Handbook for Sound Engineers

Audio Transformers 301

than TX is not suppressed and is heard in RX. A
common problem with hybrids of any kind is adjusting
network ZN to match the telephone line, which may vary
considerably in impedance even over relatively short
time spans.

If the transmitter and receiver are electrically
connected, the single transformer method, shown in Fig.
11-48, can be used. Any well-designed transformers with
accurate turns ratio can be used in hybrid applications.

11.2.2.8 Moving-Coil Phono Step-Up

Outboard boxes are sometimes used to adapt the output
of low-output, low-impedance moving-coil phono pick-
ups to pre-amplifier inputs intended for conventional
high-impedance moving-magnet pickups. These
pre-amplifiers have a standard input impedance of
47 kȍ. Fig. 11-49 shows a 1:37 step-up transformer

used for this purpose. It has a voltage gain of 31 dB and

reflects its 47 kȍ pre-amplifier load to the pickup as

about 35ȍ. This keeps loading loss on a 3ȍ pickup to

about 1 dB. The series RC network on the secondary

provides proper damping for smooth frequency

response. Double magnetic shield cans are used because

of the very low signal levels involved and the low-fre-

quency gain inherent in the RIAA playback equaliza-

tion. In these applications, it is extremely important to

keep all leads to the pickup tightly twisted to avoid hum

from ambient magnetic fields.

Figure 11-46. Repeat coil ground isolation for 600: lines.

T 1

JT–11SSP–6M

Red Gry

R 2

3.9 k 7

Brn

Yel

Org Grn

Blu

Vio R 3

619 7

3 2

1

P 1

Line

Blk Whi output

Gnd

lift

10 nF 1 k 7

J 1

Line

input

R 1

619 7

2 3

1

C 1

1300 pF

Figure 11-47. Two-transformer hybrid.

2-Wire

line

ZL

TX RX

ZN

Figure 11-48. Single-transformer hybrid.

Figure 11-49. Step-up transformer for moving coil phono

pickup.

1.4 : 2CT

2-Wire 600 7

line

600 7

600 7

TX

RX

600 7

T 1

7–7 JT-34K^ DX

M/C

pickup Red

Whi WhiBlk

Org100 pF

13 k 7

Yel 47 k 7

Mag

phono

input

Brn