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Use a small tool or pen to change the setting of this switch only if it is set to the wrong value for the country the amplifier will be used in. When changing voltages the fuse must be changed to the correct rating as shown on page 10. STEREO/BRIDGED MONO SWITCH Underneath the right input jack, this switch converts the RA150/300/500 from its normal operation as a two-channel amplifier to operation as a single-channel amplifier with approximately twice the power. Make sure this switch is set properly for your application. See page 20. If this switch is accidentally switched to Mono while wired for stereo, you will hear out-of-phase audio as the Channel 2/Right control is turned up. CHECKING FOR PROPER POLARITY To check for correct speaker polarity, briefly connect the + terminal of a 1.5V battery to the speaker cable’s “hot” or + lead, and the battery’s -terminal to the speaker cable’s “cold,” ground, or - lead. You will hear a “pop” from the loudspeaker as you connect the battery, and another as you disconnect it. Observe the direction of the speaker cone movement. If the speaker cables are wired in the common manner (and the speakers themselves are not mislabelled), the speaker cone will move forward (toward you) when you connect the battery and away from you when the battery is disconnected. If the speaker cone moves in the opposite direction, reverse the wires going to the speaker and re-test for proper polarity. Always check your speakers' polarity as not all manufacturers follow the same wiring convention. RA150/300/500 Reference Manual Chapter 3: Operation USING THE RA150 AS A HEADPHONE AMPLIFIER The RA150 is particularly suited for use driving high-impedance headphones in studio applications. However, the power is considerably greater than that provided by most "headphone amplifiers", which are often rated at 1 watt or below. Due to the high efficiency of headphones and the power of the amplifier, serious hearing damage may result from misuse of this product. Exercise caution regarding sound levels. Keep the following in mind: • Many studio-quality headphones feature impedances of 600 ohms or greater. This means that many headphones of the same type may be plugged in parallel to the output of the RA150/300/500 before the 4ohm limit is reached. • If you plug in a headphone with a lower 8ohm impedance, it will be dramatically higher in level than the others, and the high-impedance headphones may drop to inaudibility. • The TRS 1/4" jack used in headphones will tie together the grounds of the left and right amplifier channels. Use caution and only connect or disconnect head-phones when the amplifier is powered off. • Keep the RA150's front panel volume controls at a very low level and the controls of the headphone send near maximum. This will prevent accidental "blasting" during a session. CHOOSING THE CORRECT SPEAKERS Near-field monitoring through “reference” speakers has become the preferred way to monitor and mix music. With near-field monitoring, small speakers are placed so that they are a few feet from the engineer’s ears. As a result, room acoustics become less important since the primary acoustic interaction involves direct sound from the speakers rather than reflected sounds from the room. Since few home and project studios have good acoustics, near- field monitors can provide realistic monitoring in a small space at relatively low levels. Near-field monitors offer other advantages compared to large studio speakers, including smaller size, lower cost, and easier transportability to other studios for reference purpose. Because of its moderate power rating, excellent fidelity, and lack of a noise-generating fan, the RA150/300/500 “Reference Amplifier” excels in driving reference near-field monitor speakers in smaller studios. However, you should choose speakers that can handle the power the RA150/300/500 can generate. Speaker wattage ratings are often confusing, and standards by which ratings are obtained vary from manufacturer to manufacturer. If a speaker can handle 100 watts RMS continuous power, it should be able to handle the RA150/300/500. However, under conditions of clipping or other abuse of the RA150/300/500, damage to speakers is possible. For best results, use speakers designed for medium-to high-power applications. RA150/300/500 Reference Manual Operation: Chapter 3 ABOUT GROUND LOOPS A hum or buzz may be introduced into some audio systems by a phenomenon known as a ground loop. This can occur if a piece of equipment “sees” two or more different paths to ground, as shown below. Device A Device B shielded cable path 2 path 1 To AC power service One path goes from device A to ground via the ground terminal of the three-conductor AC power cord, but A also sees a path to ground through the shielded cable and AC ground of device B. Because ground wires have a small amount of resistance, small amounts of current can flow through ground and generate a voltage along the cable shield. This si...

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