All electronic parts, including cables and non-
you are making connections in your mixing sys-
powered devices (mics, passive mixers and such),
tem. The outputs of circuits
have an impedance
have impedance, measurable in ohms (symbol
S2
rating and so do inputs. What's good? What
or Z). Impedance
is
the total opposition
a
part
values are best?
It depends on the direction of
presents to the flow of signal, and
it's
important
signal flow, and in theory,
it
looks like this:
to understand some things about this value when
OUTPUTS
-
plug into
-
INPUTS
It
is
generally said that the output imped-
ance
(Z) should be
as
low as possible. 100
ohms, 10 ohms. The lower, the better, in
theory. A circuit with
a
low output imped-
ance will offer a low resistance to the passage
of signal, and thus will be able to supply
many multiple connections without
a
loss
in performance or
a
voltage drop in any part
of the total signal pathway. Low impedance
values
can be achieved economically by us-
ing transistors and integrated circuits, but
other considerations are
still a problem in
practice, such as:
1. The practical power supply
is
not infi-
nitely large. At some point, even if the
circuit
is
capable of supplying more en-
ergy you will run out of "juice".
Inputs should have very high impedance
nurnbers,
as
high
as
possible (1 00,000 ohms
1
million ohms, more, if
it
can be arranged).
A high resistance to the flow of signal at
first sounds bad, but you are not going to
build the gear.
If the designer
tells
you his
input will work properly and has no
need
for a large amount of signal, you can assume
that he means what he says. For you, a
high input impedance isan unalloyed virtue.
It means that the circuit will do
its
job with
a
minimurn of electrical energy as a begin-
ning. The most "economical" electronic de-
vices in use today have input impedances of
many
millions of ohms, test gear for ex-
ample, voltrneters of good quality must not
draw signal away from what they are
meas-
uring, or they will disturb the proper opera-
2.
Long before this happens, you may burn
tion of the circuit. A design engineer needs
out other parts of the circuit. The output
to
see
what is going on in hisdesign without
impedance may be close to the
theoreti-
destroying
it,
so he must have an "efficient"
cally ideal "ohms" but many parts in the
device to measure with.
practical circuit are not. Passing energy
through a resistance generates heat and
too much current will
literally burn parts
right off the circuit card
if steps are not
taken to prevent catastrophic failure.
3.
Even if the circuit does not destroy itself,
too high a demand for current may
seri-
ously affect the quality of the audio. Dis-
tortion will rise, frequency response will
suffer, and you will get poor results.
The classic rneasurement for output impedance
is to
load
a
circuit until the voltage drops 6dB
(to half the
original power) and note what the
load value is. In theory, you now have
a
load im-
pedance
that is the same
as
the output imped-
ance.
If you reduce the load graudally, the dB
reading will return slowly to
its
original value.
How rnuch drop
is
acceptable? What load will be
left when an acceptable drop
is
read on the me-
ter!
When the
load value (input Z)
is
approxirnately
seven times the output impedance, the
needle is
still
a
little more than
1
dB lower than the origi-
nal reading.
Most technicians say "1 dB, not bad, that's ac-
ceptable". We at TEAC must say wedo not agree.
We think that a seven-to-one ratio of input
(7)
to output (1) is not
a
high enough ratio, and
here's why:
To Next Page
Loading...
