Handbook for Sound Engineers

Sound System Design 1299

The first rule of level compatibility is to avoid
connecting devices from different classifications unless
they are specifically designed for each other.
For example, don’t connect a microphone directly to
a power amplifier because the output of the micro-
phone is too low. This connection wouldn’t damage
anything but would result in very low sound level, and
the noise from the power amplifier might be almost as
high as the wanted sound.
As an obvious example, don’t connect the output
from a power amplifier to the input of a mixer. The
power amplifier output level is far too high for the input
of the mixer. This connection would almost certainly
result in severe clipping distortion (the mixer might
even be damaged).
Many devices, however, have an input that is
compatible with one level and an output that is compat-
ible with the next higher level. For example, the input
channels of most professional mixers are compatible
with low-level devices like microphones, although their
outputs are designed for both mic-level and low- and
high-level line loads. A power amplifier, as another
example, has a line-level input and a loudspeaker-level
output. Thus, the output of a limiter or graphic equalizer
can usually be connected directly to the input of a
professional power amplifier. (Impedance must be
considered, too, but the input impedance of most profes-
sional power amplifiers is high enough to be imped-
ance compatible with the output of almost any
professional line-level device.)

The situation is complicated somewhat by variations

in the level of devices in a given category. For example,

a typical condenser microphone has a higher output than

a typical dynamic microphone. One solution to this

problem is to design the mixer for the lower-level

microphone and provide a pad for the higher-level

microphone. A more common solution on professional

mixers is to include either a built-in (adjustable) pad or

a preamplifier gain adjustment or both. By properly

adjusting these controls, the mixer’s input channel can

be optimized for either a dynamic or condenser micro-

phone (and, with some mixers, for a line-level input).

The same kind of level-compatibility problems show

up in line-level devices. Some line-level devices, mostly

special effects devices, are designed for input and

output voltages as low as 20 dBu. Others, including

some tape machines, are designed for input and output

voltages of 10 dBu. Most professional line-level

equipment, however, is designed for input and output

voltages of +4 dBu.

The level in dBu is found from

(34-34)

where,

dBu is the voltage level in dBu,

V is the voltage level in volts,

0.7745 is the reference level for dBu in volts.

dBu specifications are only used for voltage output

ratings and are not the same as dBm ratings. Many

manufacturers are now rating the input and output levels

of their products in dBu, however, so it is useful to

understand this specification.

The process of achieving compatibility with these

line-level devices is similar to the process for low-level

devices (e.g., the microphones discussed earlier).

Whenever possible, connect the output of a 20 dBu

device to the input of another 20 dBu device (the same

applies to 10 dBu devices and +4 dBu devices).

If this isn’t possible and the source device has a

higher output level than the load device, use a pad to

attenuate the level of the source device. For example, if

the source is a +4 dBu limiter, and the load is the input to

a 10 dBu tape machine, a 14 dB pad is needed to

achieve level compatibility. To design a proper pad,

impedances must be taken into account (see Chapter 23).

Without the pad, there is a risk of clipping distortion. Just

turning down the output of the source device probably

won’t solve the problem, either. This may result in that

other level-compatibility problem—electronic hiss noise.

Figure 34-62. Level watching. Courtesy Fender Musical

Instruments.

B. If the signal level is too low,

it may be "buried" in the noise.

A. If the signal level is too high,

clipping distortion may occur.

Signal Noise

Noise

Clipped

signal

dBu 20 V

0.7745

= log----------------