Handbook for Sound Engineers

Consoles 863

between windings means unwanted leakage and imper-

fect isolation, while winding self-capacitance reacts

with the winding inductances to form resonances. Reso-

nances, even if far outside the audio band, invite

response trouble, and disturb in-band phase linearity.

Combinations of these capacitances greatly affect one

of the greater advantages of transformers,

common-mode rejection (CMR).

25.10.6.2 Common-Mode Rejection

For a transformer to work and transfer wanted informa-
tion from one winding to another, a current must be
made to flow through the primary; this is ideally
achieved just by the opposing polarity (differential)
signal voltage applied across it. Again ideally, any iden-
tical signals on the two ends of the windings (common
mode) should not cause any current to flow in the
winding (because there is no potential difference across
the winding to drive it) and so no signal transfer can be
made into the secondary. So much for ideal.
Common-mode rejection is the ability of the trans-
former to ignore identical signals (in amplitude and
phase) on the two input legs and not transfer them
across the secondary as differential output information.
Principally, it is imbalanced distribution of capaci-
tance along the length of the two windings, both with
respect to each other and to ground that makes CMR
less than perfect. Co-winding capacitance has the effect
of directly coupling the two wiring masses permitting
common-to-differential signal passage, which worsens
with increasing frequency at 6 dB/octave. Electrostatic
shielding (a Faraday shield) between the windings can
alleviate co-winding capacitance coupling.
Further CMR worsening can be expected even if the
two windings are perfectly balanced with respect to
each other, if the primary winding is not end-to-end
capacitatively matched with respect to ground. Any
common-mode signal from a finite impedance source
(almost always the case) when confronted with such a
capacitatively unbalanced winding sees it as being just
that—unbalanced (becoming more so with increasing
frequency). Again, input common-mode signals are
transferred across to become output differential infor-
mation indistinguishable from the wanted input differ-
ential source.

Broadcasters particularly are concerned with
winding balance, not only on microphone transformers
but also on line-output transformers, reasoning that
common-differential transference is as likely to occur at
a source as at an input.

25.10.6.3 2Microphone Transformer Model

Fig. 25-40 gives a better idea of what the small signal of

a dynamic microphone has to suffer. The winding

capacitances (CP and CS) form lovely resonances with

the inductances, while the transformed up primary

winding resistance (RP) added to the resistance of the

secondary winding (RS) merely serves to increase the

effective source impedance of the microphone

producing loss and resultant inefficiency.

A frequency response of a less-than-ideal trans-

former fed from a 200: source and measured at high

impedance across the secondary looks something like

Fig. 25-41, where the low-frequency droop is attribut-

able to one or both of the winding inductive reactances

becoming comparable to signal impedances, while the

high-frequency peak is an aforementioned secondary

winding self-resonance. Usually the primary self-reso-

nance is fairly well damped by the source impedance,

but occasionally added cable capacitance can play cruel

tricks here, too.

The mic-amp itself, as discussed, has a high-input

impedance (hundreds of kilohms and up) while its

optimum source impedance is defined at around

5–15 k:.

It’s good engineering practice to consider how the

circuit behaves when the operating impedances are no

longer defined by the microphone (i.e., when it is

unplugged). Ordinarily, the circuit of Fig. 25-40 with

the microphone disconnected would probably oscillate,

as would any circuit with a high-gain, high

Figure 25-40. Transformer coupling model showing major

elements.

Figure 25-41. Typical transformer transmission response.

Microphone

Cable

Transformer

VS

R CoilL

CC CPLP

Primary Secondary

RS

CS

RP C

Amplifier

(high input

impedance)

LS

10 dB

5 dB

0 dB

5 dB

10 dB

15 dB

10 100 1 k 10 k 100 k

Frequency–Hz

Ratio 1:5

source = 200 7

load = >100 k 7