Handbook for Sound Engineers

360 Chapter 12

provide feedback into the Cin+ input. A current feedback
bridge circuit allows the 1646 to drive one output
shorted to ground to allow a single-ended load to be
connected. The output short increases gain by 6 dB,
similarly to conventional cross-coupled topologies.
However, it does so without loss of the common-mode
feedback loop. The resulting current feedback prevents
large, clipped signal currents flowing into ground. This
reduces the crosstalk and distortion produced by these
currents.

A typical application circuit for the THAT1646 is
shown in Fig. 12-73.

To reduce the amount of common-mode dc offset,
the circuit in Fig. 12-74 is recommended. Capacitors C 1
and C 2 , outside the primary signal path, minimize
common-mode dc gain, which reduces common-mode
output offset voltage and the effect of OutSmarts at low
frequencies. Similar capacitors are used in the ADI and
TI parts to the same effect, although OutSmarts’ current
feedback does not apply.

THAT’s 1606 version of OutSmarts provides a

differential input for easier connection to a

digital-to-analog converter’s output. A typical applica-

tion of the THAT1606 is shown in Fig. 12-75. Another

advantage to the 1606 is that it requires only a single

low-value capacitor (typically a film type) versus the

two larger capacitors required by the THAT1646,

SSM2142, or DRV134.

Active balanced line drivers and receivers offer

numerous advantages over transformers providing

lower cost, weight, and distortion, along with greater

bandwidth and freedom from magnetic pickup. When

used properly, active devices perform as well as, and in

many ways better than, the transformers they replace.

With careful selection of modernIC building blocks

from several IC makers, excellent performance is easy

to achieve.

12.3.7 Digital Integrated Circuits

Digital ICs produce an output of either 0 or 1. With

digital circuits, when the input reaches a preset level,

the output switches polarity. This makes digital circuitry

relatively immune to noise.

Bipolar technology is characterized by very fast

propagation time and high power consumption, while

MOS technology has relatively slow propagation times,

low power consumption, and high circuit density. Fig.

12-76 shows typical circuits and characteristics of the

major bipolar logic families.

Table 12-4 gives some of the terminology common

to digital circuitry and digital ICs.

Figure 12-72. THAT 1646 block diagram. Courtesy THAT
Corporation.

Figure 12-73. THAT 1646 application. Courtesy THAT
Corporation.

Din+

Cin+

Cin–

Din–

Out–

Sns–

Out+

Sns+

Vcc

In+

Vee

Gnd

Dout-

Dout+

10k

10k

25

5k

5k

10k

10k

10k

10k

20k

20k

10p

25

THAT 1646

CExt

CExt

AAD & C

(^4) In
(^3) Gnd Out–
1
5 2
Out+
8
(^67)
U1
THAT1646
In
Vcc
Vee
2
3
C 4
100n
C 5
100n
Sns+
Sns-
1
XLR (M)
Vee
Vcc
Figure 12-74. THAT 1646 CMR offset reduction circuit.
Courtesy THAT Corporation.
(^4) In
(^3) Gnd Out–
1
Out+
8
(^67)
5 2
U1
THAT1646
In
Vcc
C 4
100n
C 5
100n
Sns+
Sns-
C 1
10u
NP
C 2
10u
NP
2
3
1
XLR (M)
Vcc
Vee
Vee