Alto D4.indd

Alto D4
Power amplifier
Mike Crofts
Having sold most of my previous passive
speaker system and amp rack a couple
of years ago in favour of an all-active
system, I have occasionally regretted not
hanging on to the monitors and amps.
Finding good-quality, lightweight passive
monitor speakers isn’t a problem (I now have
various JBL and EV models), but as I usually
run four foldback mixes I really didn’t want
to go back to lugging four heavy amps
about in a rack, so I decided to check out
the Alto D series amps, which are very light
in weight and are available in four-channel
versions.
The Alto D series consists of four models:
the D1 is a two-channel, 500W-per-side into
4Ω unit, and the D3 is a more powerful version
at 750W per channel; the D2 and D4 both have
four separate power channels and are rated
PerformingMusician
Alto D4 £688
The Alto D4 is a lightweight power amp
that marries convenience with flexibility.
It packs four 750W channels into a 2U rack
that weighs less than 8kg, making it ideal
for powering passive PA speakers and
monitors at small to medium-sized gigs.
Proel International + 44 (0)20 8761 9911
www.proelint.co.uk
www.altopa.com
Active speakers are wonderful things, but for some applications —
foldback in particular — you can’t beat the simplicity of a speaker
cable, a lightweight floor monitor and an amp to power it, especially
when looking after the sound for a multi-act show.
at 4 x 500W and 4 x 750W, respectively. All the
models are 2U rack height, and the heavier
four-channel ones are still only 7.9kg — so in
went my order for a D4.
The amp arrived very well packed, and
even though I already knew the weight, it
was a pleasant surprise to lift it out of the box
without any effort. The D4 has a distinctive
silver front panel, which is perforated to
allow cooling air to flow. The power switch
is right in the middle and lights up blue
when the amp is running, and the four level
controls are in a neat, recessed panel in the
lower left-hand corner. There are three LED
indicators alongside each input level control,
labelled Sig, Clip and Prot. The first lights up
green when signal (above -20dB) is applied to
the input, the middle one warns that clipping
is occurring and the Prot light comes on when
the onboard protection circuitry has kicked in
for some reason. The D4 will self-protect if the
output is shorted or overloaded (ie. too much
current is being demanded by the load) or if
the temperature rises above a safe operating
limit.
The rear panel is well populated, with all the
input/output connectors and a large air intake
for the seven-speed automatic cooling fan. The
manual says that airflow is from rear to front
to avoid temperature rise inside the rack —
which would indeed be great, except that the
airflow actually goes the other way on the one
I bought. This may be due to an incorrectly
wired fan motor, or the manual may be wrong,
but the presence of a dust net inside the front
panel suggests to me that the air is supposed
to enter at the front, otherwise foreign bodies
would be sucked in and trapped inside the
casing, which doesn’t seem like such a good
LEDs for each channel indicate whether a signal
is present, clipping is occurring or the built-in
protection system has kicked in.
98 August 2008 | performing-musician.com

idea. Either way, access to the dust net for
cleaning can be gained by removing the six
screws that hold the front panel in place.
A bridged D4
The four inputs are over to the left side as
you look at the rear of the amplifier. The
connectors are of the Combo XLR/jack type,
which is a nice practical feature, and they
are grouped into two pairs with associated
The manual
states that
airflow is from
rear to front,
which would
minimise heating
at the back of
a rack, but in fact
the fan blows air
from the front of
the amp to the
rear.
configuration switches. The various input
options are ‘parallel’, where the inputs are
simply connected together so that anything
connected to either input 1 or 2 will be output
from both channels 1 and 2, and similarly
for channels 3 and 4. In ‘stereo’ mode the
channels are completely independent, which
is how they would be set up for four separate
mixes. Finally, ‘bridged’ mode connects the
first input of each pair (ie. Inputs 1 and 3) to
both channels of that pair operating in bridge
mode, to deliver twice the voltage swing — if
both input switches are set to ‘bridge’ the D4
acts like a stereo amp capable of delivering
1500W per channel into 8Ω loads. It’s worth
checking the position of these little three-way
slider switches before using the amp though,
as it’s possible that they could accidentally
be moved when plugging or unplugging
the input connectors. Alongside each pair of
inputs is another small slider switch, which
introduces a 30Hz high-pass filter, but it only
operates on the two paired channels together,
only convenient but it also eliminates the risk
of using an inappropriate fuse.
Light or Watt?
So far I’m happy with my Alto D4, in that it
does the job it was bought for, it seems pretty
well built and, above all, it’s convenient in
terms of space and weight. I have the D4
mounted in a 4U rack case; it’s so light that
I can use the handle on the front lid and carry
it easily with one hand — great for getting
through single doors and reducing the
Tech Spec
D4
• 4-channel power amplifier.
• Frequency response: 20Hz - 20kHz
±0.1dB.
• Input sensitivity @ 4Ω: 1V RMS.
• Input impedance: 10kΩ unbalanced,
20kΩ balanced.
• Clipping point: 10V RMS.
• SNR: 105dBA.
• 4 x 750W @ 4Ω.
• 4 x 370W @ 8Ω.
• 2 x 740W @ 16Ω (bridged mode).
• 2 x 1500W @ 8Ω (bridged mode).
• THD: 1%.
• 30Hz low-pass filter.
• Signal, clip & protection LED
indicators.
• Dimensions (WDH) 483 x 411 x 89mm.
• Weight: 7.9kg.
so you can’t, for example, have the filter
engaged for Channel 1 but not for Channel 2.
In general use this filter would normally be left
switched in, but if you’ve set a high-pass filter
on an external crossover, bear in mind that the
LF roll-off slope will be affected if you leave
this filter in as well.
Over on the other side, beyond the fan
intake, are the output connectors. A choice of
Speakon connectors or standard binding posts
is provided to suit virtually every occasion,
and there is enough space around these to
make connections easily when the amp is
fixed inside a rack. The final detail on the rear
panel is the standard IEC power connector
and the power breaker reset button — if
excessive AC supply current is detected, this
breaker will pop, and can be reset manually
by pushing it in again provided everything
is back to normal. I like this feature — it’s not
The D4 offers a choice of Speakon or binding post
connectors for hooking up your speakers.
number of trips to and from the van. So far
I have only used the D4 ‘live’ with my monitors,
and all I can say is that it drives them very
well with plenty of power to spare. It works
quite happily at 4Ω without getting very
warm, and I haven’t yet heard the fan run up
to a high speed. Although I haven’t yet tried
it in bridged mode I am seriously thinking
about getting another D4 to fill the 2U space
left in the rack — I’ll then have eight channels
of monitor power available for larger gigs, or
I could use the second amp bridged and have
a 3kW main system plus four 750W foldback
channels, with less than 16kg of amps in a 4U
rack! That’s 375 Watts per kilo, or roughly
2.7 grams per Watt — which, as should be
the case with power amps, sounds good to
me.
performing-musician.com | August 2008
99