Yamaha P-2200, P-2200/2201, P-2201

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

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)

0.5

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

300

560

1100

1710

2200

2700

3300

3900

4300

4700

5100

6200

6800

7500

8200

9100

10k

100

130

160

200

220

270

300

360

390

430

470

510

560

620

150

300

560

820

1100

1500

1600

2000

2200

2400

2700

3000

3300

3600

3900

4300

4700

5100

8.2

100

110

130

150

180

200

220

240

270

300

180k

82k

43k

27k

22k

16k

13k

11k

9100

8200

6800

5100

4300

3300

2700

2000

1500

1300

1000

820

620

510

390

330

240

200

10k

5.1k

2.7k

1.6k

1.2k

820

680

560

470

430

360

240

200

150

120

3.6

Fig. 38 - Attenuation Pad Construction and Resistor Values

for High Impedance (10K-ohm) and Low Impedance (600 ohm)

[shaded area) circuits.