The Undisputed Leader
PROFESSIONAL
In High Speed
Switching Power Supply
Technology
Sales and Factory Service:
Stewart Audio, Inc.
14407 Cuesta Court
Sonora, CA. 95370
(209) 588-8111 Fax (209) 588-8113
PA-50B/PA-100B
Owner’s Manual
WHEN THE PERFORMANCE DEPENDS ON IT
AMPLIFIERS
Congratulations on the purchase of your new Stewart Audio
PA Series power amplifier. This amplifier has been designed and built to
provide you with years of high-quality audio performance and
trouble-free operation. If after reading this manual you should have any
questions concerning amplifier installation and operation, please contact
your Authorized Stewart Dealer, or you may contact us directly at:
Stewart Audio, Inc.
14407 Cuesta Court
Sonora, CA. 95370
Phone (209) 588-8111 Fax (209) 588-8113
Your Stewart Audio PA Series amplifier is the result of years of
research and experience in the design and manufacture of amplifiers and
superior quality audio equipment. As such is provides a combination of
performance and operational benefits that simply cannot be found in
conventional amplifiers.
In order to obtain maximum performance from your PA Series Amplifier,
please take time to read this brief owner’s manual and carefully follow the
guidelines for connection and operation
Design Features
• Switching Power Supply—for efficiency, size reduction, dependability
• Superior Thermal Protection—effective protection with no noisy fan
and no untimely shutdowns
• Full Complimentary Class A/B Amplifier—utilizes integrated
Darlington Pairs for more linear operation. Sound is more musical,
more “tube-like”
• High Dissipation Output Devices—insures operation well within their
S.O.A. (Safe Operating Area) for effortless operation and device
longevity
Warranty
The Stewart PA Series Amplifiers are covered by a 2 year limited warranty.
Please make sure you fill out the warranty card provided and mail it to us.
We recommend you keep the shipping carton, packing materials, and your
bill of sale.
Specifications PA-50B PA-100B
Maximum Output , 20Hz-20Hz 25W@8? single channel 50W@8? single channel
Stereo 50W@4? single channel 90W@4? single channel
Bridged Mono 100Wx2@8? per channel 180Wx2@8? per channel
10Wx2@4? per channel 180Wx2@4? per channel
Frequency Response 20Hz - 20kHz 20Hz - 20kHz
+/-0.5dB
Bandwidth +/-3.0dB 5Hz - 100kHz 5Hz - 100kHz
Noise (A weighted, any mode) >100dB below full 98dB below full
rated output rated output
THD (8?) 20Hz-20kHz <0.08% <0.08%
Input Impedance 20K? 20K?
Input Sensitivity for Rated Output (8?) 1V 1V
Slew Rate (Stereo Mode) <30V/microsecond <30V/microsecond
Damping Factor <200/8? <200/8?
Number of Channels 2 2
Input Connectors 1/4” Phone 1/4” Phone
Output Connectors Terminal Strip Terminal Strip
Power Requirements ( 8?/Ch) 120C AC 100w 120C AC 100w
Power Supply Design High Speed Switching High Speed Switching
Dimensions Height 1.7 in (4.4cm) 1.7 in (4.4cm)
Width 8.5 in (21.6 cm) 8.5 in (21.6 cm)
Depth 6.2 in (15.7 cm) 10.2 in (25.9 cm)
Weight 3.5 lbs(1.6 kg 5.5 lbs (2.5kg)
Optional Accessories RMK-100/RMK-200 19” Rack Asemblies
for Single or Dual (side-by-side) Mounting
Stewart reserves the right to change features and specifications without notice.
9
Troubleshooting
Problem: Led not Lit.
Procedure: Is amplifier plugged into a live outlet? Is power switch pushed
“IN”? Disconnect speaker. If amplifier turns on (after a few seconds delay),
the problem is in the output connection. Check wiring and speakers for
short circuits. If amp will not turn on, return it to your dealer for service.
Problem: No output on one or both channels; LED Lit.
Procedure: Check level controls, input terminations, bridge switch, source,
input cables, speaker connector and wiring.
PA-50B/PA-100B Block Diagram
PA-50B/PA-100B
8
Contents
Safety ………………………………………2 & 3
Special Features …………………………………….4
Configuring for Stereo Operation…………………...5
Configuring for Bridged (Mono) Operation……….10
Trouble Shooting …………………………………...8
Specifications ………………………………..9
For The Record
Please fill out and return the enclosed warranty card at
your convenience. This information will provide proof of
ownership for warranty purposes and will allow us to
notify you if necessary.
In the spaces provided below, record the model and
serial number located at the rear of your amplifier.
Model No. ________________________________
Serial No. _________________________________
Purchase Date _____________________________
Retain this information for future reference.
1
Safety
Allow ample ventilation; do not enclose amplifier so air cannot move
Before using your Stewart Audio Inc. Power Amplifier, please read this
Owner’s Manual carefully to ensure optimum trouble –free performance.
WARNING: TO REDUCE THE RISK OF ELECTRICAL SHOCK,
DO NOT EXPOSE THIS AMPLIFIER TO RAIN OR MOISTURE.
DANGEROUS HIGH VOLTAGES ARE PRESENT INSIDE THE
ENCLOSURE. DO NOT OPEN THE CABINET. REFER
SERVICING TO QUALIFIED PERSONNEL ONLY.
The lighting bolt within arrowhead symbol, within an equilateral
triangle, is intended to alert the user to the presence of
un-insulated “dangerous voltage” within the product’s enclosure
that may be of sufficient magnitude to constitute a risk of
electrical shock to person.
The exclamation point within an equilateral triangle, is intended
to alert the user to the presence of important operating and
maintenance (servicing) instruction in the literature
accompanying the appliance.
Important Safety Instruction – Please Read Prior to Product
Installation.
1. All of the safety and operating instructions should be read before the
amplifier is operated. Retain these instructions for future reference.
All instructions should be followed; all warnings on the amplifier and
in the operating instructions should be adhered to.
2. This amplifier should not be used near water, for example, near a
bathtub, in a wet basement, near a swimming pool, etc.
3. The amplifier should be situated so that it’s location and position does
not interfere with its proper ventilation. For example, the amplifier
must not be placed on a rug, bed, sofa or similar surface that impedes
airflow across the chassis. Airflow through the ventilation openings
should be unobstructed.
2
PA-50B/PA-100B Input Controls
The input sensitivity of each amplifier is adjustable for optimum
performance with and line-level source. Adjustment is by means of
screwdriver-adjust rotary controls, recessed behind the front panel. With
typical program source levels present, adjust each channel to provide the
highest level without distortion. If distortion persists, check the preceding
components. The most noticeable distortion usually occurs in early stages
of the audio system, such as in the microphone preamplifier stage of the
mixer. If you are using a mixing console adjust the “gain” or trim control,
not just the channel input fader.
If the program material you are using for setting levels is not overly
compressed, the LED peak indicators (marked “O/L) should illuminate on
occasional program peaks. Desired listening levels should now be
maintained at the mixer/preamp of other source.
Rack-Mounting
Your PA-Series amplifier may be easily rack-mounted in one of several
ways. Attach the optional RMK-100 Rack Assembly to mount one
amplifier in one vertical space (1.75”) of a standard 19” equipment rack. Or
mount two PA-Series amplifiers side-by-side in a single rack space with the
optional RMK-200. The RMK-200 also allows side-by-side mounting of a
PA-Series Amplifier and any other Stewart half-rack product, such as the
ADB-4 or UDP1-A Preamplifiers.
Some Courtesies to Extend To Your Amplifier
1.
freely around it, especially above it. If the top of the unit is to be
covered, airflow must be provided.
2 .Avoid operating in direct, bright sunlight.
3. Avoid contact with moisture. Don’t plug into a wet amplifier. If
moisture has gotten inside, contact Stewart Audio Service for
instructions.
4. Avoid excessive torque on input level controls.
5. Avoid very dusty environments. A clean amplifier is a happy
amplifier.
7
, and the point where they exit
Configuring for Bridged (Mono) Operation
Your Stewart PA-Series amplifier may also be operated as a powerful
single-channel amplifier. This so-called “bridged” mode requires special
attention to input and output wiring configurations.
Input Connections
Wire the appropriate phone plug (TRS or TS) to the input cable as
described on page 4. Insert the plug into CHANNEL #2 ONLY. Make no
connection to CHANNEL #1 Input.
Output Connections
Prepare the speaker leads as described on page 3. Insert the stripped leads
onto screw terminals #1 and #3 ONLY (Channel 1 Positive and Channel 2
positive). Make no connection to terminal #2. Inspect for possible shorts
or broken wires.
Depress the front-panel button marked Bridge/Stereo. Push it in as far as it
will go. Be sure the power button is off (out), then plug the line cord into a
grounded 120v AC outlet.
The amplifier is now ready for mono operation.
Bridged Mono
+ = Speaker Positive
- = Speaker Negative
G = (No Connection
(Use Ch 2 Input Only)
6
4. Do not place the amplifier near heat sources such as radiators, heat
registers, stoves, or other appliances that product heat.
5. The amplifier should be only be connected to 120 VAC, 60 Hz
power supply. Do not defeat the ground or polarization of the power
plug.
6. Route power cord and other cables so that they are not likely to be
walked on, tripped over or stressed. Pay particular attention to
condition of cords and cables at plugs
the amplifier. To prevent risk of fire or injury, damaged cords and
cables should be replaced immediately.
7. Clean the amplifier with a clean damp cloth. Do not use solvents or
abrasive cleaners. Never pour any liquid on the amplifier.
8. When left unused for a long period of time, the power cord should be
unplugged form the outlet.
9. Damaged Amplifiers requiring service. The amplifier should be
10. serviced by a qualified service technician when:
a. The power cord or AC plug has been damaged.
b. Objects have fallen, or liquid has spilled into the unit
c. The amplifier has been exposed to rain or other moisture
d. The amplifier does not appear to operate normally or
exhibits a marked change in performance.
e. The amplifier has been dropped or the enclosure
damaged.
10. The user should not attempt to service the amplifier beyond that
described in the operating instructions. All other servicing should
be referred to qualified service personnel.
3
Special Features
Please take a few moments to read through this section of the manual, which
provides an overview of the features that make this amplifier unique.
NOTE: Observe proper polarity in interconnecting all system components.
CAUTION: Do not connect AC power until all other connections are made.
Balanced Inputs
Balanced inputs will accept either balances TRS (“Stereo,” Fig. 1) or
unbalanced TS (“Mono,” Fig. 2) 1/4” connectors. It is recommended that
cable runs over 300 feet be balanced, to reduce susceptibility to hum and
radio-frequence interference. If you must run an unbalanced signal greater
than 30 feet, a Direct Box such as the Stewart Model ADB-1 should be used to
convert the signal to a balanced line.
Tip = Positive Tip = Positive
Ring = Negative Sleeve = Ground
Sleeve = Ground For connecting an unbalanced line to a
balanced or unbalanced input
For connecting a balanced line to a
balanced or unbalanced input
Fig. 1 TRS (“Stereo”) 1/4” phone plug Fig. 2 TS (“Mono”) 1/4” phone plug
4
Configuring for Stereo Operation
Input Connections
Wire the appropriate plugs (TRS or TS) to input cables as described on
Page 4. Insert plugs into Channel 1 and Channel 2 inputs.
Output Connections
Speaker leads connect by means of a single three screw terminal block on
the rear panel.
Prepare speaker leads by stripping insulation back 5/16” (8 mm). Twist
strands together slightly to ensure that there are no stray strands. Connect to
terminals that correspond to the channel outputs as follows:
Stereo
1+ = Ch 1 Speaker Positive
2+ = Ch 2 Speaker Positive
G = Ch 1, 2 Speaker Negative
With a small, flat-blade screwdriver, tighten the screw until the leads are
held securely in place. Inspect for possible shorts or broken wires.
Check the position of the front panel Bridge/Stereo Mode Selector Switch.
The button should be in the “out” position. Be sure the power button is off
(out), then plug the line cord into a grounded 120v AC outlet.
The amplifier is no ready for stereo operation.
5
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