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Alesis POWER AMPLIFIER RA150 Reference

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Connections: Chapter 2

16 RA150/300/500 Reference Manual

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).

1 (float)

2 (hot)

3

(

cold

)

Male XLR

Connector

1/4 "

p

hone

j

ack

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.