Rotel A-1 Service manual

Technical Information

A-1

Date: April 29, 1997

Model: RB 951

Subject: Custom Installation Applications

The RB 951 Power Amplifier is extremely versatile in its ability to drive almost any
loudspeaker. This versatility is multiplied by the RB 951s ability to operate in three
different modes:
of operation to use for a specific custom installation and how to configure the amplifier
will be recommended in the following instructions.

Please do not attempt to configure the operation of the RB 951 unless you are
confident in your understanding of these instructions. Please ensure that all five (5)
pages of these instructions are included. These instructions will not be needed for
basic Stereo operation of the RB 951.

There are eleven (11) adjustments (jumpers, variable resistors, and one switch)
available to configure the operation of the RB 951. The factory

VR601/2

1.
Turn the set screws fully
head screwdriver to taper the amplifier output for precise speaker level matching.

S603/4

2.
the RCA input jacks. Use the jumper to combine
and
and S604 pins 1-2 to bypass VR602. (You must have the variable input resistors
enabled to taper the amplifier output)

Bridged Mono / Normal Stereo

3.
left of the input jacks (back panel view). Set the switch to the right for

Stereo

operation. (Recommendations regarding which mode of operation to use begins
on page 2)

S602

4.
S604. Use the jumper to combine
Stereo operation. Combine S602 pins 1-2 for Parallel Mono operation.
(Recommendations regarding which mode of operation to use begins on page 2)

: Two dual position shunt jumpers are located on the PCB at both sides of

S604 pins 2-3

and Parallel Mono operation. Set the switch to the left for Bridged Mono

: One dual position shunt jumper is located on the PCB between S603 and

STEREO, BRIDGED MONO AND PARALLEL MONO.

preset

: Two variable resistors are located on both sides of the RCA input jacks.

clockwise

to enable VR602. Combine S603 pins 2-3 to bypass VR601

for maximum signal power. Use a small flat

S603 pins 1-2

: One dual position slide switch is located to the

S602 pins 2-3

for Bridged Mono / Normal

Which mode

positions are:

to enable VR601

Normal

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4/24/03 - Page 2

P1/2/3

5.
and the power supply capacitors. Connect
Stereo operation. Connect P1 to P2 for Parallel Mono. (Recommendations
regarding which mode of operation to use begins on page 2)

S605/6

6.
the power supply filter capacitors. Use the jumper to combine

S606 pins 2-3

pins 1-2 for high input sensitivity (.775V). This adjustment will accommodate both
high and low output preamps.

S607/8

7.
transformer and the circuit breaker. Use the jumper to combine

S608 pins 2-3

1-2 for Auto Turn On active. This feature will allow the amplifier to be turned on
when a signal is sensed at the inputs. (The front panel power switch must be in to
allow Auto Turn On to operate.)

NOTE
shunt jumpers.

There are

The RB 951 is delivered from the factory in the

• No adjustment is necessary for two channel operation. In the event Stereo mode
operation does not work, please check the settings for numbers 3, 4 and 5 above.

• Specific installations may require control of the variable inputs (numbers 1 and 2
above), input sensitivity adjustment (number 6 above) and Auto Turn On (number 7
above).

: One 18AWG spade jumper is located on the PCB between the back panel

P1 to P3

: Two dual position shunt jumpers are located on the PCB at both sides of

for low input sensitivity (1.5V). Combine S605 pins 1-2 and S606

: Two dual position shunt jumpers are located on the PCB near the power

for Auto Turn On inactive. Combine S607 pins 1-2 and S608 pins

: An asterisk appears on the PCB beside the factory preset position for the

three

basic modes of operation for the RB 951:

Stereo

•

pair of 4 or 8 ohm speakers (or two pair of 8 ohm speakers connected in
parallel) with two channels of music.

Bridged Mono

•

large
Bridged Mono mode for stereo)

Parallel Mono

•

of small 4-8 ohm loudspeakers connected in parallel with one channel of
music. (Use two RB 951s in Parallel Mono mode for stereo)

- Amplifier output provides high voltage and high current to drive one

- Very high voltage and high current output to drive one

8 ohm

loudspeaker with one channel of music.. (Use two RB 951s in

- High voltage and very high current output to drive a number

STEREO

Stereo

for Bridged Mono / Normal

S605 pins 2-3

S607 pins 2-3

mode configuration.

and

and

4/24/03 - Page 3

• It is recommended that all other settings be unaltered as received from the factory
(numbers 3, 4 and 5 above) for Stereo mode operation.

BRIDGED MONO

When using two RB 951s to drive a pair of large 8 ohm

Mono

Also:

• The left channel (CH 1) RCA becomes the mono input and the + right binding post

•

•

• Variable Input resistor VR601 can be activated by setting pins 2-3 of S603 (VR602

• Input Sensitivity adjustments

• Auto On setting S607 can be set as desired (S608 can be ignored).

NOTE: Do not
with the RSS 900 Speaker Selector. The RSS 900 utilizes ground return from the
speaker that is not available when an amplifier is bridged.

mode, settings must be made according to number 3 above for both amplifiers.

is used for the - speaker lead while the + left binding post is used for the + speaker
lead (note the back panel indications).

S602 PINS 2-3 MUST

ensure this setting will damage the amplifier.

BRIDGED MONO / NORMAL STEREO P1 - P3
BRIDGED MONO MODE.

will not be in circuit while in Bridged Mono mode - S604 can be ignored).

or low sensitivity.

use any Rotel Power Amplifier in Bridged Mono Mode operation

BE COMBINED FOR BRIDGED MONO MODE

Failure to ensure this setting will damage the amplifier.

S605/6

should both be set for the

PARALLEL MONO

When using two RB 951s to drive two sets of small 4-8 ohm loudspeakers in

Mono

Also:

• Either input can be used since they are in parallel. It is recommended that only one

•

mode, settings must be made according to number 3 above for both amplifiers.

input be used to reduce the number of interconnects and possible configuration
errors. Both sets of speaker binding posts can be utilized.

S602 PINS 1-2 MUST

this setting will damage the amplifier

PARALLEL MONO P1 - P2

•

MODE.

Failure to ensure this setting will damage the amplifier.

BE SET FOR PARALLEL MONO MODE

MUST

BE CONNECTED FOR PARALLEL MONO

loudspeakers in

MUST

BE CONNECTED FOR

Bridged

. Failure to

same

desired high

Parallel

. Failure to ensure

4/24/03 - Page 4

• Variable Input resistors VR601/2 can be activated.

• Input Sensitivity adjustments

or low sensitivity.

• Auto On settings
operation.

NOTE: It may not be appropriate to use the Rotel RSS 900 Speaker Selector with the
RB 951 while in Parallel Mono mode. The very high current of the RB 951 in Parallel
Mono mode would be dissipated by the matching resistor of the RSS 900.

Four 8 ohm speakers in parallel = 2 ohms, three 6 ohm speakers in parallel = 2 ohms,
two 4 ohm speakers in parallel = 2 ohms.

FORMULA

One divided by the total resistance is equal to one divided by the first resistance plus
one divided by the second resistance plus one divided by the third resistance and so
on … or:

1/R total = 1/R 1 + 1/R 2 + 1/R 3… example:

R total for 1 eight ohm speaker in parallel with 1 six ohm speaker in parallel with 1 four
ohm speaker =
1/ (1/8 + 1/6 + ¼) =
1/ (.125 + .167 + .25) =
1/ .54167 =

1.85 ohms

Custom Install Note
recommended by Rotel for safe operation. However, depending on the type of cable
runs and the output levels specified, it may be OK! This is an area of custom
installation that depends on a case by case evaluation to determine the limits of
operation based on experience in the field.

for computing total resistance in parallel:

S607/8

: 1.85 ohms is just below the total minimum nominal impedance

S605/6

should both be set for the

should both be set for the

same

Impedance Calculations

active or inactive

same

desired high

SIGNAL LINK

Signal Link

• The number of RB 951s that can be linked will be limited to the ratio between the
input impedance of the RB 951 (32 Kohms) and the output impedance of the
preamp used to drive the system. (A 10:1 ratio is generally accepted as a
minimum for full frequency bandwidth amplification- see

capability is available in all three modes of RB 951 operation:

Example

below)

4/24/03 - Page 5

Stereo

•

of the first RB 951 to the Left and Right (CH1/2)

: Connect from the Left and Right (CH1/2) INPUT SIGNAL LINK RCA jacks

INPUT RCA jacks of the next RB

951.

Bridged Mono:

•

Connect from the Left (CH 1) INPUT SIGNAL LINK RCA jack of

the first RB 951 to the Left (CH 1) INPUT RCA jack of the next RB 951.

Parallel Mono

•

: Connect from the Left (CH 1) INPUT SIGNAL LINK RCA jack of
the first RB 951 to the Left (CH 1) INPUT RCA jack of the next RB 951. It is
possible to use the Right (CH 2) jacks instead of the Left. It is recommended that
only one be used (see

PARALLEL MONO

page 3).

• If a number of RB 951s are to be used in varying modes of operation, Link can be
used so long as the

Bridged Mono

Stereo

Parallel Mono

or

mode RB 951 is connected to the preamp before

configured RB 951s.

either

Note: Use the
impedance to the RB 951 input impedance is at least 10 to 1.

FORMULA

above to determine that the ratio of the preamp output

Example

:

RC 995 output impedance is 100 ohms
RB 951 input impedance is 32 Kohms
1/ (1/32K + 1/32K + 1/32K…) =
1/ (10 * 1/32K) =
1/ 0.0003125 =

3.2 Kohms

The ratio of 3.2 Kohms to 100 ohms is 32 to 1 which exceeds the minimum 10 : 1 ratio
needed to ensure full bandwidth amplification. Although it seems possible on paper to
drive 30 RB 951s with the RC 995, as mentioned previously, the custom installer will
determine the limits of operation based on experience.