862 Chapter 25
they do offer a good to excellent solution to impedance
matching and sundry other problems facing input stage
design. Simplistically, a transformer is a magnetically
soft core around which are two windings, the voltage
ratio between the two being equal to the ratio of the
number of turns on each. The impedance ratio is the
turns ratio squared (e.g., a 10:1 turns ratio corresponds
to a 100:1 impedance ratio) because power output
cannot exceed power input. If the voltage is stepped up
ten times, the output current must be stepped down ten
times. Impedance, which is the ratio of voltage to
current, is consequently the square of the transformed
voltage or current ratio, see Chapter 11.
Given this, it is a simple matter to calculate the ratio
necessary to match the microphone impedance to the
amplifier OSI that is realistically achievable. Since few
people are intense enough about the whole affair to
bother measuring individual microphones, the conven-
tion that 200: is a good midpoint for source imped-
ance serves well. Variations between actual
microphones make trivial differences in the larger
scheme of things. The assumption that most bipolar
input amplifiers have an OSI of between 5 k: and
15 k: indicates that the transformer ratio should lie
somewhere between 1:5 and 1:8.7.
Many consoles use higher ratios (typically 1:10),
probably in the naive belief that the noise advantage of
a step-up input transformer stems from the free gain it
affords. Although on a basic level it would seem to
make sense that the less electronic gain needed the
quieter the system must be, this fallacy is completely
belied by the truth that the transformer merely allows
you to choose and alter the source impedance for which
the amplifier is optimally quiet. Increasing the turns
ratio beyond this easily defined optimum can and will
actually render the amplifier noisier.
In practice the free gain can be more of a nuisance
than a benefit. It is not unusual for microphone inputs to
receive transients exceeding +10 dBu and mean levels
of –10 dBu, especially in a rock-and-roll stage or
recording environment. Even dynamic capsules can
deliver frightening levels that can pose head room prob-
lems in the mixer front end. A typical 1:5 transformer
has a voltage gain of 14 dB (20 dB for a 1:10 ratio),
which would mean that even with no electronic gain
after the transformer, normal mixer operating levels are
being approached and possibly exceeded. These circum-
stances make worrying about a dB or two of noise
performance total nonsense to be sure; it just serves to
point out that our microphone front end has to be
capable, if not perfectly optimized, for elephant herds as
well as butterflies.25.10.6.1 Transformer CharacteristicsTransformers have numerous limitations and inadequa-
cies resulting from their physical construction that make
their actual performance differ (in some respects radi-
cally) from that expected of a theoretical model.
The heart of the transformer is the magnetically
pliable material into and out of which energy is induced.
Virtually any material—nickel, steel, iron, ferrous
derivatives, and substitutes—have the same basic limi-
tations. They saturate at a magnetic level beyond which
they are incapable of supporting further excursion, and
exhibit hysteresis—a crossover like nonlinearity at low
levels responsible for a significantly higher distortion at
low levels than anything else likely to be found within a
well-designed modern-day signal path.
These two effects at opposite ends of the dynamic
spectrum mean that all transformers have a well-
defined range within which they must be operated and
this range is less than the range of levels the micro-
phone amplifier (mic-amp) is expected to pass. This is
especially true at low frequencies, where the core is
prone to saturation far earlier. Optimization begins here.
Is it to be designed for minimum hysteresis (butterflies)
or with plenty of material to be tolerant of monstrous
(elephantine) signal levels?
Windings are made of wire, which has resistance.
Resistance means loss and decreased efficiency and
noise performance. By the time there are enough turns
on each of the windings to ensure the inductive reac-
tances are high enough not to affect in-band use,
winding resistances can no longer be ignored.
Capacitance exists between things in close proximity
and that includes transformer windings—between each
other, between adjacent turns and piles in the same
winding, and from the windings to ground. In this given
instance it is nothing but bad news. CapacitanceFigure 25-39. Source impedance versus bandwidth gain
versus frequency for a typical follower-connected opera-
tional amplifier highlighting effects on response of source
impedance on input device.
10
5
0
5
10
15
20
2530 k 7 10 k (^7) 3 k 7
Source
resistance
100 k 1 M 10 M 100 M
Frequency–Hz
dB