Handbook for Sound Engineers

868 Chapter 25

and excellent isolation properties are really needed. And
then only really good ones. Most high-level interfacing
applications within the confines of a studio environment
justify neither their capabilities nor drawbacks.
The search was on for electronic equivalents to
transformers for both input and output applications, a
moderate degree of success being achieved early on for
input stages with classic circuits such as Figs. 25-47 and
25-48. These are simple differential input amplifier and
instrument amplifier configurations using op-amps.
Line inputs are commonly differential amplifiers,
similar to the one used in the transformerless mic-amp,
but with the resistor values elevated to bring the differ-
ential input impedance up to over the 10 k: required of
a bridging termination, Fig. 25-47. The noise of these
stages is directly attributable to these resistor values, so
the lower resistor values are better. An instrumentation
amplifier configuration would seem to offer possibly
better performance for noise (the differential amplifier
resistor values may be kept small) but it entails the use
of undesirable voltage followers (see Section 25.7) with
potential stability problems, input voltage swing limita-
tions, and unprotected (for RF) input stages. At least
with a simple differential amplifier the impedances are
comfortably low and the inputs buffered by resistors
from the outside world.
The dc-blocking series capacitors must, unfortu-
nately, be large in value to maintain an even input
impedance and sensibly flat phase response at the
lowest used frequencies. Also, being necessarily unpo-
larized, they are physically large and expensive. This is
a small price to pay, though, for such a simple but
important circuit element.
The instrumentation amplifier presents very high,
nonground-referred differential and common-mode

terminations and has the great advantage that gain may

be easily invoked between the two input amps at no cost

to the excellent common-mode rejection, Fig. 25-48.

Integrated line receivers are typically of this

configuration.

Figure 25-47. Electronic differential input amplifier.

68 pF

5.6 k 7

150 7

5.6 k 7

100 k 7

5.6 k 7

5.6 k 7

10 MF

10 MF

68 pF

TL071

A. Classic, equal value.

33 pF

2.2 MF

2.2 MF

100 pF 10 k 7

10 k 7

30 k 7

180 k 7

150 k 7

TL071

B. Optimized for matched dynamic amplifier.

30 k 7

Figure 25-48. Instrumentation amplifier-type line-input stage.

1/4 TL074

Adjust to suit

required input

impedance

(eg 3.3 k 7 for 10 k 7 input 2) 1/4 TL074

1/4 TL074

6.8 k 7

6.8 k 7

150 7

220 pF

6.8 k 7

220 pF

6.8 k 7

220 pF

6.8 k 7

220 pF

6.8 k 7

6.8 k 7