way that is pleasing to
the ear.
Silonex optocouplers offer a cost competitive, high
performance solution that fulfils these requirements.
They offer the advantage over VCA based designs that
the active element is essentially only in the circuit (and
hence potentially causing distortion) when gain
reduction is taking place. Distortion products tend to be
low order harmonics that are less objectionable to the
ear than high order crossover artifacts. The steady Figure 2
state amplitude performance of a compressor can be
expressed in terms of three parameters.
The Threshold is the input amplitude above which
compression starts to take place, see Figure 1 .
When the signal rises above the Threshold point, the
amount of compression that occurs is determined by
the Ratio (see Figure 2), which is expressed as the
quantity:
Change of input signal level
----------------------------
Change of output signal level
Figure 3
Since the action of the compressor is to make loud
sounds quieter, it is normal to have some gain after it
to restore the apparent loudness, this is termed
Makeup Gain, see Figure 3 .
The dynamic performance is usually described in
terms of the "Attack" time. This is the time between the
signal going above Threshold and the gain reduction
reaching its maximum level (or a reasonable proportion
of it), and the "Release" time, which is how long it
takes for the gain reduction to go away.
-1-
Compressor Applications for
Resistive Optocouplers
Figure 4 shows the circuit for a simple compressor with fixed threshold, the amplitude response of
which is shown in Figure 5 .
OP1 in conjunction
with R2 form a shunt Figure 4
attenuator which acts
as the gain control
element. The output is
buffered by IC1 to
prevent loading
effects. The output of
IC1 is half wave
rectified by D1 and
smoothed by R3 and
C2, and then fed to the
base of high gain
transistor Q1. As soon
as the circuit output
level starts to exceed
the Threshold set by
the turn on voltages of
D1 and Q1, Q1 will
start to conduct, turning on OP1 which will reduce the output level.
With the values shown this gain Figure 5
reduction starts at a Threshold of –10
dBu, this can be altered by adjusting R1
(smaller value = lower threshold) which
conveniently adds an appropriate
amount of makeup gain around IC1 at
the same time. For Thresholds greater
than 0 dB, a low voltage zener diode can
be added in series with D1 as shown.
The ratio is set by the coupler
characteristics and R5, and with the
value shown is approximately 3:1. The
current flo wing through the LED of OP1
also illuminates LD1, to indicate that
gain reduction is taking place. R3 and
C2 determine the Attack time, and
(R3+R4) and C2 the release, and need
to be set for the particular application.
The circuit will run off split supplies of +/-6 to +/-15 V, allowing maximum input signal levels of up to
+20 dBu RMS. IC1 can be any low noise/distortion operational amplifier, as long it has enough
output drive capability to charge C2 without distorting. The input signal needs to come fro m a
-2-
Compressor Applications for
Resistive Optocouplers
source with very low DC offset, since this would be modulated by the gain change, giving rise to
audible "thumps". If in doubt, add a coupling capacitor to the input. A selected NSL-32 coupler is
used, providing remarkably good performance, with distortion at –12 dBu input typically less than
0.005% without gain reduction taking place. Distortion whilst compressing is a function of input
frequency, the time constant of C2/R4 and the amount of gain reduction, and is typically better than
0.01% at 1 KHz/6 dB. The graded -023 version off the NSL-32 is used for reasons of repeatability in
the circuit. Unlike the limiter circuit, there is not a lot of loop gain to linearize the response. With a
NE5532 for IC1, output noise is better than –99 dBu.
Figure 6 shows the schematic for a more comprehensive compressor circuit, that offers control
over Threshold, Ratio, Attack, Release and Makeup gain.
Essentially the circuit works in a similar fashion to the limiter circuit , except that the threshold of the
comparator is set at a low level (=-20 dBu) and the preceding Threshold control VR1 attenuates the
rectified signal to alter the effective threshold. Local feedback from Q1 source to the comparator via
VR2 and R2 allows adjustment of the ratio from 2:1 to appro x 7:1, rather than the 100:1 ratio of the
limiter that is achieved by only having low frequency feedback through the optocoupler action. The
threshold range of -20 dBu to +10 dBu is shown in Figure 7 , and the ratios at a Threshold of 0 dBu
in Figure 8.
Figure 6
-3-
Compressor Applications for
Resistive Optocouplers
Attack time is variable from ~1 msec to 20
msec via VR3, and the release time from Figure 7
100msec to 1 sec via VR4. The variable gain
stage around IC2B provides from 0....+20 dB
Makeup Gain. Rather than just having an
indicating LED in series with the optocoupler,
a current mirror configured around Q2,3 and
R17,22 generates a ground referenced
voltage proportional to the current flowing
through the coupler LED. This can be used to
drive a meter (either an LED bar graph, or if
you want a "retro" look, a moving coil meter)
to give more information about how much
gain reduction is taking place. In this circuit
the graded NSL-32SR3S coupler is used,
with a 6 dB pre -attenuator to further reduce
distortion. This results in typical THD+N
figures of <0.003% at +6 dBu I/P and no
compression, and 0.03% at +10 dB and 3 dB
compression (depending on the Attack and
Release times set). Output noise with 0 dB
Figure 8
makeup gain is < -106 dBu.
-4-
