Knoll Stereo Amplifier MX1255 User Manual

Warranty  
Knoll Systems warrants MX1255 amplifiers sold in the USA and Canada by  
authorized Knoll dealers to be free of defects in materials and manufacturing.  
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.  
MX1255 Amplifier  
Installation Instructions v7.3  
12 Channel Amplifier  
Warning: To be installed and/or used in accordance with appropriate  
electrical codes and regulations.  
Description: The MX1255 is a member of the long line of Knoll multi-  
channel amplifiers. Designed primarily for the custom install industry, it  
is used to power speakers with different or similar sources in multiple  
rooms. With 12 channels rated at 55 watts per channels at 8 , the  
MX1255 packs some serious punch. Each channel has a gain control.  
The MX1255 can be turned on and off via the front panel power switch  
or via the 12 VDC trigger. MX1255 channels 2-6 can be linked to  
channel 1 individually using the link switches. MX1255 amplifiers  
designed to meet the amplifier needs of custom installed systems  
where high-quality sound is a specific requirement.  
Specifications:  
Inputs  
12 Gold RCA jacks with adjustable gain  
50k Ω  
Input impedance  
Outputs  
detachable plug  
Output power  
55 watts RMS per channel (8 )  
Bridged Power (channel 6) 180 watts RMS (8 only)  
Key Features:  
Peak output power  
Ideal impedance  
Frequency response  
S/N ratio  
100 watts per channel (8 )  
4 - 8 Ω  
1. Size efficiency. The MX1255 consists of a total of 12 power  
amplifiers in one 3-1/2” high enclosure. Each power amplifier  
channel can deliver 55 watts RMS into an 8-load.  
2. Automatic protection circuitry. Each MX1255 channel is  
individually and fully protected against low impedance,  
overheating, overloading, over voltage and under voltage. The  
protection circuitry automatically restores the amplifier channel  
as soon as its parameter returns to the safe operating area.  
3. Individual input gain adjustment. Each channel features  
an input gain adjustment pot to adjust each channel for gain  
and speaker sensitivity variations.  
4. External power on/off. The MX1255 can be turned off and  
on by external controllers via its 12 VDC trigger jack.  
5. Stackable 17" chassis. 19” rack ears available.  
6. Link Feature. Certain installations require that some or all of  
the stereo channels play the same content. Instead of using Y  
cords, the Knoll "link" cascades any or all of channels 2-6 to  
channel 1 (L/R).  
10 Hz - 40 kHz +/- 1dB (1w)  
over 105 dB A weighted (50 watts)  
less than 0.1% 20 Hz to 20 kHz  
THD distortion  
less than 0.01% 60 Hz 7 kHz 4:1  
(SMPTE)  
IMD distortion  
Trigger control  
Dimensions  
Fan Noise  
Power  
12 VDC 35mA on 3.5mm mono jack  
17" x 3.5" x 11.5" (432 x 89 x 292mm)  
quiet 26 dBA  
750 watts at 117 VAC or 230 VAC  
21 lbs (9.5 kg)  
Weight  
Note: Specifications subject to change without notice.  
Knoll Systems  
7. Bridgeable. Channel 6 can be bridged giving 180 watts RMS  
into an 8 load for use with subwoofers and speakers that need  
more power.  
145 Tyee Drive Point Roberts, WA 98281  
14-7163 Vantage Way, Delta BC V4G 1N1  
Tel: (604) 940 1689, fax (855) 734 3363  
Made in Canada/Fabrique au Canada  
 
Knoll Systems service information  
The MX1255 amplifier does not contain any user serviceable parts  
inside. If you suspect a problem that may require servicing, contact us  
at www.knollsystems.com/contact.html, or by phone at 800 566-5579.  
Installation  
Installing the MX1255 should be relatively easy. With a bit of  
planning, it will give trouble free service for years.  
Troubleshooting  
1. The most important consideration when installing the  
MX1255 is cooling. The MX1255 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.  
2. Amplifiers should always be the top components in the system.  
3. If MX1255 channels frequently shutdown due to overheating,  
install a fan directed up from the MX1255 bottom center.  
4. Never operate the MX1255 on its side, as the cooling potential  
drops significantly when operated this way.  
5. Connect the MX1255 inputs to the source component outputs  
with good quality, short as possible RCA jack cables. Connect each  
channel individually.  
6. Connect the MX1255 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).  
If a problem is encountered with the MX1255, the most expedient  
procedure is to locate the problem and if possible repair it before  
requesting service. Be sure to carefully check other system components  
such as controllers, CD players, volume controls, wiring, speakers, etc.  
that may be at fault.  
Problem  
Power LED does  
not light - no sound  
Action  
1. Check that the MX1255 is plugged in.  
2. Test the AC outlet with a lamp.  
3. If remote on/off is used, check that  
the trigger voltage is at 12 VDC.  
4. Check MX1255 power button on (in).  
Sound cuts out  
1. Verify speaker impedance is 4-16 .  
Changing speakers may be required.  
2. Check if the MX1255 feels hot. If it's  
hot increase cooling - see Installation.  
Note: Ideally the MX1255 likes 6-8 loads. Connecting  
to 4 loads won't hurt the MX1255 but those channels  
connected to 4 loads may occasionally shutdown due  
to overloading. Never connect to speaker loads less than  
4 . Channel 6 bridged mode minimum 8 speaker.  
Sound is distorted  
1. Turn the volume down.  
2. Check speakers for damage.  
3. Check inputs for proper levels.  
MX1255 gain and source output level  
may, have to be adjusted.  
4. Speakers may be less than 4 .  
7. Individually adjust the channel gains as required. Ideally, all  
gains are fully on (fully clockwise).  
8. 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.  
9. Connect trigger if being used to a 12 VDC source (about 35mA)  
using a 3.5mm mono jack. 5 VDC triggers will not work.  
10. Connect the AC power into an outlet that supplies at least  
EIGHT amps (1000 watts) dedicated to each MX1255.  
11. If more power is needed, channel 6 can be mono bridged to  
give 180 watts into an 8 load. Minimum impedance for  
channel 6 in the bridged mode is 8 . The bridged switch needs  
to be in the out position. The bridged input is 6L and the 6L gain pot  
adjusts the bridged gain. The speaker is connected to the 6L+ and  
6R+ in the bridged mode.  
MX1255 does  
not turn off  
1. Push MX1255 power switch off (out).  
2. If trigger is being used, power switch  
needs to be set to off (out) position  
or amp is on all the time.  
3. Try disconnecting trigger jack.  
Trigger does not  
work  
1. Measure the trigger voltage with a volt  
meter. It needs to be 11-15 volts DC  
to work (current is about 35mA total  
per amp). See page 5 for details.  
Speaker pops when  
amp turned on or off  
1. Speaker may need resistor placed  
across terminal. Suggest 2k0 1/4 w.  
Discharges speaker internal capacitor.  
 
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.  
The trigger is 12 VDC (35mA per MX1255) via 3.5mm  
mono jack. 5 VDC trigger outputs will not work. To  
connect the trigger to a receiver use a 12 VDC power  
supply (we suggest Knoll PS1202 power supply with an  
adaptor). Plug the power supply into the switched outlet  
on the rear of the receiver. Connect the 12 volt wire to a  
3.5mm mono jack. Plug the jack into the rear of the  
MX1255. 12 VDC polarity is not important. Make sure the  
MX1255 power switch is off (out). When the receiver is  
switched on, the MX1255 amplifier is switched on.  
User adjustments and service: There are no user  
adjustments on the MX1255. Your installer may make certain  
gain level adjustments on the rear panel.  
Caution: The MX1255 contains no user serviceable parts, so do  
not attempt to open or repair the MX1255. Refer servicing to a  
qualified technician only or contact the factory for information.  
The Link button is normally set to the “in”  
position to give all six stereo inputs individual  
sources. When the Link button is set to the  
“out” position, that input Left/Right is linked or  
cascaded (or paralleled) to input #1 (without the use of Y  
cords). Any or all combination of linked inputs 2 to inputs  
6 are possible. When all inputs are linked to input 1 the  
input impedance is about 4000 Ω.  
Channel 6 can be mono bridged giving 180 watts into 8  
ohms. Never connect speaker loads less than 8 ohms  
to channel 6 in the bridged mode. Select the  
bridge/stereo switch to the out position for the bridged  
mode. The bridged input is 6L. The 6L gain pot adjusts the  
bridged gain. The speaker is connected to the 6L+ and  
6R+ in the bridged mode.  
Caution: NEVER connect the  
MX1255 to impedances less  
than 4 or amplifier  
damage not covered by the  
warranty may occur.  
 

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