Handbook for Sound Engineers

Consoles 869

A pair of inverting amplifiers, shown in Fig. 25-49,
provides a simple, hardy, easily defined differential (but
not true floating balanced) input stage. A fascinating
circuit known as the Superbal input is depicted in
Fig. -50; this is a balanced differential virtual-earth
amplifier, referred to ground solely by one op-amp input
and capable of very good common-mode rejection,
limited by the tolerance of the components from which
it is constructed. Accepting any lopsided input signal, it
delivers a differential output perfectly symmetrical to
ground, making it an exceptionally useful input condi-
tioning amplifier.

The capacity of both these circuits to be differential
virtual-earth points makes them ideal for use in
balanced mixing bus systems.

25.10.6.10 Electronic Balanced Outputs

The simplest balanced outputs configuration is given in
Fig. 25-51. This is a pure, no-nonsense, inverter-derived

differential feed. For many internal interconnections

and especially in differential balanced mixing systems it

works well, but it should not be used to connect to the

outside world.

Ideally, there must be no discernible difference in

characteristics between the output circuit and an ideal

transformer. After all, the fate of signals in the real world

on a balanced transmission line won’t alter in your favor

simply because you’ve chosen not to use a transformer.

If transformers are being supplanted it had better be with

devices capable of affording similar benefits to the

system and its signals. Regardless of applied reverse

common- mode potential, the differential output poten-

tial must not change. Also the output should be insensi-

tive to any imbalance in termination, even to the extent

of shorting one of the legs to ground. This is the floating

test. For example, the simple inverter circuit of Fig.

25-51 fails the floating test since, if one leg is shorted to

common, the overall output has to drop by one-half

(6 dB). (The question of what happens to ground noise

with a shorted amplifier bucketing current into it will be

sidestepped here.) Two basic circuits have emerged as

being close approximations to a transformer. Not only

are they fairly closely related, but most balanced output

topologies are also derived from them. They both

depend on cross-coupled positive feedback between the

two legs to compensate for termination imbalance.

In Fig. 25-52 a unity-gain inverting stage provides

out-of-phase drive for the two legs, each output leg of

which is a 6 dB gain inverting amplifier with error

sensing applied to its reference (positive) inputs. Under

normal operation, there is no error-sensing voltage; the

two inverse outputs cancel at the midpoint of the equal

sense resistors. The two amps invert a differential

Figure 25-49. Differential mix/input amplifier.

Figure 25-50. Superbal differential mix/input amplifier.

1/2 5532

Output

1/2 5532

* Increasing these

values to 6.8 k 7

creates a unity gain

diff-input stage

6.8 k 7

6.8 k 7

6.8 k 7

150 7

100 pF

* 150 7 200 pF

* 150 7

+

33 pF

6.8 k 7

6.8 k 7

150 7

150 7

150 7

6.8 k 7

100 pF 6.8 k 7

1/2 5532

+

1/2 5532

100 pF

Output

Figure 25-51. Inverter-type differential output.

6.8 k 7

100 pF

6.8 k 7

300 7

300 7

100 pF

1/2 5532

6.8 k 7

6.8 k 7

1/2 5532

Output