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.