274 Chapter 8
Tolerance is an issue and is dependent on the constituent parts, both semiconductors.
Because the tolerance of both LDRs and LEDs is rather wide, manufactured combination
devices are likewise broadly specifi ed. The performance of both devices also varies
widely with temperature. Also, in many circuits, there is no negative feedback loop to
keep these variables within limits. Thus LDR/LED combinations are unsuited to system
gain control due to inconsistencies of say /–3 dB. They are fast enough to be used as
limiters for bass and even midfrequencies in active crossover systems, and sonic quality
is regarded as among the best. However, the above gain variation (in a population) would
translate as a spectral imbalance, making overdriven conditions in a large system unsafe
and/or uncomfortable, as well as drawing attention to the limiter action.
An LDR may also be partnered with an incandescent lamp. Even if small, the lamp is
relatively slow to turn on and off, preventing its use for clean-cut dynamics processing,
and lamp life span is more vibration sensitive and so not as certain as solid-state parts in
road-going use.
8.6.2.3 Junction Field-Effect Transistors
JFETs are the lowest cost elements and can be made operative with little support
circuitry. They are normally applied in the lower arm of an attenuator network. Without
introducing complications of increased noise, noise pick-up, and other sonic degradation
caused by introducing high ohmic value series resistors, attenuation is limited in range,
and unless added circuitry can be justifi ed, mild attenuation (around –6 dB) produces high
(1 to 10% but mainly benign, low order) harmonic distortion.^15 Low distortion control
can be attained by placing the JFET in a control loop, comprising two or more op-amps
and other active parts. However, as most JFETs ’ Ron is in the order of a few tens of ohms,
attenuation is still typically limited to –20 to –30 dB, enough for limiting, but not as a
VCA gain and mute control.
8.6.2.4 Multiplying Digital-to-Analogue Converters
Multiplying digital-to-analogue converters (M-DACs) involve a resistive ladder, usually
binary, with semiconductor switches, usually small-signal MOSFETs. They are the
solid-state equivalent of a relay-controlled attenuator ladder (see later). Types suitable
for high-performance audio must have dB steps—awkward in binary format—and
special MOSFETs for low distortion and absence of “ zipper ” noise. The latter undesired
sonic effect occurs in low-grade M-DACs; it is caused by step changes in DC levels or