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SP-20AB MANUAL
Installation
For installation as a free-standing unit,
attach the four adhesive rubber feet supplied
with the SP-20AB to the bottom of its case.
With either the HRKIT-1 or the HRKIT-2
adaptor kits the SP-20AB may be mounted in
standard 19” equipment racks. The HRKIT-1 is
for mounting one SP-20AB in the center of a
19” rack. The HRKIT-2 is for mounting two units
side by side in a 19” rack (or for mounting a
single unit in a “half rack”). Complete installation
instructions are supplied with the kits.
When installing any equipment in racks, keep
in mind that standard racks come equipped
with mounting rails with holes tapped for 10-32
machine screws. Be sure to use only 10-32
screws for this purpose (in particular, avoid
10-24 screws, which will fi t if forced but will strip
the threads). To avoid marring the pan el when
tightening the screws, use nylon washers under
the screw heads.
For best results, the input cables plugged
into the SP-20AB should be shield ed. Balanced
connections should use shielded, twisted-pair
cable. Headphone cables and speaker cables
do not need to be shielded, though no harm is
done it shielded cables (such as microphone
cables) are used.
Circuit Description
CAUTION! This section is intended to assist the
professional user with considerable experience
in electronics to achieve a better un derstanding
of the operation of the SP-20AB. Under
no circumstanc es should persons without
6
electronics troubleshooting experience and
training undertake repairs on their own. There are
no user ser viceable parts inside the SP-20AB. All
problems should be referred to the Factory or to
other qualifi ed service personnel.
The input connectors are XLR balanced. The
inputs to the SP-20AB proceed from the input
connectors to op-amp buffers wired as differential
(balanced) amplifi ers. If a mono signal is inserted
this will convert the differential op-amp to a non-
inverting amp. After the buffers, the left and right
signals proceed to the two decks of the stereo
Input Level con trol.
The SP-20AB’s power amplifi cation is derived
from a pair of LM1875 integrated circuits, which
have internal short-circuit and thermal overload
protection. They are set for a voltage gain of 26 dB
and drive the speaker outputs through a stan dard
RLC network. The network establishes the high
frequency rolloff, as well as preventing voltages
generated by the speakers from reaching back
into the IC and causing damage. The headphone
output passes through a special high-power pot
wired as a volume control.
The SP-20AB’s power supply provides ±30
volts unregulated DC (with 120VAC line input) to
the LM1875 stages, which may drop to ±23 volts
under full load. A network of dropping resistors
and zener diodes provides ±15 volts to the rest of
the circuit.
Both the Overload LED’s and the Signal
Present LED’s monitor signal levels at the output
of the LM1875, but before the headphone volume
control and the Speaker On/Off Switch.