Handbook for Sound Engineers

1300 Chapter 34

If the source device has a lower output level than the
load device, a line-level preamplifier could be
connected between them to give the required amount of
gain. In many cases, however, simply connecting these
two devices will prove satisfactory. The worst that can
happen here is additional hiss, which may be tolerable.

34.5.1.7.9 The dBV Specification

All professional equipment uses a dBu or dBm refer-
ence for any “+4” input or output. Some professional
microphones and consumer type equipment, however,
may have outputs that are rated in dB using a dBV ref-
erence. When in doubt about the dB reference, consult
the owner’s manual for the equipment or, measure the
output with a known signal.

The level in dBV is found from

(34-35)

where,

dBV is the voltage level in dBV,

V is the voltage level in volts,

1 is the reference level for dBV in volts.

34.5.1.7.10 Impedance and Power Transfer

To understand what happens to the power output of an

amplifier when different impedances are connected to it,

find the rated power output of the amplifier and its rated

load impedance. That rated load impedance is usually

the minimum acceptable load impedance of the

amplifier.

In addition, find the true minimum impedance of the

loudspeaker as well as its rated or nominal impedance.

Normally, the nominal impedance of the loudspeaker

will be used to make impedance-matching calculations

like those described in the next paragraph. The

minimum impedance of a loudspeaker, however, can

fall significantly below its nominal impedance, and a

loudspeaker with an extremely low minimum imped-

ance could even overload a power amplifier whose rated

load impedance was acceptable for rated nominal

impedance of the loudspeaker.

Some professional power amplifiers are designed to

accept impedances as low as 2: because of the very

low minimum impedance of some loudspeakers. Many

8 : loudspeakers (8: is the rated or nominal imped-

ance), for example, have minimum impedances of 6:

or even as low as 5:. Two of these loudspeakers in

parallel would have a minimum impedance of 2.5: ,

Figure 34-63. Typical sound reinforcement system gain/level chart. Courtesy Fender Musical Instruments.

Lo-Z in

Hi-Z in

14 dB

PAD

Insertion

Trim

0 dB

to

+40 dB

(gain)

Lo

Hi

Mid

10dB

at "0"

position

+10dB 10dB

at

"0"

position

+24 dB +27 dB200W

47

Input

Fader Pan LineOut G-EQ

In

Program

fader

Pre

amp

out

Pre

amp

in

Power

amp

out

1 Hi-Z in clip line

2 Lo-Z in clip line

3 Hi-Z in nominal line (trim at min)

4 Lo-Z in nominal line (trim at min)

5 Hi-Z in nominal line (trim at max)

6 Lo-Z in nominal line (trim at max)

7 Lo-Z in max gain line (trim & all faders at max)

8 Hi-Z in noise floor line

9 Lo-Z in noise floor line

Note 1: +4 dB at the power amp in jack produces 200 W/4 7 at the power amp out jack.

Note 2: Signal-to-noise ratio of lo-Z in is 74 dB; of hi-Z in is 66 dB.

Note 3: 0-dB reference is 0.775 V.

dBV 20 V

1

= log---