Wohler AMP2A-4S User manual

Title and Contents ......................................................
Introduction ......................................................................................
Section 1: General Features and Specs ..................
Description and Features ..................................................................
Applications, Specifications, and Other Options ..............................
Section 2: Operation ..................................................
Installation .......................................................................................
Front Panel Features ........................................................................
Rear Panel Features .........................................................................
Section 3: Technical Information ............................
General Technical Observations ......................................................
Level Meter Settings and Specifications ...........................................
LED Bargraph Driver PCB (919030) Description ...........................
Level Meter Bargraph "0" (Zero) Fine Adjustment ..........................
Level Meter Bargraph Peak LED Segment Adjustment ....................
Input Select Analog Switch PCB (919013) Description ...................
AMP2A-4S Interconnect Block Diagram .........................................
AMP2A-10S Interconnect Block Diagram ........................................
AMP2A-4S and AMP2A-10S
2U Stereo Audio Speaker Monitors
with Four Selectable (AMP2A-4S) or Ten Selectable
(AMP2A-10S) Analog Stereo Inputs on Phoenix,
Selected Stereo Output on Two XLR, Dual
20-Segment LED Level Meters and Phase Indication
Document P/N 821554 Rev-B
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CONTENTS
1
User Manual

2© 2007 Wohler Technologies, Inc. ALL rights reserved
Introduction
Congratulations on your selection of a Wohler Technologies product. We are confident it represents the best performance and value
available, and we guarantee your satisfaction with it.
If you have questions or comments you may contact us at:
Wohler Technologies, Inc.
31055 Huntwood Avenue
Hayward, CA 94544
Phone: [phone] Fax: [phone]
US Toll-Free: 1-888-596-4537
www.wohler.com [e-mail]
1) Read these instructions.
2) Keep these instructions.
3) Heed all warnings.
4) Follow all instructions.
5) Do not use this apparatus near water.
6) Clean only with dry cloth.
7) Do not block any ventilation openings. Install in accordance with the manufacturer's instructions.
8) Do not install near any heat source such as radiators, heat registers, stoves, or other apparatus (including
amplifiers) that produce heat.
9) Do not defeat the safety purpose of the polarized or grounding-type plug. A polarized plug has two blades
with one wider than the other. A grounding type plug has two blades and a third grounding prong. The
wide blade or the third prong are provided for your safety. If the provided plug does not fit into your outlet,
consult an electrician for replacement of the obsolete outlet.
10) Protect the power cord from being walked on or pinched, particularly at plugs convenience receptacles
and the point where they exit from the apparatus.
11) Only use attachments/accessories specified by the manufacturer.
12) Use only with the cart stand, tripod, bracket, or table specified by the manufacturer, or sold with the
apparatus. When a cart is used, use caution when moving the cart/apparatus combination to avoid injury
from tip-over.
13) Unplug this apparatus during lightning storms or when unused for long periods of time.
14) Refer all servicing to qualified service personnel. Servicing is required when the apparatus has been
damaged in any way, such as when power-supply cord or plug is damaged, liquid has been spilled or
objects have fallen into the apparatus, the apparatus has been exposed to rain or moisture, does not
operate normally, or has been dropped.
15) Do not expose this apparatus to rain or moisture.
16) The apparatus shall be connected to a mains socket outlet with a protective earthing connection.
Important Safety Instructions
CAUTION!
In products featuring an audio amplifier and speakers, the surface at the side of the
unit, where the audio amplifier heat sink is internally attached, may get very hot after
extended operation. When operating the unit excercise caution when touching this
surface and ensure that external materials which may be adversely affected by heat
are not in contact with it. There is a Hot Surface label (see diagram) attached to the
aforementioned surface of the product.

© 2003 Wohler Technologies Inc. ALL rights reserved 3
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-B
Sect. 1: General Features and Specifications
General Features
and Specifications
Description
Features
Applications
Specifications
Other Options
Section 1

© 2003 Wohler Technologies Inc. ALL rights reserved
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AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-B
Features
AMP2A-4S and AMP2A-10S
Description
Section 1: General Features and Specifications
AMP2A-10S Front Panel
The AMP2A series units are complete, exceptionally high quality stereo audio monitoring systems in compact, two rackspace
cabinets. These models contain three audiophile-quality drivers and three power amplifiers; two amplifiers (and two speakers) that
reproduce midrange and high frequency information in stereo, and a third amp/driver combination (and speaker) that handles summed
Low Frequency (LF) information below the 500 Hz crossover point. The AMP2A series unique audio design has two important
advantages. First, it provides optimally focused sound in an Ultra Near Field tm (1 to 3 feet) environment. This allows higher SPL for
the operator while reducing overall ambient sound and adjacent bay crosstalk. Second, electronic rather than acoustic cancellation of
bass frequencies provides positive audible detection of reversed polarity (“out of phase”) audio feeds. A unique LED display also
visually shows “phase” (polarity) relationships of the signals selected for monitoring.
The AMP2A-4S and AMP2A-10S models are both audio monitors capable of monitoring two analog source channels (one selected
pair) through the stereo speaker system while simultaneously visually monitoring both channels via two 20-segment tri-color LED
bargraph level meters. Both models feature a rotary switch on the front panel to allow selection of one of four (AMP1A-4S) or one
of ten (AMP2A-10S) analog stereo pair input sources for monitoring.
The rear panel for the AMP2A-4S model features four selectable stereo pairs of analog inputs on balanced Phoenix connectors. The
rear panel for the AMP2A-10S model features ten selectable stereo pairs of analog inputs on balanced Phoenix connectors. Both
models offer an analog output of the selected stereo source on two XLR connectors on the rear panel.
2U Stereo Audio Speaker Monitors
• 98 dB SPL at two feet
• Only two rack spaces high
• Excellent high frequency response for positive detection of
background whine and noise
• Audible indication of phase/polarity problems
• Thorough magnetic shielding for placement next to video
monitors
• Numerous alternate control and input options
• Quick and easy installation: simply slide in the rack and
connect audio and AC power
• AMP2A-4S model features four selectable stereo pairs of
analog inputs on balanced Phoenix connectors
• AMP2A-10S model features ten selectable stereo pairs of
analog inputs on balanced Phoenix connectors
• Both models offer front panel selection of the stereo pair input
source to be monitored
• Two 20-segment LED bargraph display level meters
• Volume and balance controls
• Stereo phase indication LEDs
• Headphone output
• Power indication LED
AMP2A-4S Front Panel
AVG
28
L
40
31
37
34
R
WOHLER
TECHNOLOGIES
POWER
SOURCE
5
16
19
25
22
7
10
13
2
1
0
3
1
PEAK
3
FAST-OUT
PHASE
FAST-IN
ANALOG AUDIO
MONITOR PANEL
AMP2A-4S
SOURCE
12 34
AVG
WOHLER
TECHNOLOGIES
L
POWER
3
R
16
28
34
37
31
40
22
25
19
3
13
10
7
5
1
0
1
2
PEAK
SOURCE
FAST-IN
PHASE
FAST-OUT
101
2
3
4
56
7
8
9
SOURCE
ANALOG AUDIO
MONITOR PANEL
AMP2A-10S

© 2003 Wohler Technologies Inc. ALL rights reserved 5
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-BSection 1: General Features and Specifications
Applications
The AMP2A-4S and AMP2A-10S are ideally suited for use in VTR bays, mobile production vehicles, teleconferencing installations,
multimedia systems, satellite links and cable TV facilities, and on-air radio studios. Designed and manufactured in the U.S., both
models are backed by a strong warranty and a satisfaction guaranteed return policy.
Units are designed to meet, at time of manufacture, all
currently applicable product safety and EMC requirements,
such as those of CE. 0 dbV ref. 0.775V RMS. Features and
specifications subject to improvement without notice.
General Specifications
Analog Input Connectors:
Analog Input Impedance:
Peak Acoustic Out (@ 2 ft.):
Power Output,
RMS Each Side (4ΩΩ
ΩΩ
Ω):
RMS Bass (4ΩΩ
ΩΩ
Ω):
Frequency Response,
Sixth Octave:
Input Level for Maximum
Output (Volume Full On):
Hum and Noise (analog):
Distortion, Electrical:
Distortion, Acoustic:
Input Overload:
Analog Out S/N:
Analog Out THD:
Magnetic Shielding:
Power Consumption
(Average Maximum):
AC Mains Input:
Phoenix Terminal Block (3-pin, male)
40KΩ, balanced
104 dB SPL
14 W transient / 10 W continuous
35 W transient / 25 W continuous
80 Hz - 16 kHz ± 5 dB)
(-10 dB @ 40 Hz, 20 kHZ)
0 dBv balanced / -10 dB unbalanced
Better than -68 dB below full output
Less than 0.15% at any level below
input threshold
6% or less at worst case frequencies
above 120 Hz, including cabinet
resonance; typically less than 1.5%
+26 dBv balanced
>90 dB
<0.008%
<0.8 Gauss any adjacent surface
45 W
100-240VAC, 50-60 Hz Universal
Physical Specifications
Typical 1/6 Octave Audio Response Curve
Level Meter Specifications
Level Meter Type:
Meter Dynamics:
Level Gain:
Dynamic Range:
Midscale Resolution:
Segment Colors:
Scale:
20-Segment LED bargraph
VU or PPM, selectable
-6, 0, +4,+8 dBu, selectable
44 dB
1 dB
Tricolor (green, amber, red)
+4 (peak) to -40 dB
B
d
0
+10
-20
-30
-10
Hz
10k
5k
2k1k50020010050 20k20
18 lbs. (8.2 kg)
3.5 x 19 x 12 inches
(89 x 483 x 305 mm)
Weight:
Dimensions (HxWxD):
Audio Response Curve
Other Options
Wohler Technologies offers by far the broadest range of standard production audio monitor units. Custom combinations of
connectors, controls and level meters are available by special order. Standard-production models or special order custom features
for the AMP2A series (2U) units include the following functions (and combinations thereof):
• Transformer coupled analog inputs
• Separate channel volume controls
• Mono, mute, and mode switches
• Alternate level meter scales and color maps
• FULL output power DC operation
• External speaker capability
• Multiple input and output connector type choices
Other custom options are possible. Call your dealer or Wohler Technologies to discuss your specific needs.
Note: See page 17 for additional bargraph information..

© 2003 Wohler Technologies Inc. ALL rights reserved
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AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-B
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© 2003 Wohler Technologies Inc. ALL rights reserved 7
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-B
Operation
Installation
Front Panel Features
Rear Panel Features
Section 2

© 2003 Wohler Technologies Inc. ALL rights reserved
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AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-BSection 2: Operation
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© 2003 Wohler Technologies Inc. ALL rights reserved 9
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-B
Installation
Section 2: Operation
Mounting
The unit should be mounted where convenient for operating persons, ideally at approximately ear level for best high frequency
response and eye level for best visual observation of the level meters. Its superior magnetic shielding eliminates concerns about
locating it adjacent to most types of CRT monitors, including even high-resolution color monitors.
NOTE: Be sure to set the level meter Input Level Gain Calibration and VU/PPM Display Mode DIP switch (accessed through the
top cover) BEFORE installing the unit into an enclosed rack or console. See page 17 for setting information.
Heat Dissipation
Heat dissipated by the speaker amps is conducted directly to the left side of the chassis; no special considerations for cooling are
necessary as long as the ambient temperature inside the rack area does not exceed approximately 40°C (104°F).
Sympathetic Vibration
Sympathetic vibration from other equipment (cables, etc.,) in the rack may be serious enough to interfere with the unit’s sound
quality out in the listening area. The use of thin card stock and/or felt or foam weather-stripping type materials between adjacent
vibrating surfaces, or tying up loose cables, etc., may be required to stop vibrations external to the unit.
Mechanical Bracing
Even though the unit is fairly heavy, the chassis is securely attached to the front panel at eight points along its surface, not just at
the four corners of the chassis ears. This feature will reduce or eliminate rear bracing requirements in many mobile/portable
applications. The weight of internal components is distributed fairly evenly around the unit.
Audio Connections
Connection of the audio feeds is straightforward. Please refer to the system interconnect block diagrams on pages 21 and 22 for
clarification of the general signal paths into and out of the AMP2A-4S and AMP2A-10S units.
Analog inputs via the 3-pin male Phoenix connectors are configured for 40K Ωbalanced connections.
Electrical Interference
Care should be exercised to avoid mismatched cable types and other similar causes of undesired reflections in RF signal systems.
If severe enough, such reflections can result in undesirable electrical interference in the audio signals.
As with any audio equipment, maximum immunity from electrical interference requires the use of shielded cable; however,
satisfactory results can sometimes be obtained without it. The internal circuitry common is connected to the chassis.
AC Power
The unit's AC mains connection is via a standard IEC inlet, with safety ground connected directly to the unit's chassis. The
universal AC input (100-240VAC, 50/60Hz) switching power supply is a self-resetting sealed type, with automatic over-voltage
and over-current shutdown. There is no user-replaceable fuse in either the primary or secondary circuit.

© 2003 Wohler Technologies Inc. ALL rights reserved
10
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-B
Please refer to Figure-2a on the following page to familiarize yourself with the front panel features of the AMP2A-4S and AMP2A-
10S units. The following sections describe these features and are referenced, by number, to Figure-2a.
Front Panel Features
Section 2: Operation
1
2
3
4
5
6
7
8
Speakers
The AMP2A series internal speaker system is comprised of two mid-range tweeter speakers (left and right) and one woofer
speaker (center). The two side channel speakers reproduce, in stereo, only the mid and high frequencies.
Power Indication LED
This LED glows green to indicate the AMP2A series unit is connected to mains power and an operation voltage is present.
Audio Level Meter LED Bargraph Displays (1-2)
Audio levels for the selected 2-channel source are visually displayed via these two 20-segment, tri-color LED bargraph
meters. Dynamic range for these meters is 44 dB and they are able to display signal levels using either PPM or VU standards.
Input Level Gain Calibration and VU/PPM Display Mode for each of the two meter bargraphs is user selectable via two
DIP switch modules accessible through the top cover of the unit (see page 17 for DIP switch settings).
Contact the factory for additional information concerning meter scales and ballistics.
Volume Control Pot
This controls the loudness of the audio reproduced by the internal speakers or connected headphone. Clock-wise rotation of
this control increases the loudness of the monitored audio.
Headphone Jack
Select the headphone audio sources as you would for the internal speakers. When you plug in headphones, the speakers will
mute. This jack accepts a standard 1/4” phone type stereo plug.
Phase Indication LEDs
These three LEDs offer instant verification of phase (polarity) conditions in the pair of channels selected for monitoring in
the Left/Right channel speakers. There are three LEDs; the two smaller LEDs labeled "FAST-IN" and "FAST-OUT" show
instantaneous phase relationships in the signal, while the larger LED, labeled "AV G ", will indicate the average phase condition.
The small "FAST-IN" LED glows (or blinks) GREEN when signals are in-phase. The small "FAST-OUT" LED glows (or
blinks) AMBER for out-of-phase signals. The larger "AV G " LED indicates the average phase condition by glowing GREEN
for in-phase conditions, or RED for out-of-phase conditions. In general, it is sufficient to regard the "AV G " LED (average
phase condition) as adequate for proper phase monitoring. While it is normal for stereo signals to contain some intermittant
instantaneous out-of-phase and in-phase conditions (small LEDs), a steady red glow of the larger LED almost always indicates
an out-of-phase alarm condition.
Balance Control Pot
This control pans the volume balance between the left and right speakers.
Source Select Switch
On the AMP2A-4S model, this 4-position rotary switch selects one of four stereo pair inputs for monitoring through the
speakers (or headphones) and level meters. The numbers on this switch correspond to the numbers silkscreened above the
Analog Input Connectors on the rear panel (Item C, page 12).
On the AMP2A-10S model, this 10-position rotary switch selects one of ten stereo pair inputs for monitoring through the
speakers (or headphones) and level meters. The numbers on this switch correspond to the numbers silkscreened above the
Analog Input Connectors on the rear panel (Item C, page 12).

© 2003 Wohler Technologies Inc. ALL rights reserved 11
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-BSection 2: Operation
Figure-2a: Front Panel Features
AMP2A-10S
Front Panel
AMP2A-4S
Front Panel
AVG
31
L
40 37 34
R
WOHLER
TECHNOLOGIES
POWER
7
19 25 22 28
10 13 16
1 0 1 5 3
PEAK
2 3
SOURCE
FAST-OUT
PHASE
FAST-IN
10
1
2
3
4
56
7
8
9
SOURCE
AMP2A-10S
MONITOR PANEL
ANALOG AUDIO
131
5
246
1
78
131
5
246
1
78
AVG
5
WOHLER
TECHNOLOGIES
L
28
R
40
37
34 31
16 25
22 19
10 13
7
POWER
SOURCE
1 3 1 0 2
PEAK
3
FAST-IN FAST-OUT
PHASE
AMP2A-4S
SOURCE
ANALOG AUDIO
MONITOR PANEL
31 2 4

© 2003 Wohler Technologies Inc. ALL rights reserved
12
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-B
D
Please refer to Figure-2b on the following page to familiarize yourself with the rear panel features of the AMP2A-4S and AMP2A-
10S units. The following sections describe these features and are referenced, by letter, to Figure-2b.
Rear Panel Features
Section 2: Operation
A
B
C
Power Connector
Attach a standard IEC-320 power cord between this connector and mains power (100 - 250VAC, 50/60 Hz). The front panel
Power Indication LED (Item 2, page 10) will glow GREEN to indicate operating voltages are present.
Analog Input Connectors - Balanced Phoenix (AMP2A-4S)
These 3-pin male Phoenix connectors (INPUTS, CHANNEL A (LEFT) and CHANNEL B (RIGHT)) accept standard
analog audio signals and are configured for balanced 40K Ωconnections. The two banks of inputs are comprised of four
connectors each for a total of eight inputs (four stereo pairs). Stereo pair numbers silk-screened above the connectors
correspond to the numbers available for selection by the Source Select Switch on the front panel (Item 8, page 10).
Connector pinout information is silk-screened just above each input connector.
Analog Input Connectors - Balanced Phoenix (AMP2A-10S)
These 3-pin male Phoenix connectors (INPUTS, CHANNEL A (LEFT) and CHANNEL B (RIGHT)) accept standard
analog audio signals and are configured for balanced 40K Ωconnections. The two banks of inputs are comprised of ten
connectors each for a total of twenty inputs (ten stereo pairs). Stereo pair numbers silkscreened above the connectors
correspond to the numbers available for selection by the Source Select Switch on the front panel (Item 8, page 10).
Connector pinout information is silk-screened between the two banks of input connectors.
Selected Output Connectors
These two 3-pin male XLR connectors are analog outputs of the Analog Input Connectors (Item B or C) as selected by the
Source Select Switch on the front panel (Item 8, page 10). The left connector outputs the left channel (channel A) and the
right outputs the right channel (channel B). Both connectors are configured for low impedance connections and the output
signals are not affected by the volume/balance controls or headphone mute. For XLR connector pinout information see the
diagram below.
High (+)
Low (-)
Gnd
(Shield)
Pin-1 Pin-2
Pin-3
Male XLR Pinout

© 2003 Wohler Technologies Inc. ALL rights reserved 13
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-BSection 2: Operation
Figure-2b: Rear Panel Features
AMP2A-10S
Rear Panel
AMP2A-4S
Rear Panel
CHANNEL A
(LEFT)
INTERNAL POWER
UNDERWRITERS LABS
LABEL ON SIDE
SEE POWER SUPPLY
NUMBER
SERIAL
50/60 Hz
100-240 VAC
++
1
+
2 3
+
4
INPUTS
+
1
+
2
+
3
AMP2A-4S
CHANNEL B
(RIGHT)
SELECTED OUTPUT
253410-01
+
4
CH A (L) CH B (R)
AB D
+++++++
100-240 VAC
50/60 Hz CHANNEL B
(RIGHT)
INTERNAL POWER
UNDERWRITERS LABS
LABEL ON SIDE
SEE POWER SUPPLY
SERIAL
NUMBER
2
(LEFT)
CHANNEL A
1 3 4 65
INPUTS
7
++ +
SELECTED OUTPUT
AMP2A-10S
253411-01
108 9
CH A (L) CH B (R)

© 2003 Wohler Technologies Inc. ALL rights reserved
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AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-BSection 2: Operation

© 2003 Wohler Technologies Inc. ALL rights reserved 15
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-B
Technical Information
•General Technical Observations
•Level Meter Settings and Specifications
• LED Bargraph Driver PCB (919030) Description
•Level Meter Bargraph "0" (Zero) Level Fine Adjustment
•Level Meter Bargraph Peak LED Segment Adjustment
•Input Select Analog Switch PCB (919013) Description
•AMP2A-4S Interconnect Block Diagram
•AMP2A-10S Interconnect Block Diagram
Section 3

© 2003 Wohler Technologies Inc. ALL rights reserved
16
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-BSection 3: Technical Information
General Mechanical Observations
Elimination of cabinet and component sympathetic vibrations (resonances) requires considerable attention to mechanical details.
Because of this, and the physical constraints of the speaker’s acoustic enclosures, even minor changes to any of the mechanical
details of the unit can seriously impair its acoustic performance. This especially applies to the speaker baffles. If mechanical
work on the unit is necessary, be sure to make adequate notes to permit accurate reassembly.
Unfortunately, the unusual and wholly proprietary method of magnetic shielding is usually degraded slightly by any disassembly of
the unit, except removal of the rear panel. Almost any maintenance or repair will require removal of the cover. If an immediately
adjacent video monitor shows magnetic interference after reassembly of the unit, it must be returned to the factory to restore the
shielding completely.
General Audio Circuitry Observations
The AMP2A series units may use one of two possible Audio Amplifier PC boards; the 919100-2 or the 919164. Both amplifier
circuits offer the same specifications and performance. Improvements of the 919164 over the older 919100-2 have to do
prmarily with manufacturability. The following descriptions are applicable to both amplifier designs.
Since a single-sided power supply is used, all amplifier sections are “biased” with a 1/2 supply reference, so all opamp signal
terminals on the main board should have a DC level of +12V, +/-0.7V. Signal inputs to the main audio board from any of the input
select circuits are via the balanced input stage, in lieu of the XLR analog inputs on the basic unit. Signal feed points for level
meters and the phase indicator are immediately after the input stage, and before the volume control section. The signal pick-off
for the headphones is after the volume and balance controls. Speaker muting is controlled by circuitry that senses connection of
headphones to the jack.
The power amps are attached to an aluminum heatsink plate (which is also connected to the circuit common for these devices).
The heatsink plate forms an operational module separate from the chassis, which allows access to the solder side of the circuit
board while power is applied to the circuitry. To avoid thermal shutdown of the power amp(s), they should NOT be operated
without their tabs being fastened to the heatsink plate.
Variations in the frequency response of different production runs of drivers sometimes requires minor adjustments in the
equalization/crossover components in individual runs of units. Some of these components may have values slightly different than
those indicated in the schematic, which are the nominal ones. If any of the drivers (speakers) are replaced, it may be helpful to
change some of these components to achieve maximum flatness of response.
The operating threshold of the woofer limiter is critical to both satisfactory reproduction of musical transients and preventing
damage to, or destruction of, the speaker itself. The side speaker output limiter circuits are similarly important, though not as
critically adjusted.
The woofer power amps are arranged in a bridge configuration; care must be taken to avoid letting EITHER speaker terminal
contact the chassis (common) OR THE GROUNDED LEAD OF ANY TEST EQUIPMENT so as not to short out the power amps.
The side speaker outputs are single-ended, so these precautions are not necessary for them.
General Technical Observations

© 2003 Wohler Technologies Inc. ALL rights reserved 17
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-BSection 3: Technical Information
Level Meter Settings and Specifications
Level Meter DIP Switch Settings
Two DIP switch modules, accessed through holes in the top cover, allow the user to set the VU/PPM Display Mode and Input Level
Gain Calibration. There are two DIP switch modules; one for each of the two meter bargraphs. There are four sections (1, 2, 3, 4)
on each DIP switch module. The first two sections (1and 2) are for setting the level meter Input Level Gain Calibration and the
second two sections (3and 4) are for setting the VU/PPM Display Mode.
Input Level Gain Calibration Settings:
DIP switch sections 1and 2set the level meter Input Level Gain Calibration, which determines the level of the input signal that
will result in a "0" reading on the meter bargraph. The factory setting is +4 dBu, but can be set for -6 dBu, 0 dBu, or +8 dBu by the
user. See the diagram below for settings.
Calibration Note: Each bargraph segment will turn on at 0.5 dB before that segment’s silk-screened scale indication. Example:
When the gain calibration is set for +4 dBu, the zero indication on the level meter will light up at 3.5 dBu. Calibration is implemented
in this way to effect a “rounding” function for more accurate indication of signal levels which occur BETWEEN the thresholds of any
two bargraph segments.
Bargraph Display Modes:
DIP switch sections 3and 4determine how levels are displayed (PPM or VU mode characteristics). The factory setting is VU. See
the diagram below for settings.
Level meter type:
Segment quantity:
Meter display modes (DIP switch selectable):
Level gain (DIP switch selectable):
Level meter scale:
Dynamic range:
Midscale resolution:
Bargraph Length:
LED segment pitch:
LED bargraph display
20
VU or PPM
-6, 0, +4, +8 dBu
+4 (peak) to -40 dB
44 dB
1 dB
2" (50.8 mm)
0.1" (2.54 mm)
Installation Note: The two level meter Input Level Gain Calibration and VU/PPM Display Mode DIP switch modules are
accessible through openings in the top cover of the unit. Any adjusments to the DIP switch modules should be performed before
installing the unit into an enclosed rack.
LED Bargraph Level Meter Specifications
-6 dBu
AMP2A 20-Segment Bargraph Level Meter
DIP Switch Settings (719030 PCB)
312 4
312 4
Gain Calibration
1234 1234
0dBu +4 dBu +8 dBu PPM
421 3
VU
1324
Display Mode

© 2003 Wohler Technologies Inc. ALL rights reserved
18
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-BSection 3: Technical Information
LED Bargraph Driver PCB (919030) Description
Input Stage and Level Calibration
The PPM-2 display driver PCB circuit (P/N 919030) provides a total of 44 dB of visual audio level indication via a 20-segment LED
bargraph.
The main PCB's input buffer amps pass unbalanced signals through a DIP switch selectable attenuator pad on each 919030 meter
driver PCB. A choice of four different reference levels, -6, 0, +4, and +8 dB for the 0 dB indication are available for selection by the
user. A chart showing these input gain calibration settings can be found on page 17 and on a label on the top cover of the AMP2A unit.
Each of the differential outputs from the meter driver's input buffer drives a half-wave rectifier circuit, IC6a for positive input
excursions and IC5b for negative ones. The outputs from the rectifier stages are combined and presented to two different filter
circuits that generate both PPM (IC3B) and VU (IC4) characteristic signals.
Display Mode (Meter Ballistics)
When DIP switch section 4is open and 3closed, the PPM signal drives the display circuitry. This signal is created by IC4B and
associated circuitry. A positive signal from the rectifiers causes the opamp output to go high turning on Q1. This charges C10
through a reverse protection diode (D2) and a resistor (R18). When the signal level decreases, Q3 turns off and C10 discharges
through R15 and R9.
Closing DIP switch section 4and opening 3allows the VU signal to drive the bargraph drivers. This signal is generated by IC3a and
associated components configured as an active filter to provide the proper VU characteristic.
Bargraph Driver
Two 10-segment bargraphs are stacked together (upper and lower) to create a 20-segment bargraph display. The signal (PPM or VU)
directly drives the LM3916, which drives the upper bargraph display. IC4a provides an additional 16 dB of gain to the lower bargraph
display driver (LM3915). D1 provides the reference supply to all opamps and both LM391x display drivers. The LED drive current
(brightness) is about 10x the current taken from pin 7. Pins 6 and 4 are the top and bottom connections to the reference dividers
inside the display drivers. The voltage across R1, P1, R2, and DP1 is the reference against which the rectifier output applied to pin 5
is compared. Refer to National Semiconductor Linear data books for a detailed discussion of the LM3915 and 3916 bargraph display
drivers.
The driver PCB connects the LEDs in each bargraph in two SERIES strings (one SERIES for the upper bargraph, and another for the
lower bargraph). The anode of the LED corresponding to the LOWEST input level in each string (-10 or -40) is connected to V+, with
the other nine LEDs (eight LEDs in the upper bargraph) following; the outputs of the 3915/16 are connected to the node between each
LED. Only ONE output of the 3915/16 is active (LOW) at any one time (“dot” mode), so if the LED string is “broken” (open) at any
point, the bargraph in that half of the display (only) will go OFF whenever the input level is AT OR ABOVE the level corresponding to
the break! IC5A is configured as a comparator, nominall y set at 6 dB above “0” to drive the topmost segment of the upper bargraph
display as a peak indicator.
Bargraph Driver Adjustment Settings
The Meter Zero "0" Level adjustment sets the meter to show a zero level for a particular input level calibration and is set by
adjusting P1 on the 919030 PCB. The Factory setting sets the "0" level for an input level gain of +4 dBu (see page 17 for setting the
Input Level Gain Calibration). To zero the meter for other input level gain selections, see page 19 for instructions.
The Peak Segment Threshold adjustment sets the meter to show a peak indication on the meter for a selected input level and is set
by P2 on the 919030 PCB. Factory setting for Peak Segment Threshold is 9.5 dBu. To set this threshold for other input levels, see
page 19 for instructions.
Calibration Note: Each bargraph segment will turn on at 0.5 dB before that segment’s silk-screened scale indication. Example:
When the gain calibration is set for +4 dBu, the zero indication on the level meter will light up at 3.5 dBu. Calibration is implemented
in this way to effect a “rounding” function for more accurate indication of signal levels which occur BETWEEN the thresholds of any
two bargraph segments.

© 2003 Wohler Technologies Inc. ALL rights reserved 19
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-B
Level Meter Bargraph "0" (Zero) Fine Adjustment
1) Removal of the Top Cover
Remove the top cover screws (2 along upper front panel, 2 per each side, 3 along upper rear panel, 1 center top). The 919030
PCBs (2 each) are vertically mounted to chassis bottom just behind the front panel mounted LED bargraph displays; one for
each bargraph.
2) Adjustment Pot Locations and Functions
Locate P1 on the 919030 bargraph driver PCB(s) you wish to adjust. The P1 trimpot faces upward from the PCB edge. Of the
two trim pots visible (P1 and P2) it is the closest to the front panel as mounted to the chassis. See the 919030 PCB layout on
page 26 for location of P1.
3) “0” Fine Adjustment Setup
To the the input of the bargraph you wish to adjust, connect an analog audio signal of an amplitude of approximately 1 kHz of the
amplitude desired to give a “0” indication on the bargraph.
4) Fine Adjustment of the Bargraph “0” Level
Turning the P1 pot clockwise increases the displayed level (sensitivity) of the associated bargraph meter. For example, to set
the zero level at +7 dBu (rather than the factory default setting of +4 dBu), adjust the level Input Gain Calibration DIP switches
(see page 17) for gain closest to that desired. Feed an analog audio signal of +7 dBu amplitude (see step 3) into the input on
the rear panel, then turn the pots counter-clockwise as you visually monitor the associated bargraph meter on the front panel
until the desired display setting (“0”) is achieved for the increased input level. To zero the meters for a lower input level, you
would turn the pots clockwise to increase the displayed level for any decreased input level.
Level Meter Bargraph Peak LED Segment Adjustment
1) Removal of the Top Cover
Remove the top cover screws (2 along upper front panel, 2 per each side, 3 along upper rear panel, 1 center top). The 919030
PCBs (2 each) are vertically mounted to chassis bottom just behind the front panel mounted LED bargraph displays; one for
each bargraph.
2) Adjustment Pot Locations and Functions
Locate P2 on the 919030 bargraph driver PCB(s) you wish to adjust. The P2 trimpot faces upward from the PCB edge. Of the
two trim pots visible (P1 and P2) it is the farthest from the front panel as mounted to the chassis. See the 919030 PCB layout
on page 26 for location of P2.
3) Peak LED Segment Setup
To the the input of the bargraph you wish to adjust, connect an analog audio signal of an amplitude of approximately 1 kHz of the
amplitude desired to give a “Peak” indication on the bargraph (top segment).
4) Fine Adjustment of the Peak LED Segment
Turning the P2 pot clockwise increases the displayed level (sensitivity) of the associated bargraph meter. For example, to set
the Peak LED Segment level at +10.5 dBu (rather than the factory default setting of +9.5 dBu), feed an analog audio signal of
+10.5 dBu amplitude (see step 3) into the input on the rear panel, then turn the P2 pot counter-clockwise as you visually
monitor the associated bargraph meter on the front panel until the desired display setting (top segment) is achieved for the
increased input level. To "peak" the meters for a lower input level, you would turn the pots clockwise to increase the displayed
level for any decreased input level.

© 2003 Wohler Technologies Inc. ALL rights reserved
20
AMP2A-4S and AMP2A-10S User Manual P/N 821554 Rev-BSection 3: Technical Information
Input Select Analog Switch PCB (919013) Description
Input selection from multiple sources is accomplished via a ten channel analog switch circuit. Connection to each source is
facilitated by installing a three position plug-in "Phoenix" style connector on the end of the source cable. The Phoenix connector has
screw clamps for the ground and a balanced pair. The signal is coupled to the circuit through a capacitor and a 47.5K ohm resistor on
each line in.
Selection of an input is made by pulling the control pins high on a 4053 analog switch. Inputs to the 4053 are normally connected to
a neutral buss (circuit bias supply) until a switch pair is selected at which time the inputs are switched to the signal busses. The 4053
has three SPDT switches, so two ICs are required for every three input pairs. There are two signal summing busses for the balanced
line. The signals are summed into inverting amplifiers and then summed into a differential amplifier, which then drives the audio
monitor in which the board is installed. Balanced outputs are available from the inverting amps for audio routing purposes.
In monaural board versions, two balanced pairs of signal summing busses are joined into one balanced summing pair by solder
bridging pads, and two opamps with their associated components are left off the board. In stereo versions, all opamps are installed
along with enough 4053 switches to accommodate the desired number of inputs. The inputs can be independently selected in the case
of mono sources or simultaneous pairs for stereo sources can be selected by installing jumpers at H3 (channel select pins).
Selection from ten stereo sources is accomplished by stacking two ISS boards and jumpering their select lines together. One board
can select from up to ten mono or five stereo inputs. Channel selection is normally done with a rotary switch mounted on the from
panel of units with the ISS option installed. This switch can be remoted as an option.
Unusual input select configurations (L only, R only, phase invert, etc.), or simultaneous multiple input select, are available as
additional options.
Trouble-Shooting Tips
Since a single-sided 24V supply is used, all opamps are “biased” with a 1/2 supply reference. The CMOS devices are powered from
a separate zener-regulated 15V source on each analog switch board. The analog switch elements are in series with an opamp summing
junction, so they can handle very high input signal levels without any "breakthrough" effect. When not selected, each 4053 input is
shunted to AC ground for improved off isolation. The inverting input of any opamp is sensitive to any stray capacitance to circuit
common. Though the summing amp design includes stabilizing elements, care should taken to avoid capacitive loading of the
summing busses. They are jumpered between switchboards in some configurations. There is an "enable" bus connected to pin 6 of
each 4053. It is available on H3 and H4, pin 12 (for bank switching or other special functions).
If it becomes necessary to disassemble a stacked audio switchboard assembly, insure that the upper board is positioned directly over
the lower one when reassembling. Should it inadvertently be displaced toward either the front or rear, the power connections may
become shorted.
Input:
Impedance:
Maximum Input Level:
Crosstalk:
Distortion:
Response:
Output Gain:
Maximum Output:
Power:
Ten balanced
90K Ω
+24 dBv
-65 dB
<0.05%
+/- 0.5 dB, 20 Hz - 20 kHz
0 dB balanced, -10 dB unbalanced
+24 dBv
Less than 30 mA with a 24 volt supply
919013 PCB Specifications