Handbook for Sound Engineers

848 Chapter 25

individual stage supply decoupling is rendered a nicety
rather than a necessity by the excellent power supply
noise rejection ratio of most popular op-amps.

Nevertheless, correct grounding paths still apply; the
removal of supply current just exposes and highlights
audio ground subtleties.
Unfortunately, although op-amps have simplified
matters in one respect, their ease of use and versatility
have been largely responsible for the creation of enor-
mous systems with so many stages, break points, mix
buses, and distribution networks that the simple daisy
chaining of ground follows signal becomes unwieldy, if
not unworkable. Alternate grounding schemes, such as
star grounding where every ground path and reference is
taken to a central ground or earth, play increasingly
important roles.

In practice, a necessary compromise between these
two prime systems occurs in most console thinking.
Daisy chain applies mostly to on card electronics (e.g.,
in the microphone amplifier sections), while systems
switching and routing rely on star connections.

25.8.4.2 Ground Current Summing

A principal grounding-related manifestation is cross-
talk, or the appearance in a signal path of things that
belong elsewhere. Other than airborne proximity-related
reactive crosstalk, the most unwanted visitations are by
the common-impedance or resistive ground path mecha-
nism. In Fig. 25-26A, R 1 represents the load of an
amplifier output (whether it’s the 10 k: of a fader or a
600 : line termination is immaterial for the present).
The resistor RG represents a small amount of ground
path wiring, loss resistance, and so on. It is quite
apparent that the bottom end of the termination is

spaced a little way from reference ground by the wiring

resistance, and the combination forms a classic potenti-

ometer network. The fake ground has a signal voltage

present of the amplifier output voltage attenuated by R 1

into RG..

Practically, with a 600: termination (R 1 ) and a

ground loss (RG) of 0.6:, the fake ground will have a

signal voltage some 60 B down. The use of the fake

ground as a reference for any other circuitry is a surefire

guarantee of injecting 60 dB worth of crosstalk into it.

Two identical terminations sharing the same fake

ground, Fig. 25-26B, happily inject a small proportion

into each other by generating a common potential across

the ground loss RG.

Should the second termination be far higher in

impedance (the 10 k: of a fader), its contribution to the

common fake ground potential will be far less (86 dB)

since the ground impedance is much smaller in relation

to the source. Correspondingly, though, this higher

impedance termination is more prone to be crosstalked

into from the lower impedance contributors to the

common ground.

25.8.4.3 Typical Grounding Problems

Here is a fairly unusual (but definitely not unknown)

grounding anomaly resulting from inattention to the

grounding paths. In Fig. 25-27 A2 is a line amp feeding

a termination of 600: into a lossy ground of 0.6:

Figure 25-25. Power-supply decoupling: A typical discrete
amplifier with the power supply isolated from audio ground.

Power supply

Decoupling

Approx 30 dB gain Supply ground

BC

258B

Vin

BC

169C

Audio

ground

Vo

220 MF

10 MF

220 nF

100 pF

100 MF

150 7

2.7 k 7

270 k 7

270 k 7 33 k^7

Figure 25-26. Ground current summing.

Identical

sources

R 1

RG

R 1

RG

B. Two terminals sharing the same ground.

Reference ground

Fake ground

(common ground

potential)

Reference ground

A. Load of an amplifier output.

Amp

termination

Ground loss

resistance

Fake ground

R 2