4
MPU (Micro Processor Unit)
The MPU circuitry measures the impedance of the
speaker load that is intended to be connected. The
MPU only permits impedances above 3.5 ohms to
be connected.
How to use the LDS and MPU
Test Functions
1. In order for the LDS2/75 to correctly carry out
its designated LDS and MPU test functions, you
must make all of your speaker connections with
the master power switch in the ‘off’ position.
2. Only when you are satisfied that you have made
all the connections correctly should you turn the
power switch to its ‘on’ position. Once the switch
is on the ‘on’ position the LDS2/75 will then pro-
ceed with its diagnostic testing and the result will
show with the rear panel LED illuminating in ei-
ther Green or Orange.
3. If the LED illuminates in Green then every-
thing has been tested satisfactorily. This happens
instantly and you could easily miss the green LED
illuminations so we urge you to watch the signal
closely.
4. If the LED continues to flash in orange then
there is a problem with either a) the speaker con-
nections or b) the load impedance of the speakers.
Fig. 3. LDS2/75 configured as a Stereo Amplifier with 4 ohm speakers.
L
R
AUDIO INPUT
4 OHM
LEFT
SPEAKER
MODE SWITCH IS “STEREO” POSITION
4 OHM
RIGHT
SPEAKER
LINE
SPEAKER
INPUT
LINE
OUTPUT
(BUFFERED)
LEVEL
LO
HI
LDS LED
SEE
BELOW
1
2
SPEAKERS
L
R
+
+
12V
POWER
120V 60Hz 3A
275
T
E
CHN
O
LO
G
IE
S
, I
NC.
She
rb
ou
rn
STEREO
BRDG
MONO
MODE
OUT
IN
+
+
MUSIC
TRIGGER
L
R
L
R
+
+
+
+
5. To determine if the problem is one of ‘connec-
tion’ you should listen to see if a low level syn-
chronized test tone is absent from any of your
speakers. If you do not hear the tone from one of
the speakers, it is almost certainly that will be the
one with the bad connection. You should turn off
the power and carefully reconnect the speaker
cables and then repeat (2) above.
6. If the same synchronized test tone can be heard
from all the speakers, then almost certainly it is
because the impedance is too low. If you are trying
to use more than two pairs of speakers, it is sug-
gested that you remove one pair, turn off the am-
plifier and then carry out another test by repeating
step (2). If this solves the problem, then it will in-
dicate that the load impedance with the two pairs
of speakers was too low. If the amplifier still does
not work, then it is suggested that you consult the
speaker manufacturer to determine the exact im-
pedance of the loudspeakers.
7. The LDS function refers to a ground potential
for measuring the speaker impedance. It does not
measure the speaker under the bridge mode opera-
tion and care should therefore be exercised in con-
necting the speaker wires when the LDS2/75 is be-
ing used in its bridged mode.
