2, Impedance Matching; Or, What Plugs Into What - Teac Tascam Series Instruction Manual

2. IMPEDANCE MATCHING — OR; WHAT PLUGS INTO WHAT
All electronic parts including cabies and non powered
devices (mics, passive mixers and such) have impedance,
measurable in ohms {symbol $2 or Z). Impedance is the
total opposition a part presents to the flow of signal, and
it's important to understand some things about this value
when you are.making connections in your mixing system.
The outputs of circuits have an impedance rating and so
do inputs. What's good ? What values are best? It depends
on the direction of signal flow, and in theory, it looks
like this:
plug into
OUTPUTS
OUTPUTS
It is generally said that the output impedance (Z)
should be as low as possible. 100 ohms, 10 ohms, the
lower, the better in theory. A circuit with a low output
impedance will offer a low resistance to the passage of
signal, and thus will be able to supply many multipte
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 using
transistors and integrated circuits but other considera-
tions are still a problem in practice.
1, The practical power supply is not infinitely large. At
some point even if the circuit is capable of supplying
more energy, you will run out of "juice".
2. Long before this happens, you may burn out other
parts of the circuit. The output impedance may be
close to the theoretically ideal "O ohms" but many
parts in the 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 may seriously affect the quatity of the audio.
Distortion will rise, frequency response will suffer,
and you will get poor results,
++ INPUTS
INPUTS
Inputs should have very high impedance numbers, as
high as possible, 100,000 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 telis you his input will work proper+
ly 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 is an unalloyed virtue. It means that
the circuit will do its job with a minimum of electrical
energy as a beginning. The most "economical" electronic
devices in use today have input impedances of many
millions of ohms, Test gear. For example, voltmeters of
good quality must not draw signal away from what they
are measuring or they will disturb the proper operation
of the circuit. A design engineer needs to see what is
going on in his design without destroying it, so he must
have an "efficient" device to' measure with.
The classic measurement for Output impedance is to
load a circuit until the voltage drops 6 dB (to half the
original power) and note what the load value is. In theory,
you now have a load impedance that is the same as the
output impedance, If you reduce the load gradually, the
dB reading will return slowly. to its original value. How
much drop is acceptable? What load will be left when an
acceptable drop is read on the meter?
When the load value is approximately seven times
the output impedance, the needle is still a little more
than 1dB lower than the origina! reading.
Most technicians says." 1 dB, not bad, that's accept-
able." We at TEAC must say we do 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.
1. The measurement is usually made at a midrange fre-
quency and does not show true loss at the frequency
extremes, What about drop at 20 Hz?
2.-All outputs are not measured at the same time. Most
people don't have twenty meters, we do. Remember,
everybody plays together when you record and the
circuit demands in practice are simultaneous, All
draw power at the same time.