Cloud Electronics MPA626 - Manual (Page 10)

MPA-626: Installation and Operation Manual 9
12 Power amplifier and line outputs
12.1 Power amplifier and protection circuits
The MPA-626 has a 150W output (at the minimum load of 4
Ω) power amplifier with a low
impedance output. In order to provide more flexibility the MPA-626 also has an output
transformer that can be switched on via a rear panel switch. When this transformer is switched
on it is strongly recommended that the MPA-626s internal high pass filter be set to 70Hz (see
section 12.2). The transformer provides one additional low impedance output, with a minimum
load of 2
Ω. This additional low impedance output is produced using an 'auto transformer' style
primary and is therefore not isolated from the rest of the mixer-amplifier circuitry. The
transformer secondary has 3 line outputs 100V, 70V and 50V, whilst these have a common 0v
connection, they are fully floating i.e. they are isolated from the rest of the amplifier. When the
transformer is in use the maximum total combined load on all outputs should not exceed 120W.
Should the installation require a 25V line the 4
Ω low impedance output should be used (150W
into a 4
Ω load is approximately 25V rms.)
The MPA-626 power amplifier section contains several protection circuits to protect it from
abuse. A VI limiter is used to protect the amplifier from short circuits and load impedances
below the rated minimum. The Dynamic clip protection reduces the signal level when the
amplifier is driven into clipping, thus limiting any clipping distortion to a set level. An output relay
is used to provide a switch on delay, eliminating switch on 'pops'. This relay is also used to
disconnect the output should the amplifier overheat or if the amplifier develops a fault that gives
rise to a D.C. output. If the amplifier cuts out because of overheating it will reset automatically
after cooling down.
12.2 High pass filter
A high pass filter is provided to protect speakers, transformers etc. from low frequency signals.
This filter has two settings, determined by the position of jumpers J10 and J11 (see diagram
below). The 70Hz setting offers the most protection and ideally it should be used whenever the
output transformer is switched on. The 20Hz setting offers less protection and should be used,
when only the 4
Ω output is used i.e. when the output transformer is switched off.
Location of Jumper J10 and J11
12.3 Aux Outputs
Two unbalanced Auxiliary outputs are provided on phono connectors situated on the rear panel.
One of the Aux outputs is at a fixed level, nominally 0dBu (775mV). The level on the other can
be varied from no signal to 0dBu by means of an adjacent rotary control. The nominal output
level is 0dBu (775mV) but the MPA-626 can operate with a wide range of signal levels up to a
maximum output of +20dBu (7.75V). The music level control affects the auxiliary outputs.
11-07-02 V4