HA-6AB directly without fi rst being preamplifi ed. The 1/4” phone inputs are stereo jacks (tip-ring- sleeve) wired for a balanced input. If a mono phone plug is inserted, the input will automatically be converted to unbalanced. If using a mono sig- nal plug into the left ¼” jack the HA-6AB will send signal to both left and right channels.
The HA-6AB is also equipped with left and right balanced female XLR connectors in addition to the standard 1/4” phone jacks. Balanced lines offer the benefi ts of cancellation of hum, noise, and RF interference which may be picked up in the interconnecting cables. For balanced con- nections, shielded twisted pair cable must be used. The wiring is as follows: Pin 1-shield; Pin 2-signal positive; and Pin 3-signal negative. The XLR inputs can not be used in mono like the ¼” input jacks.
CIRCUIT DESCRIPTION
CAUTION! This section is intended to assist the professional user with considerable experience in electronics to achieve a better understand- ing of the operation of the HA-6AB. Under no circumstances should persons without electron- ics troubleshooting experience and training un- dertake repairs on their own. There are no user serviceable parts inside the HA-6AB. All problems should be referred to the factory or to other quali- fi ed service personnel.
The inputs to the HA-6AB proceeds from the input connectors to op-amplifi er buffers wired as differential (balanced) amplifi ers.
The input connectors are stereo 1/ 4” phone jacks. In the HA-6AB, these will be paralleled by XLR connectors. If a mono plug is inserted, the ring connection will be grounded through the body of the plug, converting the differential amplifi er to a noninverting amplifi er. After the buffers, the left and right signals proceed to the two decks of the stereo Input Level control.
The HA-6AB’s power amplifi cation is derived from a pair of LM1875 integrated circuits, which have internal short-circuit and thermal overload pro- tection. They are set for a voltage gain of 26 dB. The LM1875’s drive the speaker outputs directly. The headphone outputs fi rst pass through special high-power pots wired as volume controls.
The HA-6AB’s power supply provides +/-24 volts unregulated to the LM1875 stages, which may drop to +/-20 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 (before the volume controls).
INSTALLATION
The HA-6AB is intended for mounting in standard 19” equipment racks. Standard racks come equipped with mounting rails with holes tapped for 10-32 machine screws. Be sure to use only 10-32 screws (in particular, avoid 10- 24 screws, which will fi t if forced but will strip the threads). To avoid marring the panel when tightening the screws, use nylon washers under the screw heads.
