Yamaha P-2200/2201 Product Manual page 23

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)
0.5 300 16
1.0 560 33
2.0 1 1 0 0 68
100
3.0 1 7 1 0
2200 130
4.0
2700
5.0 160
3300 200
6.0
3900
7.0 220
8.0 4300 270
4700 270
9.0
10 5100 300
12 6200 360
14 6800 390
7500
16 430
7500 470
18
20 8200 510
8200 510
22
24 9100 510
9100
26 560
28 9100 560
30 9100 560
9100
32 560
34 10k 560
10k
36 560
38 10k 560
10k 560
40
50 10k 620
Fig. 38 - Attenuation Pad Construction and Resistor Values
for High Impedance (10K-ohm) and Low Impedance (600 ohm)
[shaded area) circuits.
R2
R1 H (ohms)
150 8.2 180k 10k
300 82k 5.1k
18
2.7k
560 33 43k
1.6k
820 51 27k
22k 1.2k
68
1 1 0 0
16k 1k
1500 82
1600 100 13k 820
1 1 k 680
2000 1 1 0
2200 130 9100 560
2400 150 8200
470
2700 150 6800 430
3000
180 5100 360
3300 200 4300 240
3600 220 3300 200
3900 220 2700 150
3900 240 2000 120
1500
4300 240 91
4300 270 1300
75
4700 270 1000 62
4700 270 820 47
4700 270 620
36
4700 300 5 1 0 30
4700 390 22
300
4700 300 330 18
4700 300 240 15
5 1 0 0 300 200 12
5100 300 62 3.6