Many head units manufactured today employ digitally controlled fader electronics which are responsible for significant amounts of thermal hiss noise. Because the front and rear signals are identical in every way except for that fader noise, it is possible to reduce fader noise slightly if you are willing to dispense with the fader function.

A circuit of only 2 resistors per channel arranged to perform an average of the front and the rear signals can suppress this fader noise. The circuit shown in figure 1 develops an output which is 1/2 of the sum of the front and rear signals — which is the average of the two signals. This circuit does not significantly alter the signal level, so it is not usually necessary to readjust any level matching controls when it is installed.

Figure 1

One channel of the averaging circuit
that reduces uncorrellated noises by
3db without affecting the signal level

Averaging works by abstracting the common features of the two signals in preference to the noises which are different because of the fader electronics. The noises on the one output will cancel at some points in time, but then at other times they don't. As a result of the statistical nature of the noise, this circuit will reduce fader hiss noise by exactly 3db when the fader is optimally set for equal front and rear signal levels.

According to statistical principles better rejection is not possible without altering the fidelity of the signal in some way or without bypassing the digital fader circuits in the head unit. This is unfortunate because the fader noise is usually only a fraction of the total noise in a head unit so the actual improvement is likely to be less than 3db. For example, if 1/3 of the hiss in a head unit is fader noise, then the improvement will only be 1db. This difference is hardly noticeable, even with the quickest A/B comparisons. But when you are interested in eking out the best possible performance, and every little bit helps, the effort required to hook up 4 very inexpensive resistors may be worthwhile.

Alternatively, line drivers and preamplifiers manufactured by Anamir Electronics, Inc. are available in versions that are capable of performing this same averaging with an extremely high degrees of accuracy in addition to their many other more significant high performance gain, fidelity and cable enhancing features.