Handbook for Sound Engineers

850 Chapter 25

25.8.4.5 Reactive Ground Effects

Noise generation due to grounds is not limited to the
resistance predominant in the ground wiring at audio
frequencies. At radio frequencies (well within the band-
widths of modern op-amps) even fairly short ground
wires and buses can have very significant reactances,
dramatically raising the effective ground impedance.
This not so much reduces the isolation between the
various stages as directly couples them together. All the
inherent RF noise and instabilities of the stages become
intermodulated (by the nonlinearity of the device at
those frequencies) to make their presence felt as yet
more audible and measurable noise.

A good “shock horror” extreme example, though
described in simplistic theoretical terms, manifests itself
sometimes dramatically in practice and can be called the
standing on one leg effect.

The box in Fig. 25-29 represents a device that relies
on a wire to be connected to the ground mass. It looks
all right, and so it is, apart from the fact that at certain
radio frequencies the wire is electrically ¼ wavelength
or an odd multiple of ¼ wavelength. In accordance with
transmission-line theory our innocuous bit of wire turns
into a tuned line transforming the zero impedance of the
ground to an infinite impedance at the other end. The

result is that the device is totally decoupled from ground

at those frequencies. Practical consequences of this, of

course, vary, from instability at very high frequencies

on cards with long supply and ground leads to painful,

unreasonable susceptibility to RF in otherwise whole-

some items of equipment.

25.9 Signal Switching and Routing

Signal routing within the channel and other areas of the

system is a touchy affair that has always been an area of

much discontent for console designers, especially since

the advent of in-line consoles and remotable and assign-

able systems. There are always standard relays, but

these have lost, justifiably, a lot of appeal in the light of

ac technologies.

25.9.1 Relays

Unless they are of the expensive miniature IC package

variety, relays tend to be big, heavy, eventually unreli-

able, mechanically noisy, and a nuisance to implement

electronically. They also demand support circuitry such

as back-emf protection diodes and drive transistors for a

realistically operable system. The coils, being inductive

in nature, draw a surprisingly large instantaneous on

current and release an equally surprisingly large amount

of back-emf energy when deactivated. Both of

these—through mutual-inductance coupling, dubious

common ground paths (even as far back as the master

ground termination in separated supply systems), soft

power supplies, and even mechanical microphonic

effects—tend to impinge themselves on audio signal

paths as clicks, splats, pings, and other assorted bumps.

Of course, it’s possible to have silent relay switching.

However, after designing in separate ground unrelated

power supplies of considerable heft, spatially separating

the relays from the audio (preferably on another card),

working out the drive interfaces, and liberally sprinkling

the whole issue with diodes, resistors, and capacitors to

Figure 25-28. Virtual-earth mix amplifier as amplifier of
ground-borne noise.

R 1

R 32

RF

Ground

Conventional

virtual earth

amplifier

A. Conventional mix-amp.

30 dB

gain

Ground

RF

RS=RF/32) (all source resistors

effectively paralleled)

B. Circuit redrawn as a noninverting amplifier with

ground as signal (noise) source.

Figure 25-29. Standing-on-one-leg effect.

Device

Ground

wire 1/4L

Ground

Length

Impedance