Review: Acoustic Image Flex System

Amplifying the upright bass is a problem that has generated many solutions over the years (see the Fender Bass), but back in 1997, Acoustic Image made an impact with its answer: lightweight, high-fidelity amplifiers and fresh cabinet designs that offered portability and serious low-end dispersion.
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Amplifying the upright bass is a problem that has generated many solutions over the years (see the Fender Bass), but back in 1997, Acoustic Image made an impact with its answer: lightweight, high-fidelity amplifiers and fresh cabinet designs that offered portability and serious low-end dispersion. Acoustic Image’s current combos, heads, and cabs cater to upright bassists, although they have become popular with acoustic players of all stripes, as well as keyboard players looking for a small full-range rig. The new Flex System is AI’s newest twist, and it could prove to be the company’s most versatile product yet.

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Comprising the Flex Pre preamp and Flex Cab enclosure, the Flex system is designed to work together or separately; both halves are feature-loaded and well designed. The Flex Pre has two identical channels, with q" and 48-volt phantom- powered xlr inputs with a 10dB gain bump. The 4-band EQ uses a shelving-type low control starting at 50Hz, low mid centered at 250Hz, hi mid peaking at 1.2kHz, and a shelving EQ for the highs at 8kHz (all controls offer ±12dB of gain). Each channel has a low-cut filter with the all-important phase reverse switch, which can be converted to a –12dB per octave cut at 4kHz (for speaker emulation) by moving a jumper. While not essential for most bassists, the Flex Pre features digital reverb and delay that can be applied to each channel individually. Around back, two q" stereo-out jacks allow you to operate from channel 2 in “pseudo-stereo” mode. As a standalone piece, the Flex is powered with the included wall-wart power supply, but when used with the powered cab, the unit operates via an included proprietary balanced cable. A padded gig bag, mic-stand adapter, and international ac adapters are also included.

The Flex Cab’s semi-cylindrical shape is a hallmark of Acoustic Image’s design concept. Stiff cabinets are ideal for reproducing bass frequencies, but traditional box shapes and materials can result in a heavy enclosure. The semi-cylindrical design allows for a stiffer enclosure using lightweight materials, in this case, structural foam. Each speaker has a dedicated Class D switching amplifier: 300 watts for each of the two woofers, and 50 watts for the tweeter, making this a 650-watt powered 2x10 cab that weighs 32 pounds. The amp automatically selects the correct mode for voltages between 100V and 240V, eliminating the need for a voltage switch and the possibility of user error. The master level knob controls all three amps; the tweeter level knob offers up to 10dB of cut or 6dB of boost. To help the cab adapt to different environments, a cutoff frequency control for the downfiring woofer applies a –12dB-per-octave cut, sweepable from 30Hz to 150Hz. Although the same filter appears on the Flex Pre, the redundancy provides the function when the preamp and cab are used separately (in some rooms, you’ll be glad you have both). The DF level button drops the woofer’s output by 6dB, and a spring-loaded tilt stand is built into the cab—two more tools to ward off potential boominess. The Flex Pre is held in place atop the cabinet by strong magnets near the recessed screws. This works, but keep your smart phone, iPad, and credit cards away.

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Considering the system’s range of use, my tests involved several combinations of the “Flex Twins” with assorted pieces of equipment I use to amplify my uprights, as well as several electric basses. Taking advantage of the 1MΩ input, I plugged my copper Realist-equipped Juzek straight into the Flex Pre and heard clear, fast response, and lots of bottom. In my studio, the cab sounded best with the –6dB cut on the downfiring woofer, along with the cutoff frequency control set to about 1:00. As the Realist is a dark pickup by nature, it wasn’t activating the tweeter much, even with a treble boost. But the hi mid control added presence and bite to the attack, creating a well-balanced, direct tone with impact. I had to adjust to the way the Flex rig changes how I hear myself, but I grew to love it: Rather than having my amp up on a chair pointing at my head, or on the floor throwing the sound past my ankles, I found that the downfiring woofer creates a “3-D” sonic experience—the sound seems to come from all around. The key is to tune the lows to the environment, and the Flex system certainly has plenty of ways to do that. The plentiful low-cut filters are a boon; my strategy is to roll a knob clockwise, removing most of the bottom, and then rotate it back until I hear the low-end level that makes me happy (usually that setting is around 12:00). While the difference below that point is subtle, removing the subharmonic frequencies not only lets the amp work more efficiently, it greatly decreases troublesome resonances in bad acoustic environments. Unfortunately, my bridge-mounted Crown mic has a q" jack, so I couldn’t run it into the Flex Pre, as the necessary phantom power runs from the xlr input.

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To hear the cab’s full sonic capabilities, I used my old Fishman Blender as a preamp and ran the combined Realist/Crown mic signal straight into the Flex cab. The high end from the mic was natural and airy, with surprising headroom, and it balanced with the ample lows to give my bass a commanding presence in the room. Once I was able to feed the cab a full-range signal, its true intent came through: the sound was omnidirectional, balanced, broad-spectrum, and faithful to the instrument’s acoustic character.

The Channel Blaster pickup on my Blast Cult upright has both bridge and fingerboard transducers, so having four bands of EQ, phase control, and low-frequency rolloff available for both elements is very handy. For roots/slap-style playing, the high midrange is an important place to cut; while the Acoustic Image system is well positioned to help there, I found the slap tone through the Flex cab to be a bit strident. Slapping and plucking with a piezo pickup sends big dynamic spikes into the system, and while the Flex cab handled the abuse, ironically, it seemed too accurate. Using the identical settings, I ran the Flex Pre through a Genz Benz Streamliner/Uber 212 rig, applied a hefty mid cut at 2.5kHz, and suddenly was in rockabilly heaven. The Flex Pre’s EQ and other features are a great match for this type of playing (and my pickup system), but the Genz’s tube front end seemed more forgiving when being stuck repeatedly with the musical ice pick that is slap upright.

Of course, you can play electric bass through the Flex system, too. A passive P-Bass sounded full and round, with a pleasant “air space” around the fundamental tone. Despite its power rating, this is not an amp to play rock & roll bass with, but it will present a great fingerstyle or mellow pick tone if handled wisely. A –6dB cut is attainable with a jumper option, which helps the system deal with high-output active basses, but I still wouldn’t recommend this as a full-on slap/funk rig. Still, the amp shines in many other ways. I plugged my K&K Pure Mini-equipped Goldtone acoustic bass guitar into the Flex system and was treated to huge tone, precise detail, and no feedback. Even my old Takamine flattop acoustic sounded like a million bucks through the Acoustic Image. Using the Flex as a rehearsal PA system, I tried a few dynamic mics and found that vocals sound surprisingly good through it (and yes, the reverb was much appreciated). Acoustic Image sent us two Flex cabs to review, as it’s possible to daisy-chain them, run them in “pseudo-stereo” mode, or even run them in true stereo with another jumper option. I set one cab on either side of me while playing my Juzek and felt invincible.

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Used separately or together, the Acoustic Image Flex System really delivers the goods for upright players, but it can function as a full-range system for any acoustic instrument, keyboards, jazz guitar, or even vocals.


Acoustic Image

Flex System
Street Flex Pre, $700; Flex Cab, $1,600
Pros Pure acoustic tone, great low-end response and control, small and powerful, versatile.
Cons Cab does not agree with high-transient styles like slap.


Flex Pre
Inputs Two q" inst inputs (1MΩ), two xlr mic inputs with 48V phantom power and 10dB gain, two q" effect returns
Outputs xlr balanced direct out with pre/ post EQ, 10dB pad, and ground lift, two q" mono preamp outs, two q" stereo outs, r" headphone out, two q" effect sends, RJ-45 balanced signal/power interface
Controls Phantom power, 10dB gain, level, low, lo mid, hi mid, high, low-cut on/off, lowcut phase, low-cut freq, effect channel select, effect on/off, effect level, effect rate, mute, master, limiter
Power Powered via Cat 5 “Flex Cable” from cab, or separate power supply

Flex Cab
Frequency response 30Hz–18kHz Max SPL 118dB @ 1m
AC power 100V/240V (auto switch)
Dimensions 14" H x 15" W x 13" D Weight 32 lbs
Inputs Flex Pre interface RJ-45 connector, q" input
Output q" thru
Controls power, level, freq (DF filter), level (DF filter)
DF filter 12dB per octave (30Hz–150Hz sweepable)
DF level 0dB, –6dB switchable
Power Two 300-watt Class D amps, one 50-watt Class D amp
Speakers Downfiring 10" ceramic woofer, forward-facing 10" ceramic woofer, forward-facing 2.5" tweeter
Crossover Active, 12dB/oct