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Neutral Audio X-DREI Pro

Speaker Optimiser By Hugh Robjohns
Published August 2013

The control scheme is simple. The Function knob selects from the X-DREI's three operating modes, while the Scene knob has six settings, which introduce progressively more high-frequency boost and low-frequency cut as it is turned clockwise.The control scheme is simple. The Function knob selects from the X-DREI's three operating modes, while the Scene knob has six settings, which introduce progressively more high-frequency boost and low-frequency cut as it is turned clockwise.

Can this innocent-looking 1U rack really improve the performance of any speaker system?

In a perfect world, audio signals captured by microphones would be reproduced perfectly by loudspeakers. Sadly, we don't live in that kind of perfect world, and in practice every part of the audio signal path adds some element of noise and distortion to the captured signal. Thankfully, today's electronics technology can be made incredibly accurate and linear, adding very little noise or distortion — in some cases it is barely measurable, in fact! — although the loudspeaker remains the most non-linear element of any typical audio signal chain by some orders of magnitude. Of course, being an inherently contrary species, we often choose deliberately to introduce distortions and other non-linear artifacts because we find them musically pleasing.

Nevertheless, ever since the first audio signal was recorded and reproduced electronically, engineers have tried to find ways of counteracting and correcting distortions inherent in the system. A Spanish company, Neutral Audio Technologies, set up by Jose Manuel Jimenez in 2005, is amongst the latest to be running with that particular baton. Its DREI (Dynamic Reduction of Electronic Interactions) technology first appeared in a domestic hi-fi product in 2008, and the company's X-series professional line was launched in 2010. Since then the system has apparently been embraced by a lot of high-profile artists in their live touring rigs, and the company have been very active in bringing their products to the market's attention.

Neutral Audio claim that their entirely proprietary DREI system "improves” the sound quality of any system in which it is used, fundamentally by optimising the signals sent to the loudspeakers to compensate for their inherent generic physical limitations. The company's web site presents lots of what I like to call pseudo-science, and while it might impress gullible hi-fi enthusiasts, I was left rather more bemused and confused and with a distinct whiff of 'snake oil' in my nostrils! However, the company seem to have acquired quite a range of respected and credible endorsees, so I asked them for a review unit to see and hear for myself what exactly this product does.


Jimenez seems to be quite secretive about the technology involved, and as English isn't his native tongue, the details and explanations that he does provide are often rather confused or confusing. What I can tell you, though, is that this is a wholly analogue, multi-band processor (the audio is split into three separate frequency bands for the proprietary processing entirely in the analogue domain), and it is immediately obvious that key parts of the processing involve a lot of phase-rotation and subtle but significant equalisation.

The company's web site published some details about the DREI system, explaining that there are three separate processing elements: re-interpreting the waveforms, de-intermodulation, and sound-stage manipulation. I'm sure it's no coincidence that 'drei' is also German for three! The first of these processes — reinterpreting the waveforms — is claimed to modify the input signal to make it as "sinusoidal as possible”, on the basis that speakers can't cope with signals that change direction quickly and that they essentially become inefficient trying! There's no doubt that the DREI system makes the output more sinusoidal — triangle and square waves both become noticeably more rounded, as the test plots I acquired using an Audio Precision measurement system reveal. On closer inspection, these results are initially very similar to the mathematical process of differentiation, better known to us as high-pass filtering, but actually I think they are the result of phase rotation.

Phased & Confused

To test my supposition, I created a very asymmetrical waveform using Adobe Audition's signal generator and passed that through the X-DREI. The output was not only symmetrical, but also about 4dB louder. With broadband signals, the total amount of energy in the signal appears to be maintained, and it would seem that this waveform manipulation is achieved by band-splitting the input and then manipulating the relative phase within each band, before recombining their outputs to ensure that the overall spectral balance is broadly maintained.

The de-intermodulation processing is claimed to reduce intermodulation distortion that has already been introduced in the source signal by previous non-linear processes. When two signals of different frequencies pass through a non-linear process, amplitude modulation can occur, creating new frequency components that aren't present in the source signals. These new modulation products are at frequencies which are the arithmetic sum and difference of the originals. For example, if the input signals are pure tones with frequencies of 9kHz and 10kHz, the intermodulation products will be at 8kHz and 11kHz. Such intermodulation distortions are usually at a very low level, but still contribute to the sound and are inherently a non-musical form of distortion, to which our ears are often very sensitive.

The X-DREI Pro's rear-panel connections comprise two XLR inputs and two XLR outs.The X-DREI Pro's rear-panel connections comprise two XLR inputs and two XLR outs.

In theory, the non-linear behaviour of any system can be modelled and the intermodulation products resulting from specific input signals can be calculated. Once known, any intermodulation distortions in the source could theoretically, therefore, be reduced... and that's what the DREI system is said to do. Unfortunately, I was unable to find an effective way of demonstrating whether or not it managed to achieve that. I suspect that the intention is to pre-process the signal to compensate for the expected non-linearity of the loudspeakers, hopefully to give the impression of greater clarity. With reasonably good monitor speakers, the result is presumably actually to introduce unwanted distortion artifacts instead, and I was aware of a subtle character change that didn't appeal during my testing — it seemed to cloud the signal and I generally preferred the input to the output!

The third and final aspect of the DREI processing is to change the way the stereo sound-stage is perceived, modifying the spectral balance and the resulting impression of width and depth. The publicity claims that this is not equalisation, but my bench tests showed that there is some modest but significant equalisation involved. Specifically, there is a 2dB shelving boost above 300Hz and 3dB cut below, in the most extreme setting. A distinct 1dB notch with a Q of about 0.66 (two octaves) centred at 5kHz is also present in all modes. This is clearly a deliberate 'voicing' that changes the perceived character of the sound stage, as any loudspeaker design will confirm.

Inside The Box

There are several different Neutral Audio products incorporating the DREI processing, as well as OEM modules. However, the company supplied the X-Drei Pro model for review. This 1U rackmount unit is housed in a simple white-painted steel case, extending about 160mm behind the rack ears. There are just two rotary switches and six LEDs on the right-hand side of the front panel.

Lifting the lid reveals that the X-DREI's earth is bonded to the chassis via the main PCB.Lifting the lid reveals that the X-DREI's earth is bonded to the chassis via the main PCB.

The first rotary control knob selects between normal stereo, bypass and 'duplex mono' operating modes. The last accepts only the left channel input, and provides a duplicated mono signal from both the left and right outputs. The LEDs indicate the processor's status, with a red light for power on, and two blue lights to indicate when the DREI circuitry is active, and that the system is working correctly. The bypass and duplex modes are indicated with separate flashing green LEDs, while another blue LED illuminates steadily for the normal stereo mode.

The second rotary switch determines the required sound-stage, with six options. The mastering setting (fully clockwise) supposedly introduces no change to the sound stage. The other positions weren't labelled on the unit itself, although a shaded coloured background is employed, spanning red at the mastering end to green at the opposite end. The handbook suggests the green setting corresponds to an 'indoors' mode, with 'half' at the blue/mauve mid-region, and 'outdoors' at the orange area adjacent to the mastering position. Apparently, this colour coding is consistent with other Neutral Audio products.

The rear panel has a plate carrying two pairs of XLRs for the balanced analogue line-level inputs and outputs, and an IEC mains inlet with integral fuse-holder and on-off switch. The unit ships configured internally to either 230 or 115V AC. Unusually, its lid can be removed after unscrewing just two bolts — most modern products employ at least six screws around the lid to ensure adequate EMC shielding! Inside, the main circuit board carries conventional, full-sized components, but occupies only the right half of the floor area. A large sealed unit measuring roughly 125x95x25mm contains the proprietary DREI circuitry, taking up about a quarter of the main circuit board's area. It seems that this circuit board is employed in other Neutral Audio products that are housed in hi-fi-style cases, and there is provision on the PCB for RCA/phono sockets and a headphone amp (none of which were populated in the review model).

The left-hand side of the circuit board carries a small torroidal transformer and a linear power supply, with a small slide switch to select the mains input voltage (this was 'hot-glued' into place at the 230V setting on the review model). Curiously, the mains safety earth from the IEC connector is taken directly to the main PCB and connected to the metal chassis, via contact between the PCB trace and one of the mounting bolts supporting the circuit board. Most manufacturers bond the safety earth directly to the chassis adjacent to the IEC input to ensure a solid connection, and unfortunately the arrangement employed here, combined with how the metalwork is painted, means that getting a decent earth on the outside of the unit is trickier than it should be. Consequently, the review unit failed its first PAT test because, depending on where the case was measured, the earth bond resistance varied between 0.1Ω and 5.6Ω! The PAT earth-bond limit is 0.1Ω plus the resistance of the IEC mains cable, so perhaps this aspect of construction should be reconsidered.

A small vertical daughterboard is mounted on the back of the two front-panel switches, and carries the status LEDs. This board is connected via a ribbon cable that runs underneath the main circuit board to a header plug at the back of the unit. Sealed relays are used to bypass the audio circuitry, and Burr-Brown INA134 and DRV134 balanced line input and driver chips provide the analogue interface circuitry. Four other op-amps appear to be employed in the signal path, but their markings had been obscured with black enamel paint and silver dots. The proprietary DREI circuitry was completely concealed within the large metal enclosure.


Testing Times

Having spent some considerable time using the intriguing X-Drei Pro, it seems to me that it provides little benefit to high-quality systems, but can be extremely effective with audio systems that are the least accurate — which might explain why it has interested live sound engineers.

Initially, I wired the X-DREI into an insert loop within my Crookwood mastering console, just to make it very easy to switch it in and out, so that I could directly compare the input and output signals, before and after processing. The key benefit of this approach was that I could trim the insert monitor level to precisely adjust for the roughly 3dB gain boost introduced by the X-Drei. Without that compensation, simple in-out comparisons would be totally meaningless; louder is always better, as we all know! I mainly used Neumann KH310 sealed-cabinet monitors and AKG K702 headphones.

Rotating the sound-stage control knob anti-clockwise introduced even more gain above 300Hz, reaching a rather considerable +5dB at the 'indoors' setting. The signal level below 300Hz is also progressively attenuated as the control is moved through the different mode positions from 'mastering' to 'indoors', the maximum attenuation being about 3dB. My Audio Precision test plot (see the 'For Good Measure' box) illustrates the mastering and indoor positions, plus a middle setting of the control.

This tonal rebalancing was actually very obvious, damping the bass slightly and pushing the mid-range forward, enhancing the perceived clarity, presence, and sense of reverberation. The effect was not dissimilar to the way many budget 'monitor' speakers are voiced to make them sound more detailed and revealing than they really are! Rotating the soundstage control delivers an effect which is subjectively similar to moving closer to a stereo source — a bit like moving a simple stereo microphone array closer to an orchestra, for example. The amount of presence builds and the bass becomes slightly more diffuse or unfocused.

Neutral Audio X-DREI Pro

Although I didn't find the sound-stage control particularly useful when auditioned on the sealed cabinet speakers (the same sense of forwardness can largely be achieved with subtle EQ and dynamic control, after all), I discovered that it is actually surprisingly handy with small ported loudspeakers! I rigged up some fairly horribly ported lo-fi speakers borrowed from an old kitchen mini-system, and found that the bass end appeared to tighten up as the sound-stage control was advanced, giving the impression of being better controlled. The familiar boominess of such systems was usefully reined in — something that might indeed prove very helpful in a large live sound rig!

The phase rotation aspect of the X-DREI is probably its primary strength. Phase rotation has long been the secret weapon of FM radio stations around the world, allowing them to achieve greater levels than would otherwise be the case. A lot of audio signals are inherently asymmetrical, and thus one side of the waveform will reach the clipping level before the other side. By processing the signal to make it more symmetrical, the energy is redistributed and the output can generally be increased before clipping occurs. The perception of such processing is that systems can be turned up louder before clipping than was previously possible — again, something that would be very useful in the live sound arena, as well as in mixing and mastering.

I was unable to hear any direct evidence of intermodulation distortion reduction... but then intermodulation distortion is relatively rare today, because most decent audio equipment is extremely linear. Again, perhaps this is something that becomes more apparent on large live sound rigs which employ loudspeaker systems that are rather less well behaved than studio monitors!

Worth A DREI?

Neutral Audio have an interesting product in the X-DREI, and one which does have genuine value as a corrective audio processor, particularly with systems that are less accurate and linear than good studio monitoring chains. However, in my opinion the company really does itself no favours at all with all the pseudo-science and secrecy. Phase rotation is a well-known and very useful technology, as are the techniques and advantages of multi-band processing, and spectral voicing to change the perceived sound-stage. Although less familiar in the pro-audio world, the same also applies to the concept, at least, of intermodulation correction. All of these things can be explained in a sensible way without revealing the proprietary detail of how they are actually achieved in the X-DREI circuitry, and doing so would, in my view, greatly enhance the company's credibility with technically-minded professionals. The 'magic box' approach might impress some of the hi-fi fraternity, but a quick google search exposes many forum threads where the product is dismissed outright as snake oil (or worse) amongst the pro-audio industry — purely because of the nonsense in the company literature — and that's a great shame.  


Though there are some broadcast processors that employ phase rotation, none do so across multiple bands, and nothing combines the X-DREI's three processes in one box.

For Good Measure

I measured the noise, distortion, group-delay, and frequency and phase responses of the X-DREI Pro under various operating conditions, using an Audio Precision analyser. You can see the results for yourself at /sos/aug13/articles/neutral-xdrei-media.htm.


  • Phase rotation is a very useful way of allowing increased system output level.
  • De-intermodulation may be beneficial on less-linear PA speakers.
  • Sound-stage manipulation appears to help rein-in the worst resonances and boominess of poorly controlled ported speaker systems.


  • The review model's chassis earth bonding is less than ideal.
  • The processing introduces a significant level boost.
  • The marketing literature and faux-secrecy does the company no favours at all.


Designed to counter the distortions inherent in loudspeakers, the X-DREI Pro is capable of useful results — but is most effective on less-than-ideal systems, such as live PAs.


€2891.90 including VAT.