Handbook for Sound Engineers

Consoles 847

A system composed of many separately powered
units will almost certainly hum, buzz, and sound gener-
ally uneasy if not earthed, which is seemingly in direct
contradiction to the earlier statement that “the system
will operate perfectly regardless of what potential it is
tied to.” Being tied to a lot of different self-generated
potentials at a lot of different points along a system path
is definitely not in the recipe.

Each different power transformer will have different
amounts and permutations of leakage and, hence, propa-
gate different potentials and degrees of
power-line-borne noise into our otherwise perfect
grounding path. Assorted ground potentials mean
assorted ground currents, meaning assorted noises.

Tying the entire grounding path to earth is the best
shot at “swampout” of leakage impedances. A connec-
tion to a (nearly) zero impedance makes nonsense of
most other potential-creating paths, most of which have
reactances in the kilohms.

Regardless of earth termination in such a multisupply
circumstance, significant currents exist along the ground
reference lines. The resultant interelement noise and
hum voltages (developed across the inevitable line resis-
tances) quickly become intolerable in unbalanced
systems. Any wobbling of the ground reference
becomes directly imposed upon the desired signal.

Balanced, or pure differential, transmission helps to
obviate these perturbances by rendering them common
mode in a system that is (theoretically) only sensitive to
differential information. In reality, practical trans-
formers can afford a good 70–80 dB common-mode
isolation at low audio frequencies. They deteriorate in
this respect at 6 dB/octave with increasing frequency up
to the winding resonance frequencies unless consider-
able effort is made to fake a more accurate balance
externally. Although transformer balancing does effect a
dramatic improvement in noise levels, it is far greater
for fundamental hum (50–60 Hz) than it is for other
power-line-borne noise. This explains why in tricky
systems, lighting dimmer buzz, motor spike noise, or
any source with a high-frequency energy or transient
content is so persistent.

The golden rule is to treat the grounding of any
balanced system as if it were unbalanced. This mini-
mizes the inevitable reference ground currents.

There is one overarching good reason only glanced
by earlier for grounding to earth. The consequences of a
piece of the gears’ chassis becoming inadvertently at the
power-line potential are obvious. We would much rather
see death to a fuse or breaker than to one of us.

25.8.4 Console Internal Grounding

Let us assume that the grounding for the studio control

room is all sensible and that our console has a solid

earth termination. What about the intraconsole

grounding paths? For most console builders this is

perhaps the ultimate unbalanced signal path.

Conventional amplifier stages rely on a voltage

difference between their input and reference in order to

produce a corresponding output voltage (referred, natu-

rally, to the reference of the input). If the input is held

steady while the reference is wobbled, a corresponding

(amplified) inverted wobble will appear at the output.

It is plain that any signal the reference sees that is

not also common to the input (e.g., ground noise) will

get amplified and summed into the output just as effec-

tively as if it were applied to the proper input. The

obvious (and startlingly often overlooked) regimen to

render extraneous noise unimportant is to ensure that

the point at which an amplifier source is referred is tied

directly to the reference, while that amplifier output is

only taken in conjunction with the reference. Successive

stages daisy chain similarly—source reference to desti-

nation reference, and so on. This philosophy is called

ground follows signal.

25.8.4.1 Ground Follows Signal

“Ground follows signal” is a classic maxim and one that

has dictated the system design of nearly every console

built. It was particularly true in the era of discrete semi-

conductor design, where ground was often not only

audio ground but also the 0 V power-supply return;

ideally the audio and supply grounds should be sepa-

rate. As an added complication, the power-supply posi-

tive lines, being heavily regulated and coupled to

ground, were an equal nightmare as they too became

part of the grounding path. This could be fairly simply

avoided by spacing each circuit element away from the

supply line by an impedance considerably greater than

that offered by the proper ground path—achieved by

either separately regulating or simply decoupling by a

series resistor, parallel capacitor network, as shown in

Fig. 25-25. This actually gives the lie to the notion that

single-rail supply systems are easier than differential

rail arrangements; to do them properly results in almost

indistinguishable numbers of parts and degrees of effort.

Accelerating technology has for once actually made

life a bit simpler—specifically, the trend toward IC

op-amps with their required differential (+Ve and Ve)

power supply. This, thankfully, removes electronic

operating current from the audio system ground, while