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Regarding Input Impedance and Terminations

There is sometimes a misunderstanding regarding the

nature of matching or bridging inputs, the use of termi-
nating resistors, and the relationship between actual
input impedance and nominal source impedance. Most
electronic outputs work well when "terminated" by an
input (connected to an input) having the same or a
higher actual impedance. Outputs are usually overloaded

when terminated by an impedance that is lower than the
source impedance. When the actual input impedance of
the following device is nearly the same impedance as the
source, it is known as a "matching" input. When the
input of the following device is ten times the source

impedance, or more, the input is considered to be a
"bridging" input. There is hardly any loss of signal
level when an input bridges the source device, but a

matching input may cause a loss of 3 to 6dB in level.
Such losses, however, are normal and usually present
no problem.

It seldom is necessary to place a 600 ohm "termin-

ating resistor" across any high impedance input (the

P-2200's input can be considered to be high impedance).
In fact, most 600-ohm outputs operate normally when
bridged by a high impedance; it is as though no load

were connected to the source device.

The only instance where a terminating resistor may

be required is when the manufacturer of the source

device specifically states that a terminating resistor is
necessary. In such cases, there is usually a special type
of output transformer in the source device, or the device

is constructed primarily of precision, passive com-
ponents (no transistors or tubes), such as a passive

equalizer. In these cases, the terminating resistor assures
optimum frequency response in that device. Input
terminating resistors are not needed for the P-2200 to
operate correctly. If a 150 ohm or 600 ohm resistor is

specified for the source device, it should be installed at
the end of the cable nearest the P-2200 in order to

minimize possible hum, noise or signal losses in the cable.

Fig. 36A - The Actual Voltage reaching the Load Device

is given by the Formula: (also see Appendix)

Fig. 36B - Where to Insert a Termination Resistor when one

is required.

CABLING AND IMPEDANCE MATCHING
Attenuation Pads

A "pad" is a resistive network that lowers the level in

an audio circuit. The most common professionally used
pads are "T-pads" and "H-pads." T-pads unbalance true
balanced lines (and floating lines), but work well in
unbalanced circuits. H-pads are best for balanced or
floating lines, but should not be used in an unbalanced
circuit since they will insert a resistance in the return
lead (ground). For a discussion of other types of pads,
refer to the AUDIO CYCLOPEDIA by Howard M.
Tremain (Pub. Howard W. Sams).

Fig. 37 - Where to Install a Pad when one is required.

Always install a T-pad near the input of the device it

feeds, with as short a length of cable as possible on the
low level side of the pad. This maintains a high signal
level in the longer transmission cable, minimizing any

induced hum and noise.

The low impedance pad values illustrated in Figure 38

are designed for 600-ohm lines. Commercially manu-
factured pads are available; consult your Yamaha dealer.
When connected between a 600-ohm or lower source
and a 600-ohm or higher termination, pad attenuation
values will remain fairly accurate. For higher impedance
circuits, resistor values must be changed. A 600-ohm pad

inserted in a high impedance circuit may overload the

device feeding the pad (the source device). Multiply the
given values by the output impedance of the source
device, and divide that answer by 600 to achieve the
desired value. The high impedance values listed for the
T-pads in Figure 38 are close approximations of average
hi-fi pads, based on 10,000-ohm nominal impedances.

For low level circuits, use 1/4 watt resistors. For out-

puts with continuous sine wave levels above +24dBm,
use 1/2 watt resistors; for continuous sine wave levels

dB Loss R1 T (ohms) R1 H (ohms) R2

0.5

1.0
2.0
3.0

4.0

5.0
6.0
7.0
8.0
9.0

10
12

14
16
18
20
22
24
26
28
30

32
34
36
38
40

300

560
1 1 0 0
1 7 1 0
2200