Interfacing and Processing 247alike, an amplifi er’s individual input impedances may be ultra-matched. Since with
voltage matched systems, the wanted input signal is a voltage, the ideal, “ noninvasive ”
amplifi er input or load impedance would appear to be very high, say 1 M Ω. Then only
minuscule current would be taken from the source.
8.1.2.5 High Impedances
Some high-end hi-fi makers have taken the high impedance route, claiming better sonics.
This may be inseparable from the circuitry used to create the high-Z conditions, and not
necessarily down to the high-Z conditions per se.
In power amps with low (or zero) feedback, and using bipolar junction transistors
(BJTs) in the input section, high input impedances (above 10 k Ω ) can be more diffi cult
to implement consistently. On this basis, the early transistor amplifi ers sometimes had
their inputs rated inμ A of input current drawn! In contrast, there is usually no diffi culty
attaining impedances as high as 1 M Ω or more, when the input stage parts are valves,
JFET or insulated gate FET (MOSFET) or any combination of these—whether loop or
local feedback is zero, low, or high.
When un terminated, such high impedance circuits are noisier (hissier) and far more liable
to allow parts to be microphonic than lower ( “ normal ” ) impedance ones.^7 Demonstration
is simple enough: try tapping the appropriate capacitors with an insulated tool while
listening with full-range speaker(s) connected. High impedance inputs can also be
the cause of diffi culties and compromises with direct coupling. However, unless the
input is direct coupled, or is at least coupled via very large capacitors, LF and subsonic
microphony and electrostatic noise pick-up will not “ see ” the lower source impedance
and will persist in accordance with the high impedance.
8.1.2.6 Low Impedances
As input impedance is lowered, there is less microphony and electrostatic noise pick-
up when the amplifi er inputs are disconnected, even with unshielded cabling. However,
loading is increased, as is ultimately thesusceptibility to magnetic fi eld noise pick-up,
which is much, much harder to shield against.
8.1.2.7 Loading
A single load of (say) l k f Ω may or may not compromise the source’s performance. But
two or a few of such loads almost certainly will, unless the source is rated appropriately
(see later). Low impedance inputs are also the most easily damaged if one amp’s output