Resistive optocouplers can be used in either series (Figure 1), shunt (Figure 2) or series/shunt (Figure 3) configurations.The hybrid series/shunt (Figure 4) is a cross between the shunt and series/shunt and offers some advantages.

In most normal solid state audio circuits, the optimum source and oad resistances are going to fall somewhere in between the Ron and Roffof the couplers. There is likely to be a significant asymmetry between the "up" (decreasing attenuation) and "down" (increasing attenuation) times of the series and shunt configurations. If this a problem, it is best to use a coupler with a relatively fast response, for example the NSL-32SR3. This device also shows the best distortion performance of any of the NSL-32 series.This is an important consideration when being used as a linear attenuator, as it will not be driven hard ON or OFF as in a switching circuit. It is better to drive the coupler LED from a constant current source, to minimize the effects of variations in LED forward voltage from device to device and temperature. A simple circuit that gives 1mA per Volt input is shown in Figure 5.

The measurements of the various circuits were taken with an input level of +10 dBu at 1 KHz, and the distortion (THD+N) measured with a 30 KHz bandwidth. Although some of the distortion figures may seem less than ideal, three factors should be kept in mind:

1. These measurements were taken at fairly high levels, typically corresponding to -6 dBFS on professional equipment. They will decrease rapidly with input level typically being 3 to 4 times less at 0 dBu, and virtually unmeasurable at -20 dBu.
2. The distortion products produced by an Audiohm coupler are all low order, second and third harmonics, which are less objectionable than high order harmonics produced by crossover artifacts.
3. In the series/shunt and hybrid series/shunt attenuator configurations the worst distortion occurs at high attenuations. The output signal level is so low that the distortion is unlikely to be noticeable.

Please note that the horizontal scale is in mA, for example 300 m = 300 microamps. Input signal was 1 KHz, +10 dBu. The high dark resistance of the NSL-32SR3 gives quite good attenuation (typically about 80 dB at 1 KHz). However the circuit's usefulness is limited by the non-linear relationship between LED current and attenuation, and high distortion levels at any attenuation level below -10 dB. This is because the voltage across the non-linear cell resistance increases as the coupler turns off. Time constants are approx 0.5 and 12 msec for increasing and decreasing gain respectively. At frequencies above a few hundred Hz, the maximum attenuation is mostly determined by the parasitic capacitance and decreases with frequency.

This configuration (Figure 3) achieves better OFF attenuation and symmetrical time constants, at the expense of an additional coupler. The circuit could be driven by complementary current sources, but the simple arrangement shown in Figure 10 saves complexity and current consumption, and also provides a reasonable dB/linear control law over a good proportion of the range. With the values shown, the circuit imitates a 5 KOhm potentiometer, although there is some overall variation in total resistance with attenuation as it should be fed from a low impedance source. C1 sets the up and down time constants at about 20 msec, which achieves a fast but smooth audible response. The attenuation and distortion curves are shown in Figure 11.

Note the distortion null at 6 dB attenuation where both couplers have the same resistance.

The hybrid series/shunt arrangement shown in Figure 12 gives an improvement in distortion performance, Figure 13, by using resistor R3 to drop a proportion of the signal at low to medium levels of attenuation. The "bent" attenuation law is useful, giving greater resolution in the normal working area of the control, and a sharper response towards cut-off. To further improve on this distortion performance there are two possibilities:

1. Use a coupler with a higher voltage rating, for example the NSL-37V51.
2. Use two couplers in series to reduce the voltage across each cell.

All the above circuits have a control law, which is determined by the inherent characteristics of the couplers and supporting devices. Although consequently non-linear, it is useable for many simple applications. If a true linear response is desired the circuit of Figure 15 can be used. A shunt attenuator is placed in the feedback loop of IC1, which sets the current through the coupler OP1 so that the feedback voltage on IC1 pin 3 is equal to the control voltage applied to R1. Matched coupler OP2 has the same current flowing through the LED as OP1, forming a shunt attenuator linearly controlled by V_CONTROL. OP1 should feed a high load impedance for best linearity and insertion loss. C1 provides frequency compensation around the loop for stability. The output attenuator is fully floating and can be run hundreds of volts away from the control circuit. This feedback control could equally be applied to a series/shunt circuit giving a floating linear potentiometer output.