Handbook for Sound Engineers

288 Chapter 11

There is essentially no intrinsic impedance associ-
ated with the transformer itself. With no load on its
secondary, the primary of a transformer is just an
inductor and its impedance will vary linearly with
frequency. For example, a 5 H primary winding would
have an input impedance of about 3 kȍ at 100 Hz,
30 kȍ at 1 kHz, and 300 kȍ at 10 kHz. In a proper
transformer design, this self-impedance, as well as those
of other internal parasitics, should have negligible
effects on normal circuit operation. The following appli-
cations will illustrate the point.
A 1:1 output transformer application is shown in Fig.
11-25. It has a winding inductance of about 25 H and
negligible leakage inductance. The open circuit imped-
ance, at 1 kHz, of either winding is about 150 kȍ. Since
the dc resistance is about 40ȍ per winding, if the
primary is short circuited, the secondary impedance will
drop to 80ȍ. If we place the transformer between a
zero-impedance amplifier (more on that later) and a
load, the amplifier will see the load through the trans-
former and the load will see the amplifier through the
transformer. In our example, the amplifier would look
like 80ȍ to the output line/load and the 600ȍ line/load
would look like 680ȍ to the amplifier. If the load were
20 kȍ, it would look like slightly less than 20 kȍ
because the open circuit transformer impedance,
150 kȍ at 1 kHz, is effectively in parallel with it. For
most applications, these effects are trivial.

A 4:1 input transformer example is shown in Fig.
11-26. It has a primary inductance of about 300 H and
negligible winding capacitance. The open circuit imped-
ance, at 1 kHz, of the primary is about 2 Mȍ. Because
this transformer has a 4:1 turns ratio and, therefore a

16:1 impedance ratio, the secondary open circuit imped-

ance is about 125 kȍ. The dc resistances are about

2.5 kȍ for the primary and 92ȍ for the secondary.

Since this is an input transformer, it must be used with

the specified secondary load resistance of 2.43 kȍ for

proper damping (flat frequency response). This load on

the secondary will be transformed by the turns ratio to

look like about 42 kȍ at the primary. To minimize the

noise contribution of the amplifier stage, we need to

know what the transformer secondary looks like, imped-

ancewise, to the amplifier. If we assume that the

primary is driven from the line in our previous output

transformer example with its 80ȍ source impedance,

we can calculate that the secondary will look like about

225 ȍ to the amplifier input. Actually, any source

impedance less than 1 kȍ would have little effect on the

impedance seen at the secondary.

Transformers are not intelligent—they can’t magi-

cally couple signals in one direction only. Magnetic

coupling is truly bi-directional. For example, Fig. 11-27

shows a three-winding 1:1:1 transformer connected to

drive two 600ȍ loads. The driver sees the loads in

parallel or, neglecting winding resistances, 300ȍ. Like-

wise, a short on either output will be reflected to the

driver as a short. Of course, turns ratios and winding

resistances must be taken into account to calculate

actual driver loading. For the same reason, stereo L and

R outputs that drive two windings on the same trans-

former are effectively driving each other, possibly

causing distortion or damage.

11.1.3.6 Transformer Noise Figure

Although the step-up turns ratio of a transformer may

provide noise-free voltage gain, some 20 dB for a 1:10

turns ratio, it’s important to understand that improve-

Figure 11-25. Impedance reflection in a 1:1 transformer.

Brn Org

150 k 7 150 k 7

Red Yel

Open circuit

Brn

80 7

Red Yel

Short circuit

Brn Org

18 k (^780 7) Line 20 k 7
Red Yel
Brn Org
680 7 80 7 600 7
Red Yel
Line driver applications
Line
Org
Figure 11-26. Impedance reflection in a 4:1 transformer.
Yel
Red
2 M (^7) Brn
Org
Whi Blk
125 k 7
225 7
80 7 42 k 7
Org 2,430
Line receiver application.
Line
Yel
Red
Brn
Whi Blk
"Open" circuit.
7