Preamplifi ers and Input Signals 209Although the RCA design employs MOSFET input devices, which offer, in principle, an
input impedance that is perhaps 1000 times better than this fi gure, the presence of on-chip
Zener diodes, to protect the device against damage through misuse or static electric charges,
reduces the input impedance to roughly the same level as that of the junction FET device.
It is a matter for some regret that the design of the CA3140 series devices is now so
elderly that the internal MOSFET devices do not offer the low level of internal noise of
which more modern MOSFET types are capable. This tends to rule out the use of this
MOSFET op-amp for high-quality audio use, although the TL071 and its equivalents,
such as the LF351, have demonstrated impeccable audio behavior.
7.15 Performance Standards .........................................................................................
It has always been accepted in the past, and is still held as axiomatic among a very large
section of the engineering community, that performance characteristics can be measured
and that improved levels of measured performance will correlate precisely, within the
ability of the ear to detect such small differences, with improvements that the listener will
hear in reproduced sound quality.
Within a strictly engineering context, it is diffi cult to do anything other than accept the
concept that measured improvements in performance are the only things that should
concern the designer.
However, the frequently repeated claim by journalists and reviewers working for
periodicals in the hi-fi fi eld—who, admittedly, are unlikely to be unbiased witnesses—
that measured improvements in performance do not always go hand in hand with the
impressions that the listener may form, tends to undermine the confi dence of the circuit
designer that the instrumentally determined performance parameters are all that matter.
It is clear that it is essential for engineering progress that circuit design improvements
must be sought that lead to measurable performance improvements. However, there is
now also the more diffi cult criterion that those things that appear to be better, in respect to
measured parameters, must also be seen, or heard, to be better.
7.15.1 Use of ICs
This point is particularly relevant to the question of whether, in very high-quality audio
equipment, it is acceptable to use IC operational amplifi ers, such as the TL071, or some