Handbook for Sound Engineers

Audio Transformers 293

symmetry will be determined largely by the balance of
line and load impedances.

11.2.1.5 Inter-Stage and Power Output

Inter-stage coupling transformers are seldom seen in
contemporary equipment but were once quite popular in
vacuum-tube amplifier designs. They typically use turns
ratios in the 1:1 to 1:3 range and, as shown in Fig.
11-35, may use a center-tapped secondary producing
phase-inverted signals to drive a push-pull output stage.
Because both plate and grid circuits are relatively high

impedance, windings are sometimes section-wound to

reduce capacitances. Resistive loading of the secondary

is usually necessary both to provide damping and to

present a uniform load impedance to the driving stage.

Although uncommon, inter-stage transformers for

solid-state circuitry are most often bi-filar wound units

similar to line output designs.

The classic push-pull power output stage, with many

variations over the years, has been used in hi-fi gear, PA

systems, and guitar amplifiers. The turns ratio of the

output transformer is generally chosen for a reflected

load at the tubes of several thousand ohms

plate-to-plate. A typical 30:1 turns ratio may require

Figure 11-32. Typical line output application circuit.

Figure 11-33. Universal isolated output application.

2 k 7

1 k 7

R 1

100 nF

2.2 M 7

30 k 7

4.64 k (^7) 330 pF
499 7
IN
RS < 100 7
R (^2) C
1
R 6
R 4
A 3
OP97
+^3
C 2
R 3
2
3
A 1
LME49710
6
6
+1
R 5
X 2
Brn
T 1
JT
11
EM
D 1
D 2
18 V
Red Yel
(^32) P
1
1
Line
output
Org
A 2
LME49600

• 4
  +18 V
  X 1
  2
  2
  X 1 and X– are JT-OLI-3 or equivalent 2
  3
  2
  IN + A^1
  R 2
  2 k 7
  R 1
  499 7
  C 1
  470 pF
  18 V
  D 2
  D 1
  X 1
  X 2
  3 4
  T 1
  JT–11–HPMC
  1 2
  3 2
  J 1 Unbalanced
  output
  P 1 Balanced
  (^1) output
  +18 V
  1