CORUS Series 4plus Combo Amplifier

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Regular Price: $1,529.00
Special Price: $1,329.00
To Continental US (Lower 48)

Product Overview

The Corus combo (and its matching extension cab) are a specially modified version of Acoustic Image's well-loved Coda combo amps. What's different? While the Coda (and its bigger brother, the Ten2) are truly full-range amps, with the bottom end to be excellent for low-pitched instruments like upright bass and acoustic bass guitar, the speaker array in the Corus combo is "voiced" for higher pitched instruments. Its output has a more "forward" sound, which makes it an excellent choice for instruments like acoustic and "jazz-box" guitar, violin, mandolin, cello, and more. It even works wonderfully with keyboards - check out this review of the Series III Corus (Dec 2008) from Keyboard Magazine!

How is this "more forward" sound accomplished? The difference is the special design woofer, which replaces the standard subwoofer-style speaker in the Coda. The Corus woofer is more efficient, and the crossover network is modified for a different balance between the woofer and the midrange/highrange front panel speaker (which remains the same as the one in the Coda). The Corus, thanks to the increased efficiency (more volume per watt) sounds louder than the Coda at a given level setting and its character sounds a bit more "in your face." The trade-off to the improvement in efficiency - there's always a compromise! - is slightly less bass response.

The Corus is designed to better match the needs for guitar and other instruments that occupy the sonic space above basses. Many keyboard/piano/organ players have found that they prefer the voice of the Corus to the Coda as well, despite the extended range of most keyboards. Frequency response: 60-16kHz (+/- 3 dB) versus 40-16kHz for the Coda, and Efficiency: 93 dB/m versus 90 dB/m for the Coda.

The Corus S4+ combo incorporates a 650W Clarus 2-channel head, mounted into the cabinet using the Cabrio Docking System, which allows you to separate the head from the cabinet for independent use.

A cool built-in "kickstand" mechanism allows the cabinet to be tilted to better direct sound to the player’s listening position. For larger venues, the Ten2 EX (Series 4) extension cabinet is an ideal match.

The Corus amplifier is an all-in-one "combo" amp, meaning that it contains a preamplifier, power amplifier, and speaker system in one compact unit. We've broken down our description below into sections so that you can understand the various parts of the whole.

The Preamp:
A key ingredient in Acoustic Image gear is the very careful design of the electronics. They have always been engineered to deliver the natural sound of your instrument, no artificial flavorings or colors. Please answer this question... do you want to hear your instrument or the amp??

Me, too. My specifications for an ideal sound is for a pickup/mic and amp to simply make my bass sound bigger.

Besides the amazing transparent sound, useful features have been designed into the Acoustic Image preamps to provide the most flexibility for various playing situations. For instance, you get -- on each channel -- a parallel effects loop, and a selectable low-cut (aka high pass) filter to help with feedback or “boominess” elimination (see explanation in features list below). Both channels have on/off access to the built-in digital effects. You also get a balanced direct out with ground lift, as well as pre/post setting, to allow connection to a mixing board for PA support, or for recording purposes. There's even a Phase Switch, which alters the phase by 180°, which can often help with acoustic feedback.

The front end also features the input options for either microphones (XLR) or instruments (¼"), and switchable 48v phantom power -- and a 10db gain switch (for low/high output mics) -- is available on the XLR input. Separate Input Level (gain) and Output Volume Controls allow for precise tonal vs. volume control, and a Master Mute Switch allows you to cut the output for silent tuning or for breaks.

The two-channel, Series 4PLUS Corus has two separate channels which can be used to blend two sources - two different pickups or mics (or a combination of the two), or two different instruments. The XLR and ¼" inputs are on separate jacks. Additionally, they are separately buffered, which means that you could effectively use both at once, on each channel. You would only have independent control if at least one of those sources had its own volume control (like a keyboard, or an instrument with an outboard active preamp) but it does open up the possibility of actually "mixing" up to four different sources.

The preamp features a new four-channel equalizer, as follows:

  • Low: Shelving, ±12dB at 50Hz
  • Lo Mid: ±12dB at 250 Hz
  • Hi Mid: ±12dB at 1200 Hz
  • Treble: Shelving, ±12dB at 8 kHz

The built-in Digital Effects can be assigned to either channel (or both/neither), and the amount of the effects in your mixed signal can be adjusted with the level knob. The rate setting is now configurable (!) and the effects include the following presets:

  1. Reverb
  2. Delay
  3. Reverb PLUS Delay

One last new feature - illuminated on/off buttons for the front panel controls to indicate whether the button is on (lit) or off (unlit). This is really helpful on a dark stage!

The Power Amplifier:
More Power!! (please insert your own Tim Allen Home Improvement TV show grunts) Yes, it can put out up to 650 watts (at 4 ohms). That said, the combos are not rock and roll amps — a small combo shouldn't be expected to compete with your 4x12 Marshall half-stack! And it doesn't sound like a Fender Twin.

With that sort of power, this combo can get quite loud. But the wattage is there to ensure smooth, clean response. The somewhat percussive nature of stringed instruments can be very demanding. The "extra" energy (commonly called "headroom") is available to cleanly handle those peaks and percussive plucks and heavy strums, so you don't get that "squashed" sort of attack many amps often deliver when they run out of gas.

The wattage is certainly handy to have when you have larger gigs, too. Lots of players gig Acoustic Image Clarus heads (which is identical to the preamp/power amp section of this combo) along with a more conventional and larger speaker cabinet for bigger venues. And since the head "pops out" easily and quickly -- 30 seconds or less, no tools required -- you have that option available to you with this combo! I, myself, have taken the head out of my Coda combo, for use with a larger speaker setup, on electric bass/classic rock gigs. It did an admirable job driving a pair of Euphonic Audio Wizzy 10 cabs -- it makes for a formidible, yet still very light and highly portable, rig.

The Speakers:
The design and performance of the Acoustic Image speaker array is quite revolutionary. The lowest bass frequencies are omnidirectional. As we all know, our acoustic instruments do not radiate only in one direction — their sound moves about the room. The ten inch down-firing woofer in the Acoustic Image combos spreads that sound around naturally, as our ears might expect. There is still plenty of definition, it's just not as annoyingly directional as so many other amps can be. With other amps, you might walk across the front of the stage, and suddenly you walk into a small "zone" where THE guitarist's amp is SHOUTING at you. Not with this amp.

But that's only one of the differences between it and other bass amps. These combos have a complement of drivers that results in a full range speaker cabinet. In addition to the woofer, you'll find a five inch cone mid/high driver. Plug your MP3 player in and be amazed at the result! Nice tight bass response, with balanced mids and highs; real, uncolored sound. As pure and clear as the sound in front of your instrument, no added "shading."

What you get is flat frequency response across the spectrum, extended, tight, well-controlled natural bass sound, and omnidirectional output of the lows (so you can be heard, without monitors, all over the bandstand) — all in a package that's about foot tall and less than 23 pounds in weight! In fact, the amp virtually disappears on stage leaving behind the natural sound of your instrument - it's as if your instrument got a lot bigger and makes more acoustic sound!


  • Cabrio Docking System allows you to separate the amp and speaker components for separate use!
  • 1/4" or XLR input; input impedances are 1 megohm or 600 ohms based on jack chosen, separately buffered
  • Effects Loops via 1/4" phone jacks for each channel
  • Phantom power (switchable) on XLR inputs
  • Channel Input and master level controls
  • Balanced XLR Line Output with Ground Lift
  • PRE- or POST- switch for XLR output
  • Mute switch
  • Headphone output
  • AC Power On/Off switch with LED Indicator
  • 650-Watt (@4 ohms) "Digital" Power Amplifier -- clean power! (Internal speakers are 8 ohms)
  • External Speaker Output (Neutrik Speakon)
  • Two-way Sealed Speaker System
  • 10" Long-throw Low-frequency Woofer
  • 5" Mid/High-frequency Driver
  • Detachable AC Input Connector with Built-in Fuse
  • Dual voltage operation (115v/230v, 50/60 Hz) via automatic switch (comes with USA Cable; to use in other countries, you can source an IEC-C13 AC Cable, also commonly used for computer power supplies.)


  • Molded polymer results in a stiff but light enclosure
  • Lightweight- less than 23 lbs! with Durable Molded Enclosure
  • 12" H x 15" W x 13" D
  • Molded Carry Handles
  • Fitted slip cover with cable pocket and shoulder strap included

Includes Vinyl Cover -- Included with all of the combo and extension speaker (EX) models is a lightweight "dust cover" with shoulder strap and accessory pocket. Made of lightweight vinyl, it covers the top and sides of the cabinet, with openings to allow access to the amplifier's integrated handles. It also comes with a carry strap which loops through the openings and the amp handles, allowing you to "shoulder" the amp. The cover is handy - it even has a small pocket for cables, tuners, and other small gigging accessories. But don't expect it to substitute for a case, as it offers only minimal "protection" from slight bumps and other minor hazards.

Warranty Information

Acoustic Image's Warranty, Simply Stated

We stand behind our products with a full warranty of five years from the date of purchase. Speaker components are warranted for 180 days. Should a problem arise, please call us before returning your amplifier or enclosure. Naturally, our warranty does not cover products that have been damaged through misuse.

Bottom line, we don't say that ours is a "lifetime warranty" or even a "limited lifetime warranty". We do, however, commit to treat you the way we would like to be treated. For good.


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