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MXR Omni

Working with separate effects devices conventionally linked together can be something of a headache when you need to switch quickly between them on stage. The Omni effects unit from MXR goes some way to relieving the tension by offering a semi-programmable effects facility.

The Omni is a 19" rack-mounting multiple effects system with effect chain preselection capabilities provided by a floor-mounted remote control footswitch unit. The rack unit occupies a 2U high space and has a grey patterned front panel that is neatly divided into six sections with controls for Compressor, Distortion, Equalizer, Delay, combined Flanger/Chorus and an external effects link.

Main unit

A high impedance (500 kilohms) mono input socket is provided on the top left of the front panel which will happily accept signals from most guitars, keyboards and microphones. Directly below this are two further jack sockets labelled Out 1 and Out 2. These provide low impedance (470 ohm) outputs which are capable of driving fairly long lengths of cable if required. Out 1 is the main output whose level is determined by a Master Level knob on the top right of the panel, giving a maximum of 6dB gain and should be used to match the Omni to an amp or mixing desk's operating level.

Output 2 provides an inverted output (180 degrees out of phase) when a mono jack is connected. With a stereo lead connected and the Delay effect selected access can be obtained to the 'delay only' signal at the 'ring' and an inverted dry output at the 'tip'. Using both outputs allows stereo image effects to be created by feeding individual outputs either to separate amplifiers or to separate channels of a mixer.

All front panel knobs are identical being large and black with a smooth action to them; their knurled edges making them easy to manipulate. Unfortunately MXR haven't calibrated them for cosmetic rather than financial reasons one suspects. To some extent this omission is satisfactory as it forces the user to 'tune' his sounds by ear rather than automatically turning a knob to control mark 7 say, and leaving it. However, calibrations of some sort (other than the tiny, central red dot provided) are necessary for live performance when a player may wish to vary the settings of an effect quickly and accurately.

Each front panel section features a brightly lit, red LED which indicates that the effect is on. Now to the Omni's effects themselves.


A compressor is basically an averaging device which attenuates signals above a certain level or threshold and amplifies lower level signals. A compressor used on a guitar signal for example, can artificially increase the guitar's natural sustain properties by continually boosting a note as it gradually decays thus making it appear to sustain longer.

This circuit is based on a current-controlled op-amp and provides automatic variation of the circuit attack and release times according to the type of input signal. Two controls are provided: Sustain and Level which effectively allow the user to select the amount of signal compression that takes place. Sustain sets the threshold level and turning this control clockwise gradually lowers this threshold and increases the circuit sensitivity to low level signals. Once the Sustain control has been set, the Level control can then be used to match the volume of the compressed signal to the original.

The compressor proved efficient in use. Vocals worked best with Sustain set around 12 o'clock as the peaks from loud, erratic vocals were compressed, resulting in a smoother performance. With a guitar input and Sustain on full, plucked harmonics sounded almost as loud as power chords, which means that your lead solo can be heard at the same level as your rhythm work. A positive boon for live gigs! One point to remember about compressors in general is that a low threshold amplifies very quiet sounds along with cable noise, hum and, in the case of microphones, handling noise - so be warned!


This section has controls marked Level and Drive. Level controls the volume of the distorted signal whilst Drive controls the amount of 'clipping'. The distortion produced is a 'broken up' fuzz sound with little sustain of its own. You really need to use the Compressor in conjunction with the Distortion if you want to achieve those Santana-like soaring leads on guitar. The Drive control itself had no appreciable effect until the knob was advanced past the 2 o'clock position.

A pushbutton switch next to the Drive control lets you route the Distortion into the Equalizer or the reverse when the button is pushed in. This fairly unique feature does have an appreciable effect. The distortion sound becomes very raspy and biting when the Equalizer is placed before the Distortion as the signal harmonics are being emphasised. The overall character and tone of the distortion can be drastically modified by the combined use of the Equalizer's controls.

Internal layout of the main unit.


This uses second order 'shelving' type filters to split the audio bandwidth into three bands - Bass, Mid and Treble. Bass covers the frequency range 10Hz to 460Hz (+/-12dB), Mid covers 300Hz to 6.1kHz (+/-12dB) and Treble overlaps from 2.1kHz up to 15kHz (+9dB/-12dB). Since the system bandwidth of the Omni only extends up to 13kHz, the Treble control is reduced in effectiveness. As with all three controls the centre position gives flat equalisation, clockwise and anti-clockwise movements giving boost and cut respectively.

Bass gives a full, boomy sound on clean guitar in the boost position and a useful cut on vocals which counteracts the 'bass proximity effect' produced by many cardioid microphones. The Mid control acts over a wide range and boost gives an effective 'lift' to synthesiser sounds whilst adding punch to guitar chords.


Analogue circuitry based on the bucket brigade principle enables the input signal to be delayed to produce an echo effect. The Delay control is adjustable to give time delays from 30mS to 300mS enabling multiple echo effects, 'flutter echo' and metallic reverb treatments to be achieved when combined with the Regeneration control features.

When the Mix control is set central, an equal ratio of 'dry' and 'delayed' signal is obtained at the main output. Clockwise setting gives 'delay' only, anticlockwise 'dry'. The Delay circuitry reduces the dry signal bandwidth to 10kHz and the delay signal to 2kHz. Since BBD technology always means a trade-off between increased delay time and reduced bandwidth, MXR are probably quite right in opting for the former as most users would put up with poorer quality sound if they could get the echo length they desire.


This dual function section houses three controls: Width, Rate and Regeneration with a pushbutton to select either Flanger or Chorus operation mode. When pushed in, Chorus is selected and the Regeneration control is disabled to prevent undue degradation of the Chorus effect. Rate controls the sweep frequency of an internal LFO whose range is variable between 0.08Hz and 8Hz. This gives a long, slow flange sweep at the minimum setting and a medium speed Leslie sound near maximum.

The amount of time delay modulation is determined by the Width control which ranges from 0.8 to 16mS. When set fully anticlockwise with near full Regeneration, the characteristic 'hollow' comb filter sound is produced and increasing Width gradually brings in the oscillator sweep. A good 'yawning' flange can be achieved that is noise-free and very quiet in operation. The Chorus sounds are very rich and really 'beef up' any input signal, especially guitars and electric piano.

External Loop

This final section permits an external auxiliary effect to be patched into the signal chain via the front panel Loop In and Loop Out jack sockets. Two black pushbuttons can then be used in four combinations to place this external effect after either the Compressor, Distortion, Equalizer or Delay. This is an extremely useful feature and by sending the Loop Out to a second amplifier with different tone settings, you can switch quickly between amps using the External Loop footswitch on the remote control unit.

Remote footswitch unit interior.

Remote Control

The MXR Omni comes with a remote control footswitch unit that connects to the main unit's rear panel socket using the detachable 12 foot guitar lead provided. FET switching circuitry ensures silent operation and no problems were encountered during operation. Surely, it would have been more sensible to have had the footswitch connection on the front panel though, where it would still have been accessible with the main unit rack mounted?

The remote unit itself is 17¼" long and 2" high sloping to 1" at the front. Visually it has a similar finish to the main unit and contains, from left to right, seven robust footswitches for Compressor, Distortion, Equalizer, Delay, Flanger/Chorus, External Loop and Master Bypass. The latter switch has an associated green LED above it, the others are all red. Foam rubber strips on the underneath of the unit prevent sliding, but did not function too successfully on smoother surfaces.

As you have probably already noticed, no mention has been made so far of the effect selection procedure. This is because all such switching can only be done from the remote control unit which means that your Omni is completely useless for example, if you accidentally forget to bring the remote unit along to a gig or worst still, lose it!


The advantage the Omni has over separate effects devices is its effect programmability. By pressing the Master Bypass footswitch the effects are removed from the signal chain and only the direct input is heard. By pressing individual effect foot-switches you can pre-select the effects you require. In this instance the red LEDs above each selected effect glow at half-brightness to tell you exactly which effects will be in circuit when you eventually depress the Master footswitch again.

It is a convenient method but still requires you to push several footswitches at some point in the procedure. Things are made slightly easier by the fact that, with the side of your foot (and a little practice) you can select several adjacent switches at once.


The MXR Omni is certainly a versatile unit capable of producing a wide range of effects. The rack unit is well made, compact, visually pleasing and ergonomically designed, but is let down slightly by the absence of any knob calibration or a mains power switch.

The advantages over individual effects are fairly evident - no need to replace batteries, it only requires one mains plug and you don't suffer signal degradation from the various interconnecting leads.

However, there is one main disadvantage; the Omni is non-modular and as such you have no choice over the effects used in the system, you either like them or don't and this ultimately is the crucial factor. The Omni is by no means cheap and prospective customers may well be dissuaded from buying the system if they dislike one or more of the effects, however good the overall system is. Having said that, there are bound to be people who'll like the treatments available and will buy it.

As with all equipment, you should always get a full demonstration at your local music store and make your decisions based on that (with, of course, the aid of your E&MM reviews). The MXR Omni is certainly worth a trip to your local dealer, so try one and see!

The recommended retail price of the MXR Omni is £472.50 including VAT. It is distributed in the UK by Atlantex Music Ltd. (Contact Details).

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Hans Zimmer

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Aria U60 Deluxe BBS

Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jul 1983

Gear in this article:

Studio FX > MXR > Omni

Gear Tags:


Review by Ian Gilby

Previous article in this issue:

> Hans Zimmer

Next article in this issue:

> Aria U60 Deluxe BBS

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