Principal Features Congratulations on your purchase of the Alesis RA-100 Reference Amplifier. With all the wondrous advances in both analog and digital electronics in the past few years, it’s easy to forget that an audio chain is only as good as its weakest link. With low-cost digital devices now offering sound quality unheard of only a few years ago, Alesis has applied its expertise toward creating a sonically accurate, stable, and affordable stereo power amplifier that’s suitable for the digital age. Optimized for studio monitoring applications and moderate-power live performance setups, the amp’s main features include: • 100 watts per channel into 4Ω, 75 watts per channel into 8Ω • Dual clipping indicators alert you of any type of non-linear operation, not just clipping • Output short circuit protection to minimize down time and protect the amplifier’s circuitry • Massive, conservatively rated, custom-designed extruded heat sinks (individual for each channel) for cool operation • No ventilation fan is needed, allowing for quiet operation and reduced ambient noise in the studio • Extremely low noise and distortion; suitable for quiet applications such as recording studios, church installations, and museums However, some of the RA-100’s most important features are hard to put on a spec sheet. In a quest for a “musical” sounding power amp that can stand up to a tough life on the road as well as continuous operation in the studio, Alesis has made some changes—some minor, and some very significant—to the standard power amp. • The RA-100 doesn’t produce any audio transients during power-up or power-down. Nonetheless, there’s a 2-second mute on power-up to compensate for any pieces of equipment plugged into the RA-100 that produce transients upon power-up, and receive power at the same time as the RA-100. Also, because the amp doesn’t produce power on/off thumps itself, the mute circuit can be pre-power amp. This allows simpler circuitry than what would be required for shutting down the power amp stages. • The RA-100’s power supply has plenty of reserve capacity to handle percussive transients, but if someone kicks the RA-100’s plug out of the wall, you want all amplification to cease immediately. Therefore, if the amp senses any loss of power, it instantly mutes the audio. • Because power-up and -down are noiseless, the on/off switch doubles asmute or “panic” button. • The output stage uses full complementary-symmetry circuitry throughout. Although there are cheaper ways to design an output stage, this time-tested approach is generally considered superior to non-complementary- symmetry types. The output devices are rated at 30 Amps of peak current, and boast a 20 MHz FT (transition frequency) for superior bandwidth. • Extremely good stability with reactive loads (e.g., speakers and crossovers). Stable operation is essential when presented with the changing load characteristics of typical speakers; the RA-100 uses several stabilization techniques to maintain consistent feedback network characteristics. • Alesis recognizes that amplifiers will sometimes be driven into clipping— even though this is not good practice—during a recording session or under the pressures of live performance. As a result, a great of deal of research went into creating the best possible clipping entry/recovery characteristics. Amps with lots of overdrive and saturation tend to create overshoot and ringing during the clipping process, which degrades the sound; the RA-100 has been designed so that it exhibits very little saturation, even when overloaded. As a result, it enters and exits clipping very cleanly. Clean clipping allows the RA-100 to be driven harder, which makes the overall perceived sound louder. a Monitoring a sine wave being clipped confirms this action. With the RA- 100, the amp will follow the original input signal as soon as clipping ceases (a). With some conventional amp designs, the signal will “hang” at the clipped level for a while before catching up with the input signal (b. This saturation-caused overshoot can sometimes sound worse than the clipping itself. • No “speaker stress” during shutdown. With traditional protection circuitry, the act of clamping the output devices to a safe value can make the amp unstable with particular notes that interact with the speaker resonance, thus producing oscillation or “motorboating.” This could damage your speakers. The RA-100 uses an elegant clamping design that observes the load at all times but also integrates the signal characteristics into the clamping action. Thus, the clamping action is gentle and, after clamping to the maximum current encountered in normal operation, will slowly (over 1 to 2 seconds) clamp based on the load and signal characteristics. If the source of the problem (e.g., a short across the speaker terminals) is removed, the signal recovers instantly. • Alesis also recognizes that whatever else, “the show must go on.” As a result, under conditions of extreme abuse (such as ultra-low speaker impedances or mounting that allows for no ventilation), the RA-100 will limit its signal rather than simply shut down. Taken to an extreme, this will produce distortion but sound will still come out.