Knoll Systems Stereo Amplifier MA252 User Manual

Trigger does not 1. Measure trigger voltage with a voltmeter. It  
work  
needs to be 11-16 volts DC to work (current is  
about 35mA total per amp). See page 5 for details.  
MA252  
Stereo Amplifier  
Installation Instructions v1.0  
Speaker pops  
when MA252 is  
turned on or off  
1. Speaker may need resistor placed across terminal.  
Suggest 2000 ohm 1/4 watt. This discharges the  
speaker internal capacitor if it has one.  
Warning: To be installed and/or used in accordance with appropriate  
electrical codes and regulations.  
Warranty  
Knoll Systems warrants MA252 amplifiers sold in the USA and Canada by authorized Knoll  
dealers to be free of defects in materials and workmanship. This warranty extends for three  
full years from the date of purchase by the original consumer. Any products returned to  
Knoll Systems and found to be defective by Knoll Systems within the warranty period will be  
repaired or replaced at Knoll Systems decision, at no charge. Knoll Systems will not be  
responsible for the actual cost of installation or removal of the product, nor for any  
incidental or consequential damages. Some states do not allow the exclusion or limitation of  
incidental or consequential damages, so the above limitation may not apply to you. This  
warranty gives you specific legal rights. You may have additional legal rights that vary from  
state to state.  
Description: Designed primarily for custom installations, the MA252 is  
used to power stereo speakers, usually in a single room. Each channel  
has a gain control. The MA252 can be turned on and off using the front  
panel power switch, composite video detection, when music is sensed  
or the 12 VDC trigger. A special feature of the MA252 is that whole  
house music from another amplifier can be heard on the speakers  
connected to the MA252. When the local source needs amplifier power  
the MA252 senses this, disconnects the other amplifier and amplifies  
the local source. The MA252 amplifier is designed to meet the amplifier  
needs of custom installed systems where high-quality sound is a  
specific requirement.  
Specifications:  
Line level inputs /outputs Gold RCA jacks  
Input impedance  
Speaker inputs /outputs  
Output power  
Peak output power  
Ideal impedance  
Frequency response  
S/N ratio  
50k ohms  
detachable plug  
Key Features:  
50 watts RMS per channel (8 ohms)  
100 watts per channel (8 ohms)  
4 - 8 ohms  
Manual and Automatic Power On and Off. The MA252 stereo  
power amplifier is designed to be turned on when a video signal  
is detected, music inputs are sensed, a 12-volt trigger is  
activated or the front panel power switch is pushed on. Power  
off is automatic in all modes except the front panel power  
button.  
10 Hz - 40 kHz +/- 1dB (1w)  
over 95 dB A weighted (50 watts)  
less than 0.1% 20 Hz to 20 kHz  
less than 0.01% 60 Hz 7 kHz 4:1 (SMPTE)  
12 VDC 35mA on 3.5mm mono jack  
17" x 3.5" x 11.75" (431.8 x 89 x 298mm)  
0.5 v p-p minimum  
THD distortion  
IMD distortion  
Trigger control  
Dimensions  
Whole house music override. In homes, yachts, businesses  
and motor home applications (typically with plasma or large  
screen TV's) the MA252 features amplifier inputs so that whole  
house music system can be heard on the local speakers (without  
the MA252 turned on). When the local source (TV, dish, etc.)  
needs amp power, the whole house music switches off  
automatically and the MA252 amplifier plays the local source.  
Automatic protection circuitry. Each MA252 channel is  
individually and fully protected against low impedance,  
overheating, overloading, overvoltage and undervoltage. The  
protection circuitry automatically restores the amplifier channel  
as soon as its parameter returns to the safe operating area.  
Individual input gain adjustment. Each channel features an  
input gain adjustment pot to adjust each channel for gain and  
speaker sensitivity variations. The gain adjustments are on the  
rear of the MA252 so installers normally set the gain.  
Video trigger  
Power  
175 watts at 117 VAC or 230 VAC  
11 lbs (5 kg)  
Weight  
Note: Specifications subject to change without notice.  
Knoll Systems  
145 Tyee Drive Point Roberts, WA 98281  
12140 Horseshoe Way, Richmond, BC V7A 4V4  
Tel: (604) 272 4555, fax (604) 272 5595  
Made in Canada  
 
Hum and noise  
Installation  
In most cases, one of the following suggestions should help you  
solve a hum noise in your system. Please try these steps in the  
order shown, proceeding from one step to the next if the prior  
suggestion does not eliminate the problem.  
Installing the MA252 should be relatively easy. With a bit of  
planning, the MA252 will give trouble-free service for years.  
1. The most important consideration when installing the  
MA252 is cooling. The MA252 has a lot of power packed into a  
small chassis size. When installing it in an equipment stack, it  
should be the top component. It needs at least 3"-5" of space  
above the amplifier to allow for adequate convection cooling.  
Suggestion #1  
To determine if a cable TV connection is responsible for the hum, first  
turn off all the components. Disconnect the cable TV feed to your  
system at the first place where it connects to your components.  
Alternatively, disconnect the cable TV wire where it is connected at the  
wall outlet. Turn your system back on, and listen if the hum has  
disappeared. If removing the cable TV feed has eliminated the hum,  
you will need to insert a Ground Loop Isolator (Knoll model GB634)  
before reconnecting the cable TV feed, or contact your cable TV  
operator to see if they can better isolate your cable feed.  
2. When installing the MA252 in a rack, we suggest adding a  
3-1/2" blank above and below the MA252. Amplifiers should  
always be the top components in a rack system.  
3. If MA252 channels frequently shutdown due to overheating,  
install a fan directed up from the MA252 bottom center.  
Suggestion #2  
Turn off all components in your system and then disconnect the input  
cables at the amplifier. Turn the amplifier back on to see if the hum is  
still present. If the hum disappears, the fault may be in the input cables  
used. Try replacing them with cables that have better shielding, and  
make certain that the input cables are not running on top of any AC  
power cords. Change the cables one at a time to determine if one or all  
cables are responsive. If the hum disappears when the input cables are  
disconnected but returns after the cables are changed and the system  
reconnected, your source device may be causing the problem.  
4. Never operate the MA252 on its side, as the cooling potential  
drops significantly when operated on the side.  
5. Connect the MA252 inputs to the source component outputs  
with good quality, short as possible RCA jack cables. Connect  
each channel individually.  
6. Connect the MA252 speaker outputs to speakers using good  
quality speaker wire. Minimum 16 gauge copper wire is  
recommended with 14 gauge minimum for runs over 30' (10m).  
Suggestion #3  
Poor grounding of the electrical system in your home may also cause  
ground loop problems, particularly when there are multiple components  
with three prong, grounded, power cords. Unplug these components  
one at a time, and see if one or all of them is causing the problem. The  
ultimate solution to this type of problem is to rewire your house with an  
isolated, star type-grounding configuration. Knoll understands that this  
may be impractical and expensive. In some instances, the use of an  
approved AC Power Isolation Transformer (Knoll model PLB200) of  
sufficient capacity may solve this problem.  
Note: Ideally the MA252 likes 6-8 ohm loads. Connecting  
to 4 ohm loads won't hurt the MA252 but it may occasionally  
shutdown. Never connect a MA252 to less than 4 ohms.  
Do not attempt to mono bridge the MA252.  
7. Make sure the speakers in each room are connected in phase  
with the amplifier + going to the speaker + . Out of phase  
speakers give unstable imaging and poor bass response.  
Warning: If you suspect that the grounding system in your  
home’s electrical wiring is causing the hum problem, it is  
important that you do not make any changes to the wiring. Only  
a licensed electrician should make any changes to household  
wiring and they must be made in full compliance with all local  
building, safety and electrical codes.  
8. If connecting the MA252 to a whole house sound  
system or another amplifier make sure that the other  
amplifier is a common ground amplifier and that you use  
good quality speaker wire, minimum 16 gauge.  
 
9. If a video signal is being used to trigger the MA252 on and  
off, connect good quality short as possible video wire with a RCA  
jack to the MA252 “Video Input”. Make sure the music sense  
button is OUT.  
12. If the 12 volt trigger is being connected, use a 12 VDC  
source (about 35mA) with a 3.5mm mono jack. Knoll PS1202  
with a new 3.5mm jack works well. 5 VDC triggers will not work.  
10. If the music sense or video input is being used to turn the  
MA205 on and off, set the “Music Sense” button to OUT. Adjust  
the Sens (sensitivity) so the MA252 turns on when quieter sound  
passages are playing. Adjust the “Delay” so the MA252 turns  
itself off 25-120 seconds after the sound has stopped.  
Caution: Never listen to sound that is distorted. If distorted  
sound is heard, turn the volume down immediately or speaker  
and/or amplifier damage could occur that is not covered by the  
warranty. If this problem persists, contact your dealer.  
User adjustments and service: There are no user  
adjustments on the MA252. Your installer may make  
certain gain level adjustments on the rear panel.  
11. Individually adjust the channel gains as required. Ideally,  
the gains are fully on (fully clockwise).  
Caution: The  
MA252 contains  
no user  
Caution: DO NOT CONNECT BRIDGED  
AMPLIFERS or amps that do not have  
common grounds to the AMP INPUTS”  
as damage may occur.  
MA252 REAR PANEL (condensed)  
Connect the AMP INPUTS”  
Connect these  
unbuffered stereo  
line outputs to  
other amplifier  
inputs as needed.  
speaker level inputs to the  
whole house audio system  
or other amplifier outputs.  
The AMP INPUTSpass  
directly to TO SPEAKERS”  
when the MA252 isnt turned  
on.  
serviceable  
Connect a composite  
video output from a  
VCR, plasma TV, etc.  
to the Video Inputif  
the MA252 is to be  
trigerred on when a  
video signal is present.  
Make sure the music  
sense button is  
parts, so do not  
attempt to open  
or repair the  
MA252. Refer  
servicing to a  
qualified  
technician only  
or contact the  
factory for  
Connect stereo  
line level inputs  
here.  
Connect the TO SPEAKERS”  
to the 4-8 ohm speakers.  
12VDC  
TRIGGER  
INPUT  
MA252  
switched to the OUT  
position.  
Input  
Output  
Stereo Amplifier  
L
AMP  
TO  
Connect this 3.5mm  
trigger jack if the  
MA252 is to be  
triggered on when a  
12 VDC signal is  
present. Polarity  
can be reversed.  
It needs about  
INPUTS  
L
SLPEAKERS  
Knoll Systems  
For video triggering  
without music sensing  
turn the Senspot  
Point Roberts WA USA  
R
R
information.  
Richmond, BC Canada R  
+
+ +  
+
- -  
- -  
fully counterclockwise.  
Video  
Input  
Sens Delay  
L Gain R  
Music sense  
CAUTION  
Out = Active  
35 mA.  
RISK OF ELECTRIC SHOCK  
Made in Canada  
Fabrique au Canada  
120 VAC / 1.5 A  
DO NOT OPEN  
POWER  
When using music sense only,  
adjust Sens so the MA252 turns  
on when the source music is at  
a medium low volume. Sens is  
not used for video on or trigger  
on. For video sensing without  
music sensing turn Sens fully  
counterclockwise.  
After MA252 is on,  
Delay is adjusted to  
turn off MA252 after  
the source music ends.  
Ideal delay adjustment  
is about 20-40 seconds.  
Delay is used with video  
on and music sense only.  
Adjust the gain  
The music sense (auto turn on when  
when the system is  
on. Best signal to  
noise ratio is when  
MA252 gains are on  
full (clockwise).  
music is present) button should be  
OUT (active) when the MA252 power  
is to be triggered either from music  
sense or the composite video signal.  
The music sense button should be IN  
when the amp is turned off and on from  
the front panel switch or the 12 VDC  
trigger.  
Ideal speaker impedance is 4-8 ohms.  
 

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