Alesis RA500 User Manual - Page 18
To connect a balanced line output to, feed the RA150/300/500, To connect an unbalanced source to,
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Connections: Chapter 2 BALANCED OR UNBALANCED? With long cable runs (e.g., over 6 meters/20 feet) in noisy electrical environments, the cable itself can act as an "antenna" and pick up RF fields, AC hum, or other types of interference. To avoid these problems, many professional studios and live sound companies use balanced line connections. The average application will probably not require balanced lines, but using balanced connectors between the mixer and the RA150/300/500 means one less possibility for ground loops and hum elsewhere in the system. Balanced lines carry a pair of signals, each out of phase with respect to the other but otherwise identical. To be converted back into a single, unbalanced line, both balanced lines feed a differential amplifier input or transformer that responds to the difference in levels between signals. Thus, the out-of-phase signals are recombined into an unbalanced signal, but interference induced into the cable will not be out of phase. Since there is no difference between these signals, the differential amplifier or transformer will reject the interference to a great degree. This tendency to ignore interference is called Common Mode Rejection. To connect a balanced line output to feed the RA150/300/500: You have two options: TRS-to-TRS cable Most modern mixing consoles, such as the Alesis Studio 32, feature balanced outputs on 1/4" TRS jacks. This is the same balancing connector used by the RA150/300/500. Get a 3-conductor cable with a 1/4" male TRS connector at each end. XLR-to-XLR cable A slightly more expensive balanced connector is the XLR type, most commonly sold as a "microphone cable". The main advantage of this is that the connector locks in place, making it more resistant to accidental disconnection. If your mixing console has XLR outputs, and you have the RA300 or RA500 amplifier, you may connect it with an XLR female to XLR male cable. (The mixer will have an XLR male jack, the amp has an XLR female jack.) If you have the RA150, which doesn't have an XLR input, you may use an XLR-F to TRS-M cable or adapter. The 1/4" input jack of the RA150 is balanced. (See Figure 5 on page 17.) To connect an unbalanced source to the amplifier input: Unbalanced cable Simply use a standard, shielded 1/4" patch cord. Or, if the source has an RCA/phono output, use a shielded phono-to-phono cable. Unbalanced-to-balanced cable with telescoping shield In some installations, you may be able to eliminate a ground loop even though the source is unbalanced by wiring a cable or adapter that has the shield connected at only the unbalanced end, using the inner conductor wires to carry the "hot" signal to pin 2 and the ground to pin 3 of the RA150/300/500. This may keep any ground potential difference between the source and the amplifier out of the signal path. The following diagram shows an adapter that assumes pin 2 of the XLR connector is "hot"; note that the shield of the phone jack is NOT connected to pin 1 (the ground of the amplifier), but is left floating (disconnected). Male XLR Connector 2 (hot) 1 (float) 3 (cold) 1/4 " phone jack If you have an RA150 without an XLR input, then make a 1/4" mono to 1/4" TRS adapter. Wire the "hot" of the input signal to the tip, the shield/ground to the ring, and leave the sleeve disconnected at the RA150. 16 RA150/300/500 Reference Manual