Handbook for Sound Engineers

Audio Transformers 287

output appears as 0.5 Ei in series with RG /4 or 150ȍ.
When RL is connected, a simple voltage divider is
formed making EO = 0.25Ei or a 12.04 dB loss. For a
real transformer having RP=50ȍ and RS=25ȍ, the
equivalent circuit becomes 0.5Ei in series with
(RG+RP)/4 +RS or 187.5ȍ. Now, the output
EO= 0.222Ei or a 13.07 dB loss. Therefore, the inser-
tion loss of this transformer is 1.03 dB.

11.1.3.4 Sources with Zero Impedance

One effect of using negative feedback around a high
gain amplifier is to reduce its output impedance. Output
impedance is reduced by the feedback factor, which is
open-loop gain in dB minus closed-loop gain in dB. A
typical op-amp with an open-loop gain of 80 dB, set for
closed-loop gain of 20 dB, the feedback factor is
80 dBí20 dB = 60 dB or 1000, will have its open-loop
output impedance of 50ȍ reduced by the feedback fac-
tor (1000) to about 0.05ȍ. Within the limits of linear
operation—i.e., no current limiting or voltage clip-
ping—the feedback around the amplifier effectively
forces the output to remain constant regardless of load-
ing. For all practical purposes the op-amp output can be
considered a true voltage source.
As seen in Fig. 11-19, the distortion performance of
any transformer is significantly improved when the
driving source impedance is less than the dc resistance
of the primary. However, little is gained for source
impedances below about 10% of the winding dc resis-
tance. For example, consider a typical line output trans-

former with a primary dc resistance of 40ȍ. A driving

source impedance well under 4ȍ will result in lowest

distortion. The line drivers shown in Fig. 11-28 and Fig.

11-29 use a paralleled inductor and resistor to isolate or

decouple the amplifier from the destabilizing effects of

load (cable) capacitance at very high frequencies.

Because the isolator’s impedance is well under an ohm

at all audio frequencies, it is much preferred to the rela-

tively large series, or build-out, resistor often used for

the purpose. It is even possible for an amplifier to

generate negative output resistance to cancel the

winding resistance of the output transformer. Audio

Precision uses such a patented circuit in their System 1

audio generator to reduce transformer-related distortion

to extremely low levels.

11.1.3.5 Bi-Directional Reflection of Impedances

The impedances associated with audio transformers may

seem confusing. Much of the confusion probably stems

from the fact that transformers can simultaneously

reflect two different impedances—one in each direction.

One is the impedance of the driving source, as seen from

the secondary, and the other is the impedance of the

load, as seen from the primary. Transformers simply

reflect impedances, modified by the square of their turns

ratio, from one winding to another. However, because of

their internal parasitic elements discussed previously,

transformers tend to produce optimum results when

used within a specified range of external impedances.

Figure 11-24. Insertion loss compares the outputs of real and ideal transformers.

× ×

RG

Ei Np NS EO

XFMR

IDEAL

RL

RG

Np

NS

NS

Np

Ei EO RL

2

NS

IdealEO = Ei Np

RG

RL

NS

Np RL

2

Insertion loss (dB) = 20 log

RG

Ei Np NS EO

XFMR

IDEAL

RL

RP RS

RP

NS

Np

2

RG+

RS

NS

Ei Np

EO RL

RP

NS

Np

2

RG+

NS RL

Np

Real EO= Ei ×

+ RS + RL

× ×

× ×

× +

Real EO

Ideal EO

×

×