TOA Electronics Stereo Amplifier A 903MK2 User Manual

Operating Instructions  
TOA 900 SERIES II  
MIXER POWER AMPLIFIER  
A-903MK2  
A-906MK2  
A-912MK2  
THE  
LIGHTNING  
FLASH  
WITH  
ARROWHEAD WITHIN A TRIANGLE IS  
INTENDED TO TELL THE USER THAT  
PARTS INSIDE THE PRODUCT ARE A  
RISK OF ELECTRIC SHOCK TO  
PERSONS.  
TO REDUCE THE RISK OF ELECTRICAL  
SHOCK, DO NOT REMOVE COVER.  
NO USER SERVICEABLE PARTS INSIDE.  
REFER SERVICING TO QUALIFIED  
SERVICE PERSONNEL.  
THE EXCLAMATION POINT WITHIN A  
TRIANGLE IS INTENDED TO TELL  
THE  
USER  
THAT  
IMPORTANT  
OPERATING  
AND  
SERVICING  
INSTRUCTIONS ARE IN THE PAPERS  
WITH THE APPLIANCE.  
Please follow the instructions in this manual to obtain the optimum results from this unit.  
We also recommend that you keep this manual handy for future reference.  
TOA Corporation  
 
General Description  
The TOA A-903MK2, A-906MK2 and A-912MK2 Mixer Power Amplifiers control and mix up to eight  
independent input signals. The A-903MK2 delivers up to 30 watts of output power, the A-906MK2 60 watts  
and A-912MK2 120 watts.  
Optional plug-in modules are available for use with these mixer power amplifiers to provide versatility for a  
wide range of operating applications. Edge connectors on the rear of the unit permit the selection of the TOA  
plug-in modules.  
The mixer power amplifiers have output terminals to match 4- or 8-ohm speaker systems, or speaker  
distribution systems may be connected to 25- or 70-volt terminals, and can be rack mounted by using  
optional rack mounting bracket MB-25B. The optional perforated panel PF-511 provides suitable ventilation,  
finished in color to match the mixer power amplifiers.  
Features  
1. 8-channel mixer power amplifier  
2. Wide frequency response : 20~20,000 Hz 1 dB  
3. Low distortion and noise level  
4. Remote master volume control  
5. Excellent output regulation  
6. Bass and treble controls  
7. Bridging input/output  
8. Self-protection circuitry design  
9. Signal processing input/output  
10. Varied output impedances : 4 ohms, 8 ohms, 25 and 70 volts  
11. A full range of plug-in modules  
12. Portable or rack mounting type  
–3–  
 
Nomenclature  
Front Panel  
Power Switch  
Input Volume Controls  
Adjust gain of input #1 ~#8 respectively.  
Applies line power. Two-position push button  
switch for on/off modes.  
Bass Control  
Modifies bass response. Turn clockwise to boost,  
counter-clockwise to attenuate the bass  
response. Tone is flat at center.  
Power Indicator LED  
Green LED lights when the power is switched  
"ON".  
Signal Indicator LED  
Treble Control  
Modifies treble response. Turn clockwise to  
boost, counter-clockwise to attenuate the treble  
response. Tone is flat at center.  
Green LED lights when more than –30 dB signal  
level is fed to the inputs by means of the input  
and master volume controls.  
Tone Switch  
Normal Indicator LED  
Yellow LED lights when fed to the proper signal  
Selects ON/OFF of the BASS and TREBLE  
controls. When slided to OFF, BASS and  
TREBLE controls are active. When slided to ON,  
they become inactive to make tone flat.  
level.  
Peak Indicator LED  
Red LED lights when amplifier approaches  
clipping level. If steady lit, the input level control  
should be turned down until the LED flashes only  
intermittently.  
Low-Cut Switch  
Cuts off unnecessary low frequency.  
Master Volume Control  
Adjusts overall gain of the unit.  
Protection Indicator LED  
Remains lit for about 5 seconds after power  
switch is turned ON.  
–4–  
 
Rear Panel  
AC Power Cord  
Preamp Out  
Connects to power source.  
Connects to a signal processing equipment such  
as a limiter, an equalizer, etc. The input  
impedances of the equipment should be of more  
than 600 ohms.  
AC Outlet (Unswitched)  
Provides AC power for auxiliary equipment with  
power consumption of up to 500W.  
Bridging Input/Output  
This terminal is used as a mixing bus. Mixing is  
achieved when the similar terminal of another  
amplifier is connected to this terminal. The output  
level taken from this terminal is independent of  
Master Volume Control, Bass and Treble  
Controls so that the terminal can also be used as  
recording output.  
Output terminals  
Connect to speakers.  
Impedance Select Switch  
Selects the desired impedance.  
Enables unbalanced output of low impedance  
when this switch is set to "DIRECT".  
The input impedances of the equipment to be  
connected here should be of more than 10k  
ohms.  
Module Input Ports  
Accepts optional PLUG-IN MODULES.  
Module selection is determined by application.  
For details, refer to Plug-in Module Instruction  
Manual.  
Remote Volume Control Terminals  
Connecting a 10k linear taper potentiometer  
across these terminals will allow remote control  
of preamplifier output.  
AUX Out  
Provides connections for a booster amplifier or  
tape recorder.  
Mute terminals  
Mutes input signals fed to plug-in modules  
employing muting function by short-circuiting at  
these terminals.  
The input impedance of the equipment should be  
of more than 10k ohms.  
Power Amp In  
Connect to accessory equipment.  
–5–  
 
Input Connections  
There are eight INPUT PORTS for PLUG-IN MODULES.  
Select the desired modules for each application. Plug the modules  
into the INPUT PORTS.  
Slide them between the guide rails and secure with two screws.  
CAUTION : Modules should not be inserted or removed while  
the amplifier is turned on.  
Cover unoccupied INPUT PORTS with blank panels. Secure blank panels with screws.  
Output Connections (A-903MK2)  
The speaker outputs of the amplifier are 4 ohms, 8 ohms, 25V and 70V.  
Connect speakers to any one of these outputs.  
Class 2 wiring may be used.  
There are two types of output;  
4 ohms, 25V and 70V via output (matching) transformer  
8-ohm direct output.  
The connecting method differs in each case. Refer to the following diagrams.  
Note : Impedances indicated below imply total speaker system (load) impedances.  
To connect speakers to outputs of : 4 ohms, 25V or 70V (BALANCED OUTPUT)  
(A-903MK2)  
167-ohm  
70V line  
21-ohm  
25V line  
4-ohm  
Loosen the screw.  
Place the impedance switch in 4-ohm position.  
Tighten the screw.  
Note : Low-cut switch should be in "CUT" position.  
Cut off unnecessary low frequency in TRANS OUTPUT to obtain best acoustic condition. The  
900MK2 series amplifiers are characteristically flat even in the low frequency range. Thus, in TRANS  
OUTPUT the acoustic effect and frequency-response characteristics may be altered.  
–6–  
 
To connect speakers to 8-ohm output (UNBALANCED DIRECT OUTPUT)  
(A-903MK2)  
8-ohm  
Loosen the screw.  
Place the impedance switch in 8-ohm position.  
Tighten the screw.  
Output Connection (A-906MK2/A-912MK2)  
The speaker outputs of the amplifier are 4 ohms, 8 ohms, 25V and 70V.  
Connect speakers to any one of these outputs.  
Class 2 wiring may be used.  
There are two types of output ;  
8 ohms, 25V and 70V via output (matching) transformer  
•4-ohm direct output.  
The connecting method differs in each case. Refer to the following diagrams.  
Note : Impedances indicated below imply total speaker system (load) impedances.  
To connect speakers to outputs of : 8 ohms, 25V or 70V (BALANCED OUTPUT)  
(A-906MK2/A-912MK2)  
83-ohm(A-906MK2)/  
41-ohm(A-912MK2)  
70V line  
10.4-ohm(A-906MK2)/  
5.2-ohm(A-912MK2)  
25V line  
8-ohm  
Loosen the screw.  
Place the impedance switch in 8-ohm position.  
Tighten the screw.  
Note : Low-cut switch should be in "CUT" position.  
Cut off unnecessary low frequency in TRANS OUTPUT to obtain best acoustic condition. The  
900MK2 series amplifiers are characteristically flat even in the low frequency range. Thus, in TRANS  
OUTPUT the acoustic effect and frequency-response characteristics may be altered.  
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To connect speakers to 4-ohm output (UNBALANCED DIRECT OUTPUT)  
(A-906MK2/A-912MK2)  
4-ohm  
Loosen the screw.  
Place the impedance switch in 4-ohm position.  
After all connections are made, tighten the screw.  
Muting and Remote Volume Control Connection  
POTENTIOMETER  
10k OHMS  
MUTE 2  
MUTE 1  
Remote volume control connection limiting  
front panel master volume control range.  
Rack Mounting  
To mount the amplifier in a standard 19" equipment rack, use the optional MB-25B rack mounting bracket.  
MB-25B  
Remove 4 screws securing case.  
Fix the brackets with attached 4 screws. The length  
of screws should not exceed 12mm (1/2").  
Amplifier  
PF-511 perforated panel (1-unit size)  
Use the perforated panel PF-511 to provide space for ventilation,  
when two or more units are mounted in an equipment rack.  
Contact your local TOA dealer for the perforated panel PF-511.  
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Operation  
After all connections are made, turn power switch ON, illuminating the Power Indicator LED. The amplifier  
comes into operation approximately 5 seconds after the power is turned on.  
Volume Adjustment  
Obtain desired output level by adjusting the individual input and master volume controls. For normal music  
playing or announcement, adjust volume using their controls until the normal LED intermittently lights. Sound  
quality is deteriorated when the peak LED remains lit.  
Tone Control Adjustment  
Set the tone switch to the OFF position. This activates the bass and treble controls. The frequency response  
characteristics of each control are flat at center, boost in CW and attenuation in CCW positions. When tone  
controls are unnecessary, set the tone switch to ON mode.  
Installation  
Never block ventilation holes.  
The amplifier should never be placed in areas  
exposed to direct sunlight.  
susceptible to vibration.  
with high humidity or dust levels.  
with high ambient temperatures.  
with poor ventilation.  
adjacent to heat-generating equipment.  
CAUTION : Do not remove the case or you may encounter an electric shock.  
Note : When the temperature of heat sink exceeds 105 °C the protection circuit is activated and the output is  
disconnected from the circuit. Output is automatically restored when the temperature return to normal  
operating parameter.  
Servicing  
Unpacking  
Upon receipt of the amplifier shipment, please inspect for any damage incurred in transit. If damage is  
found, please notify your local TOA representative and the transportation company immediately.  
State date, nature of damage, whether any damage was noticed on the shipping container, prior to  
unpacking.  
Please give waybill number of shipping order.  
Failure  
Should amplifier fail, contact your nearest TOA authorized contractor or service center.  
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Specifications  
A-903MK2  
A-906MK2  
A-912MK2  
Type  
8-channel mixer power amplifier  
30W RMS  
Output Power  
60W RMS  
120WRMS  
(D)20~20,000Hz, 0.5%THD  
(T) 50 ~20,000 Hz, 0.5% THD  
Power Band Width  
(D) 20 ~20,000 Hz, 1 dB  
(T) 20 ~15,000 Hz, 1 dB  
(T) 20 ~20,000 Hz, +1 dB, -3 dB  
Frequency Response  
Total Hamonic Distortion  
Inputs  
0.02% at 1 kHz, rated output  
Eight Input Ports : Each port accepts any input module.  
One Bridging Input/Output  
Input Port #1 to #8 : 100 mV/10k ohms  
Bridging Input/Output: 100 mV/3.3k ohms  
Input Sensitivity/Impedance  
Preamp OUT/Power Amp IN  
1000 mV into 600 ohms/1000 mV, 10k ohms  
Main (T): 4 ohms, 25  
& 70 volts, balanced  
Main (D): 8 ohms,  
unbalanced  
Main (T) : 8 ohms, 25 & 70 volts, balanced  
Main (D) : 4 ohms, unbalanced ,  
Aux : 10k ohms, 1V  
Outputs (T)=Transformer  
(D)=Direct  
Aux : 10k ohms, 1V  
(D) Less than 0.5 dB, no load to full load  
(T) Less than 1.0 dB, no load to full load  
Output Regulation (1 kHz)  
Signal to Noise Ratio  
(Band Pass 20 ~20,000 Hz)  
Tone Defeat SW ON  
Master volume min : 90 dB  
Master volume max : 77 dB  
Power amplifier only : 105 dB  
Tone Controls  
Bass : 10 dB at 100 Hz, Treble :  
10 dB at 10 kHz  
8 Input gain controls  
1 Bass tone control  
1 Power ON/OFF switch  
1 Master gain control  
1 Treble control  
1 Tone defeat switch  
Controls  
1 Low-Cut switch (60 Hz, 6 dB/octave)  
Indicators  
Protection  
1 Power LED, 1 Protect LED, 1 Signal LED, 1 Normal LED, 1 Peak LED  
Self-protection, with AC fuse (inside)  
Input No.1 to No.8  
Bridging, Mixer preamp. output  
Power amp. input, Aux output  
Output  
Mute, Remote VR  
AC power cord/plug  
AC outlet  
Card-edge connector  
RCA phono jack  
Connectors  
Screw-terminal strip  
Screw-terminal strip  
SJT, 3-prong type  
3-pin grounding type  
180W  
100W  
Power Consumption  
Temperature Range  
60 W  
-10 °C ~ +60 "C [12°F~140°F]  
420 x 99.1 x 318 mm  
[16.5 X3. 9X 14.1 in.]  
11.4 kg [25.1 Ib.]  
420 x 99.1 x 358 mm  
9.6kg [21.2lb.]  
Dimensions (W) x (H) x (D)  
[16.5 X3. 9X 12.5 in.]  
7.8 kg [17.2 Ib.]  
Black  
Weight  
Color  
4 Volume control covers (YA-920)  
Output disconnected for approx. 5 seconds after switching power on.  
Standard Accessories  
Other Features  
* Specifications are subject to change without notice.  
–10–  
 
Appearance (A-903MK2)  
Front View  
420  
Rear View  
Top View  
Side View  
–11–  
 
Appearance (A-906MK2/A-912MK2)  
Front View  
420  
Rear View  
Top View  
Side View  
TOA Corporation  
PRINTED IN TAIWAN  
133-12-114-60  
KOBE, JAPAN  
 

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