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Impedance Matching
Impedance matching went out with vacuum tubes, Edsels and beehive
hairdos. Modern transistor and op-amp stages do not require impedance
matching. If done, impedance matching degrades audio performance
.
Modern solid-state devices transfer voltage between products, not
power . Optimum power transfer requires impedance matching. Optimum
voltage transfer does not. Today's products have high input impedances
and low output impedances. These are compatible with each other.
Low impedance output stages drive high impedance input stages. This
way, there is no loading, or signal loss, between stages. No longer
concerned about the transfer of power, today's low output/high input
impedances allow the almost lossless transfer of signal voltages.
What then, does impedance matching have to do with unity gain? Well,
it shouldn't have anything to do with it. But because of different
manufacturer's definitions, it is one way (brute force) of correcting
gain discrepancies between products. Impedance matching introduces
a 6 dB pad between units . Let's see how this works.
Look at Figure 3. Here we see a real world interface between two
units. The positive and negative outputs of the driving unit have
an output impedance labeled R OUT . Each input has an impedance
labeled R IN . Typically these are around 100 ohms for R OUT and
20k ohms for R IN . Georg Ohm taught us that 100 ohms driving 20k
ohms (looking only at one side for simplicity) creates a voltage
divider, but a very small one (-0.04 dB). This illustrates the above
point about achieving almost perfect voltage transfer, if impedance
matching is not done .
If it is done, you lose half your signal. Here's how: impedance
matching these units involves adding 100 ohm resistors (equal to
R OUT ) to each input (paralleling R IN ). The new input impedance
now becomes essentially the same as the output impedance (100 ohms
in parallel with 20k ohms equals 99.5 ohms), therefore matching.
Applying Ohm's law to this new circuit tells us that 100 ohms driving
100 ohms creates a voltage divider of 1/2. That is, 1/2 of our signal
drops across R OUT and 1/2 drops across R IN , for a voltage loss
of 6 dB. We lose half our signal in heat across R OUT . Not a terribly
desirable thing to do; yet, it does fix our unity gain problem.
Back to Figure 2 . By selectively impedance matching only between
Units A and B, we introduce a 6 dB pad. This cancels the 6 dB gain
resulting from using balanced outputs with this unit. This changes
the output of Unit A to ± 1/2 V, or +1 V balanced. Since
Unit B already is unity gain balanced, then we do not impedance
match, and its output is also ± 1/2 V. We do impedance match
Unit C's output and now Unit C passes this +1 V signal to its output
as ± 1/2 V, and finally we get a true unity gain result from
all three boxes. One volt in, produces one volt out -- balanced.
Preferred Alternative to Impedance Matching
The preferred alternative to impedance matching is ridiculously
easy -- turn the level control down 6 dB. Of course this means the
unity gain mark, or detent position, loses its meaning, but this
is far better than losing half your signal.
Many users do not view this issue as a problem. There are so many
other variables that require turning level controls up or down that
this just becomes part of the overall system gain setting. Most
units have sufficient headroom to allow for an unexpected 6 dB of
gain without hurting anything.
Besides, the unity gain mark/detent is only a reference point. The
whole reason manufacturers give you level controls, is to allow
setting the gain you need for your system. If it were important
for them to remain at unity, they would not be there. They are yours.
You paid for them. Use them.
Summary
Unity gain and impedance matching: a strange dichotomy. One solves
the other, but badly.
Impedance matching is not necessary and creates many ills. It reduces
signal levels and dynamic range by 6 dB (and possibly signal-to-noise
by the same amount). The large currents necessary to drive the low
matching-impedance usually degrades total harmonic distortion. And
the extra current means excess heat and strain on the power supply,
creating a potentially unreliable system.
Simply turning the level control down (or up, as the situation dictates),
is the best solution for unity gain disparities.
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