248 Chapter 8
is accidentally connected to another’s input. Added protection would add complexity,
increase the cost, and likely degrade sonics.
8.1.2.8 In Tandem
In professional (and even a few domestic) applications it is normal for each signal source
to drive more than one amplifi er input. The loading of amplifi ers driven in tandem is
cumulative: each added amplifi er reduces the impedance (or increases the loading) pro-
rata in accordance with its impedance. Assuming conventional power amplifi ers with
10 kf2 input impedance, the reciprocal pattern is shown in Table 8.4.
Note that there are very few types and models of the likely sources (e.g., active
crossovers, delay lines, preamps) that are rated and able to drive impedances of below
600 ohms without degraded performance. Much pro-gear is rated and even specifi ed for
600 ohms, but still gives its best measured and sonic performance into 2 k or even higher.
For large tandem systems, existing equipment usually has to be retro-fi tted with special
line-driver amplifi ers, or these are added as independent units, in line. Line drivers used
in live sound practice do not expand the allowable loading by much, usually down to
300 ohms and possibly as low as 75 ohms. To be sure, only 50% of this rating would
be used. The rest allows for tolerances, variables (see later), add ons, and the cable’s
capacitance loading at hf. In a major concert where 100 or more power amps have been
Table 8.4 : The reciprocal pattern of conventional
power amplifl iers (with 10 k Ω input impedance)No. of amps in tandem Total Z (^) in
l 10 kΩ
2 5 kΩ
3 3.3 kΩ
4 2.5 kΩ
5 2 kΩ
6 1.7 kΩ
l0 1 kΩ
(^15) 666 kΩ
20 500 kΩ