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MPA Series User Manual v1.1 11

General Notes

Multi-zone Applications

Where the sound system specication calls for separate

control in several zones, MPA Series ampliers can be used

in multiples.

Signal sources can be connected to several inputs as

required, but care must be taken to ensure the output stage

of the signal source is capable of driving the resulting lower

input impedance.

The impedance of the line inputs (music inputs) is 47k ohms

and it is reasonable to assume that most op-amp based

signal sources are able to drive a 10k ohm load, allowing up

to ve ampliers to be paralleled.

The input impedance of the mic inputs is 2.4k ohms, making

them suitable for microphones with a nominal impedance

of 600 ohms or less. A single 600 ohm microphone

could therefore typically be connected to four paralleled

mic inputs. If this guideline gure cannot be adhered to,

the use of suitable mic or line distribution ampliers is

recommended.

To avoid any problems associated with differences in

mains supply earthing, we recommend that all MPA Series

ampliers used in a multi-zone application should be co-

located and connected to a common mains supply.

Note that when using multiple MPA Series ampliers in a

19” rack, suitable ventilation arrangements must be made to

ensure that lower ampliers do not cause those above to

overheat (see Ventilation for further information).

EMC Considerations

MPA Series ampliers fully conform to the relevant

electromagnetic compatibility (EMC) standards and are

technically well behaved. You should experience no problems

interfacing units to other items of equipment and under

normal circumstances, no special precautions need to be

taken.

If the unit is to be used in close proximity to potential

sources of HF disturbance such as high power

communication transmitters, radar stations and the like, it is

suggested that input signal leads be kept as short as possible.

Always use balanced interconnections wherever possible. If

the MPA Series amplier is mounted in a 19” rack, do not

locate the unit in close proximity to a powerful amplier of

any kind, which may radiate a strong magnetic eld from the

power transformer.

Earthing

When several mains powered units are connected together

via their signal cables, there is a risk of one or more earth

loops which may cause an audible hum on the system even

with the gain controls set to minimum.

The 0 V rail of an MPA Series amplier is directly coupled to

the chassis ground. No interconnection problems should be

encountered, but if there is any hum or other extraneous

noise when source equipment is connected, the situation can

generally be remedied by observing the following guidelines:

Always connect sources using balanced connections •

wherever possible, with the cable screen only connected

at the receiving end (amplier input).

Use audio isolating transformers (readily available from •

trade suppliers) at the inputs if necessary. These will

ensure that the amplier is electrically isolated from the

source equipment.

The signal source units should be located as close as •

possible to the ampliers and the metal housing of

the various units should not be electrically connected

together through the equipment rack. If this is a

problem, rack isolating kits are available from specialist

hardware suppliers. If the problem persists, try to

connect all interconnected units, including power

ampliers to a common power source to ensure a

common ground is provided.

Ventilation

MPA Series ampliers are force cooled by a thermostatically-

controlled fan. The fan is operative at all times, remaining

at low speed at internal temperatures below 50 ºC, then

increasing in speed above this temperature to a maximum

speed at 70 ºC.

Always allow adequate space around the amplier(s) to

allow a free ow of air through the unit(s). In 19” rack

applications we recommend leaving 1U of rack space above

and below each unit. Plain 1U blank panels, not slotted

ventilation panels should be used, as the latter reduce the

effect of forced-air cooling.

The direction of airow in MPA Series ampliers is from

front-to-rear; it is recommended not to mix the ampliers

with other equipment employing forced-air cooling which

acts in the opposite direction within the same rack.

In free standing applications we recommend tting the

feet supplied and placing the unit on a at surface and

leaving the ventilation slots on top of the unit free from any

obstructions.

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