This Article Was Originally Published On: February 3, 2015 #Issue 15. 

What if I told you that there is this company that not only pioneered the use of class-D output sections, but also pretty much started the whole micro head thing? What if I told you that this company has been offering such features as high-pass filtering, phase reversal, and balanced XLR inputs (with available phantom power) for over a decade? What if I told you that this company made their speaker enclosures out of injection-molded structural foam? And what if I told you that this company is about 17 years old and based in the United States for all of those years? It may sound like I am discussing a mythical company, which could not possibly exist in reality. But I am not. I am talking about Acoustic Image, and believe me, I’ve only scratched the surface…

We’ll talk more about the impressive Acoustic Image history, below, but first up, I’d like to introduce their impressive latest offerings. Namely, the Flex™ System (Flex Pre and Flex Cab) which AI sent us for review. This product won a Bass Gear Magazine 2013 Winter NAMM Best of Show Award, and we’ve been itching to get a long, hard look at this rig ever since.

728×90 Fender Rumble

Forming the Image

Acoustic Image, or “AI,” has been designing high end, high fidelity musical instrument amplifiers since Rick Jones formed the company back in 1997. Rick, himself, is an interesting chap. His feet were set on the path to AI back in the ’80s, when he was looking for a new rig for his upright bass, and – like so many other great gear innovators – he tried what he could find on the market, was not completely satisfied, and decided to build something himself. Trying his hand at speaker design, he experimented with various approaches, including different vented and sealed-box designs (including isobaric designs), before eventually adopting the down-firing woofer approach. He spent a few years optimizing the box, driver and crossover designs, but during this time period, he also co-founded a telecommunications company (which designed fiber optic systems) that went public in 1993, and obviously took up a lot of his time.

Working out of Rick’s garage, AI’s first product was a “D-shaped” speaker enclosure, called the D-10 (which bears strong resemblance to their current enclosures). Rick wanted to add a power amp to the enclosure, and found a small company making OEM class-D amps. This was before Bang & Olufson (“B&O”) took the class-D market by storm, mind you. Once a preamp and power amp had been developed, the D-10 morphed into the CD10 combo.

Things got more serious for AI in 1999, when they brought some prototypes to the Winter NAMM Show. Later in 1999, Paul Ingbretsen and Chris Jones (Rick’s son) joined the company, and they remain with AI, still. In October of 1999, Acoustic Image shipped their first production units (the Contra combo). While the combo was well received, some folks asked for a head unit. So, they took the electronics out of the Contra and created the Clarus head. These first units were single-channel designs. When they added the second channel (in 2000), the 2-channel combo became the Coda, and the 2-channel head became the Clarus 2R.

Remember that OEM class-D manufacturer I mentioned? Well, Rick being Rick, he not only utilized their amps, he got involved in the company and used his background in high tech startup companies to help them raise money. Eventually, he became part of the company. Of course, a short time later, the B&O juggernaut came into its own and this small amplifier company went out of business. Seizing opportunity out of ruin, however, Rick would eventually incorporate B&O amplifier modules into AI designs (more on this, below).


Rick parted ways with his telecom company in 2001 and took a job as a professor at Duke. Well, as they say, “it didn’t take,” and before long, AI started a major product redesign; by mid-2002, Rick left Duke and has been working full-time at Acoustic Image ever since (he had been working at AI part-time up until this point).

Their combos, cabs, and heads have always been on the compact side, but the first truly “micro” head which I recall playing through was the Clarus SL (the “SL” stands for “Smaller Lighter”). This head incorporated a switch-mode power supply, and put out 250 watts into a 4-ohm load (it could hit 350 watts into a 2-ohm load). It did not have an XLR input, though it did feature both high and low-impedance ¼” inputs. At the time of its introduction in 2004, this was a truly diminutive head, but what really caught my attention was the superior tone and greater than expected volume from “only” 250 watts. I distinctly recall playing through a bunch of heads at a bass-only shop and being amazed that the best-sounding head was also the smallest! Another milestone AI head is the original (Series II) Focus, also introduced in 2004. It was the first compact head (weighing it at under 4 lbs) to hit 600 watts (into 4 ohms), and it was packed with features few, if any, other brands could match. The Series III iteration of the Focus head would up the output to 800 watts. With the recent refreshing of the AI product lineup, the Focus name has been dropped, but the Clarus head now puts out 650 watts (4 ohms), effectively filling its shoes.

The AI mantra is that clear, natural, “flat” sound comes first … followed closely by portability and practicality. The cylindrical shape allows for a very compact design, and these enclosures are built from lightweight materials. So the portability part seems covered. But what about those top priority goals? Well, for starters, those small, cylindrical enclosures pack a down-firing woofer in addition to the front-firing driver. The enclosures are made from acoustically inert polymer materials, which, in addition to fighting unwanted vibrations in the cab, also allow for smaller, lighter enclosures. The preamps have always been designed to be “instrument-neutral,” and will work with a wide variety of instruments, from guitars, to basses, to violins, to keyboards. Many of their designs feature 2-channel front ends, and have an intentionally uncolored voice when set “flat,” but good tone control – including low cut (high pass filtering) and phase reversal options.

Flexing Some Engineering Muscle

Let’s take a look at what the Flex System has to offer, starting with the Flex Pre. While this preamp has obviously been designed to work in conjunction with the Flex Cab, I’d like to point out that this is a great stand-alone preamp in its own right. The Flex Pre is a 2-channel preamp, with identical controls for each channel up through the input and tone stack stages. There are separate 1/4″ and XLR inputs for each channel. In keeping with the AI philosophy of building rigs which can be used by a wide variety of instruments, the 1/4″ input is optimized to work well with piezo-equipped instruments, and features a 1 MOhm input impedance. The balanced XLR input has a low-impedance input suitable for microphone use. In addition, this input also has the option of bumping up the gain by 10dB, as well as 48v phantom power, for those mics which require it. Following the inputs, we find the input gain Level control. If you have to deal with an especially hot input, there is an internal jumper (one for each channel) which can be adjusted to provide 6dB of gain reduction. Note: both inputs on each channel can be used at the same time, allowing for the possibility of handling four simultaneous inputs.

Moving on to the tone stack, AI has added an additional band of EQ, moving from a 3-band to a 4-band setup. The Low control is a shelving-type, affecting frequencies below 150Hz. The Lo Mid control affects frequencies between 70Hz and 700Hz (centered at 250Hz). The Hi Mid control affects frequencies between 700Hz and 3kHz (centered at 1.2kHz). The Hi control is a shelving-type, affecting frequencies above 3.5kHz. All four bands offer 12dB of boost or cut. In addition to this more conventional EQ, the Flex Pre also offers a Low Cut feature. This control is basically a variable high-pass filter, but with what AI hopes is a name that more closely relates to the function. The Freq control allows you vary the starting point of the 12dB/octave rolloff between 30Hz and 150Hz. These types of controls are very useful when it comes to controlling unwanted vibrations or a boomy room, and it’s one of my favorite features on the Flex Pre.


Another feature, which is sure to appeal to the player who routinely blends a mic’d signal with a pickup, is the phase reverse switch. It is not uncommon for a mic’d signal to be out of phase in relation to the signal from the pickup, and this can cause phase-related interactions (cancellation in some frequencies, augmentation in others, etc). In addition, mic feedback can sometimes be reduced by reversing the phase of the signal. A nice option to have on hand. However, if you find that you do not have a need for the phase reverse option, there is another internal jumper which can be adjusted so as to convert the phase reverse switch to a speaker emulation filter (which is a feature that is likely more attractive to guitar players than to bass players).

After the two channels, we have an onboard stereo 24-bit digital effects processor, which may be applied to either channel, or to both. A 4-position switch determines which effects (if any) are applied. The four options are off, reverb, reverb and delay together, or delay. Controls are also provided for Level (your wet/dry mix control) and for Rate (reverb decay time or delay time). The Master volume control comes right after the balanced XLR Direct Out (which conveys the combined output of both channels). The input Level control affects the level of the DI signal, whereas the Master volume does not. Above the DI, we have push-button control for Post EQ (the signal is pre-EQ if the Post EQ switch is not engaged), Mute, DI Pad (10dB), and Ground Lift.

The Flex Pre has a rather unique shape, with a slanted front panel which is designed to work well when the player is positioned higher than the Flex Pre and looking down on the unit. AI has provided elegant solutions for either placing the unit on top of the Flex Cab (where it is magnetically held in place) or mounting it to a mic stand, and placing the Flex Pre wherever the player wants it, while placing the Flex Cab in its optimal location. If those options aren’t enough, it can also be placed upon any flat surface (resting upon four small rubber feet), or even mounted on the side of a music stand or mic stand, via an available mounting bracket.

It does have a small, but feature-packed, back panel, which belies the Flex Pre’s pseudo-stereo configuration. From left to right, we find two rows of 1/4″ jacks (one for each channel) for mono Preamp Out, Effects Loop return and send, and Stereo Out. There is also a 1/8″ stereo headphone output jack, labeled “H/P.” Plugging into this jack mutes the other preamp outs.

One of the more unique features of the Flex Pre is the CAT5 “Ethernet” (RJ-45) output jack. This is the primary connection between the Flex Pre and the Flex Cab, and when used in conjunction with the Flex Cab, power to the Flex Pre is provided via this input. However, if the Flex Pre is being used independently, power can be provided via the supplied external power supply, which operates anywhere from 100V to 240V. Appropriate adapters are provided to allow worldwide use. The final control on the back panel is the Limiter switch, which reduces the peak signal being sent to the preamp outputs.

A Measure of Reason

Although Acoustic Image initially cut its teeth by designing their own class-D output sections, AI eventually altered course and began using B&O’s ICEpower® modules in their amplifiers (and powered enclosures). Curious as to the reasoning behind this decision, I asked Rick Jones to let me in on why they made this change. On par with their other design and logistical decisions, it was certainly a well-reasoned decision. Here is Rick’s “brief summary” of why they made the change to ICEpower modules:

  • The power amp/power supply module has become a commodity. Just like it makes no sense to develop your own processor for a PC, it makes no sense to produce your own module. And that’s coming from someone who was among the first to use class D amps and SMPS.
  • As long as you connect the feedback to the correct point in the circuit, class D amps sound the same. Due to the nature of the amplification process, the output stage does not contribute to the sound of the amp, it is just a switch. You’re listening to the sound of the output filter and the effect of the feedback. B&O’s design is the same as our old designs in both regards. So, we had no advantage to exploit with a “proprietary” approach. Once again….a commodity.
  • The advantage of building in large volumes is huge. Reliability improves, costs go down, etc. We would build a few hundred amps at a time a couple of times a year and, as a result, would essentially start over on the learning curve with each production run.
  • It turns out that we could still have some proprietary advantages that we could add to the module, like automatic voltage switching and improved heat sinking (still no fans!). Plus, we could exploit the small size and create the compact powered speaker. [I wrote a white paper that summarized the advantages of the powered speaker, which I have attached.]

With regard to that powered speaker comment, when you make a powered head (the Clarus 611IA) with the same features as the Flex Pre, and you make a non-powered cab (the Ten EX 640ex) with similar driver configuration to the Flex Cab, it begs the question, “why put the power amp(s) in the cab, as opposed to the head?” Here is Rick’s response (which he alluded to, above), originally published in a 2011 white paper:

There are four advantages that a powered speaker system has over a passive speaker system: better performance, better protection and safety, better sound, and better value. Each is explained below.

  • Better Performance
    • By collocating the amp with the speaker it is driving, long cables are eliminated. Such cables degrade the performance of the system by reducing the damping factor and raising the Q. Damping refers to how well the amp controls the movement of the speaker cone. If the damping is low (which can be caused by added resistance of a long speaker cable), the speaker cone continues to move after the amp signal ceases. This results in poor transient response. The added resistance of a speaker cable can also increase the Q of the speaker system (the Q describes the nature of the low frequency resonance of the speaker; high Q generally means the resonance is less controlled, leading to poor bass response). Higher Q leads to a “hump” in the bass response and also can deteriorate the transient response of the speaker.
    • Dedicating an amp to a speaker means that the passive crossover used in a multi-speaker system can be eliminated. An active crossover, operated at lower signal level and placed ahead of the power amp in the signal path, is used, instead. As a result, lower power components can be used (op amps versus inductors, capacitors and power resistors), resulting in a crossover that can be more precisely tuned to get the most out of the speakers.
  • Better Protection and Safety
    • Because the output of the power amp is dedicated to the speaker, it is not accessible, and therefore is not exposed to the hazards of the outside world. The output can’t be accidentally shorted, damaging the amp. Today’s high power amplifiers are capable of very high output currents, up to 50 Amps, for brief periods of time. It turns out that the amp output, with its high current capability, is not dangerous to humans, but the spark resulting from shorting the speaker output to a piece of metal can leave a good sized divot in the metal.
    • When the amp and speaker are separate, lots of effort goes into protecting the amp from the speaker, and vice versa. Passive approaches, such as fuses and in-line power sensing resistors, are often used. These types of devices don’t operate quickly, which can result in damage. Faster acting active approaches can be used, but these have to be over-designed to take into account the widely varying circumstances that the separate power amp and speaker present. When the amp and speaker are integrated, active approaches such as clipping limiters and overcurrent sensors can be better used. The designs can be optimized for the specific circumstances, leading to better performance and higher reliability.
  •  Better Sound
    • The power amp and speaker are matched in the powered cabinet, thereby eliminating the guesswork of deciding which speaker to use with which amp. As a result, the advantages listed above (higher damping factor, less effect on Q) produce better sound from the combination. Additionally, the use of an active crossover and dedicating an amp for each speaker in a multi-speaker system results in better control of each driver, which results in better sound.
    • The use of less intrusive, active protection schemes in the integrated system produces better sound, too. Passive, in-line devices inject resistive elements between the amp and speaker. Such devices lower the damping and raise the Q, both of which degrade performance.
  • Better Value

    • Collocating the amp and speaker in the same cabinet eliminates the separate chassis for the power amp. Typically, that chassis is large and relatively heavy. So, there is a significant cost savings.
    • In many circumstances, such as instruments with active pickups (guitars, basses and keyboards are examples), the instrument can be directly connected to the powered speaker. No preamp or amp is needed between the instrument and the speaker. Again, a significant cost savings.
    • As mentioned above, use of active crossovers and protection systems eliminates expensive passive devices. Again, reducing cost.
    • The ability to optimize the design of the amp and speaker as an integrated unit can result in cost savings in each component.

Ah, but you ask, why is it, then, that cost of the Flex Pre and Flex Cab exceeds the cost of the Clarus 611IA head and Ten EX 640ex cab by $200? Well, there are three power amps in the Flex Cab (versus two in the 611IA head), plus the Flex Cab employs an active interface module (versus the passive crossover in the 640ex cab).


The Flex Cab

While the Flex Pre is definitely worth getting excited about, all by itself, it really shines when you combine it with the Flex Cab. The Flex Cab is a powered enclosure, with two 300-watt class-D amplifiers – one for the front-firing 10″ driver, and one for the downward-firing 10″ driver – and a 50-watt class-D amp for the 2.5″ tweeter. The Flex Cab isn’t quite as complex as the Flex Pre, but it is much more than just a powered speaker enclosure.

One of the most immediately visible design elements is the cylindrical shape of Flex Cab (and AI’s other speaker enclosures). AI’s own FAQ on this topic explains:

“One of the biggest problems with speaker cabinets is the fact that the panels of the typical square cabinet vibrate and add unwanted sounds to the output of the amp. The result is that the amp will lose focus and not sound as defined. A cylinder is inherently stiff and does not vibrate like the panel of a typical speaker cabinet, which means our cabinets have better definition and more focus.”

As previously mentioned, the primary connection between the Flex Pre and Flex Cab is the RJ-45 connector – labeled “To Preamp” on the Flex Cab. This connection not only transfers the signal from the Pre to the Cab, it also conveys power from the Cab to the Pre. A separate 1/4″ mono Input is provided, with a 1/4″ Thru output jack located immediately above (allowing for “daisy chaining” multiple cabs). The Master Level predictably controls the output of the two 300-watt amplifiers. A separate Tweeter Level knob controls the 50-watt tweeter amplifier. In between these two controls, we find the DF Cutoff Freq control. This control is a sweepable 12dB/octave low-cut filter which reduces the low frequency output of the downward-firing woofer. The starting frequency of this low cut is adjustable from 30Hz (fully counter clockwise) to 150Hz (fully clockwise). A push-button switch allows for a -6dB adjustment to the gain of the downward-firing woofer’s amp.

Another simple, effective AI feature is the built-in tilt-back feature. There are two legs on the front part of the bottom of the cab which can be extended to tilt the enclosure. In addition to helping to aim the front-firing driver and the tweeter more towards your ears, the tilt mechanism can also help with room-effect boominess. Of course, in some rooms, the Flex Cab may work better for you without being tilted, so it’s definitely a feature to experiment with for best results.

Cut the Engineering Mumbo Jumbo, How Does It Sound?

I’ve played through a number of AI products in the past, so I had a decent idea what to expect: relatively neutral tone; solid, but not hyped, lows to low mids; smooth highs; amplification which does not draw attention to itself. I am happy to say that all of these exemplary traits exist in spades with regard to the Flex System. However, AI didn’t stop, there. The Flex rig has really taken clarity to the next level, and while the rig still doesn’t draw attention to itself, the upper mids to highs really reach out and touch you in a way that I hadn’t noticed before with prior AI rigs. Each time I plugged a new instrument into the Flex System, I was impressed with the transparency of the amplification. Each individual instrument spoke very uniquely, and I always had the impression of hearing that particular instrument, only louder, as opposed to hearing the “rig.”

From a volume perspective, the Flex System is not going to be the loudest 650-watt rig you’ve heard. However, when you factor in its diminutive form factor, it may be one of the loudest “small combo-sized rigs” out there. In the context of the trade-off where you only get to pick two of the three “L traits” – low, light, or loud – AI has pulled a move reminiscent of Kirk and the Kobayashi Maru. The Flex rig definitely goes low, but with a slew of control options (in addition to the EQ stack, you have the Low Cut feature, the choice of tilt-back or not, and further control over the down-firing woofer). The Flex Cab isn’t “beach ball light,” but it is a quite manageable 32 lbs – and remember, that’s for two 10’s, a tweeter, and three amps! In terms of loudness, the Flex System manages to let your instrument be heard in a way that fills a decent-sized stage, but it doesn’t pummel you (or the guitarists, unfortunately) in the process. By treating each of the three L’s as an opportunity for well-reasoned compromises, AI has produced a rig which grabs much of the prize, while minimizing the costs along the way. Well played, Captain.

The move from a 3-band to a 4-band EQ is a big improvement, in my opinion. When you employ shelving controls for bass and treble, you leave a huge chunk of the frequency range available for the bandpass control(s) to handle. If you only employ one midrange control, the frequency selection and Q are critical choices, and choices which make sense for one instrument, say violin, may not work well for another (like electric bass guitar). The midrange frequency centers of 250Hz and 1.2kHz are especially well chosen for both electric and upright bass (though I’m not sure how the violin players feel about these choices), and I really appreciated the move to two midrange tone controls.

I’m not normally a big reverb or delay user, but the effects built into the Flex Pre are really, really good! I especially enjoyed the effect of using both reverb and delay with fretless bass guitar. Spending some time playing through these effects, solo, had me thinking of songs to work into the setlist which would let me take advantage of these superb onboard effects.

Given AI’s history and the large numbers of upright players using their gear, I was not surprised to find that the Flex System really excelled at double bass. My ’50s era Kay (with a Barbera Multi-Transducer bridge) sounded like it was jumping right off of a Chess Records recording. Phil Maneri’s 150-year-old European upright sounded massive, yet sublime. And I loved how the tone just filled the room; your ears aren’t really drawn to the physical position of the rig. That whole omni-directional thing really does work! My Epiphone El Capitan acoustic bass guitar has seen a lot of gigs, and it’s been run through a ton of different rigs. The Flex Pre/Cab combo presented the most “acoustic” tone yet from that bass, and had a very pleasing, natural tone.

Moving on to electric bass, the Flex System really made a P-bass sound like a P-bass, and a J-bass sound like a J-bass. That sounds simplistic, but when you hear it done really well, it’s always eye-opening to realize how much unique tonal ground Leo managed to cover with those two designs. One of my favorite basses to play through the Flex System is my new Zon Legacy Elite IV. The mahogany body and Bartolini pickups give it a meaty, solid tone, and the composite neck and Polyfusion preamp present a sweet, singing high end. That particular Zon tonal recipe was more obvious and glorious with the AI rig than with any other rig I have played through since picking it up. Switching to a Fodera E5S, the Emperor’s balanced tone and “warmth with clarity” was immediately obvious. As I mentioned, above, the volume and output is not going to slam you across the room, like Michael J. Fox. But it does do a great job of filling space effectively.

 To further demonstrate its wide range of capabilities, I even used the Flex System as a small vocal PA. And guess what? It performed this task flawlessly.

One final observation I will add is that this rig is incredibly lacking in noise or hiss. You can really crank the gain, and if you are not plucking the instrument, you can hardly tell it’s on. Very impressive.

The Bottom Line

Rick Jones and Acoustic Image have brought a phenomenal range of technological innovations to market. They’ve produced uniquely gigable amps, cabs and combos – especially for double bass players. The Flex System represents the best amplification system which AI has ever produced. Used independently, the Flex Pre and Flex Cab are highly competent and rewarding products. Used together, the two make for an especially compelling rig. If you value transparent amplification, effective tone and room control, flexible placement for both the enclosure and the controls, and stage-filling capabilities in a compact package, AI has you covered. The American-made Flex System is something special; try one for yourself.

Flex AMP In-hand

Tonal Flexibility:4
Ease of Use:4

EDM-1 In-hand Score 4.17average


The Flex Pre packs a super-intelligent set of features and controls. Great EQ, plus a high-pass filter, mean that you can deal with about any instrument or room. The onboard effects are quite nice.

Flex Cab In-hand

Tonal Flexibility:4
Ease of Use:3.5

EDB-2 In-hand Score 4.25average


The Flex Cab packs a lot into a small package. It is very well made, and fills a room without drawing attention to itself. The clarity, sweetness, and lack of noise/hiss are exceptional.