My audiophile association with DEQX started in 2006 with their fist unit, the PD-2.6P. The unit’s unique hardware and software opened up a new understanding of speaker behavior and what was possible in speaker/room correction. In April 2011 I reviewed their next iteration, the HDP-3 . It was a significant improvement over the PD-2.6P and received a well deserved, positive review.

Kim Ryrie, the brains behind DEQX, gave me a heads-up on the upcoming HDP-4 and offered one for review. He mentioned the exterior faceplate looked identical, but there had been many hours of research and development leading to upgrades in parts, circuitry and programming. Knowing Ryrie’s commitment to sound quality, I had no doubt that this new model would offer further gains in areas of transparency, clarity, accuracy and smoothness. I also knew that the HDP-3 was already excellent in these areas and wondered how much better it could be. About three weeks after our discussion it arrived and I was ready to start the burn-in process (about 100 hours).

A précis on DEQX.

The DEQX is a speaker corrector, room corrector, and preamplifier with two analogue inputs, one RCA, one XLR, and five digital inputs, S/PDIF, XLR, BNC, USB and TOSLINK. It contains a DAC, an ADC, and active steep slope crossover networks. It has a remote control for volume and function that can also be used to control a real time parametric equalizer and can store up to a 100 different settings. It is a one box unit that is the proverbial Swiss Army Knife of audio functionality. By the time of publication, an optional, USB input will be available as standard. For those who already own the HDP-4, a user friendly USB card is available.

DEQX HDP-4 rear panel.

The following is taken from my 2011 review of the DEQX HDP-3 which describes the operation and installation of the DEQX which applies equally to the HDP-4:

First, load the supplied DEQX CD into your computer. Then an optional microphone (there are two microphone packages available) with cable is connected to the DEQX and an included USB cable is also connected from your computer to the DEQX. The next step is to set up the microphone about 1 meter from the middle of your speaker. The instructions in the manual are well written and comprehensive. You’ll need a mic stand for this which is not supplied. It is important that you bring the speaker as far away from any walls as you can so the microphone only reads direct sound and minimizes the effects of any reflected sound. A frequency sweep test tone is produced that the computer graphs of the full frequency response of the speaker. You’ll observe all kinds of squiggles, peaks and valleys far from the neutral axis. You then apply the ‘corrective’ filter and, voila, you have a flat response. You should do this for each speaker even though they’re supposed to be identical, they usually are not. There are infinite adjustments available and you can tailor the sound to your preferences.

Once you’ve corrected your speakers you can perform a similar process for the room equalization. We all know that the acoustic impact of the room on the sound of your stereo is enormous and can have a deleterious effect. Place your speakers in their normal listening position and place the microphone as close to your listening position using the mic stand. Then, another frequency sweep test tone is generated and the graph produced will show you the irregularities, or more properly, the sonic properties of your room. In my case, there was a bass hump at around 50Hz which interfered with midrange performance. Also, adjustments needed to be made in the 9-10,000Hz area that had a large impact on the soundstage and transparency of the system. The program handled the adjustments beautifully and the improvements in the sound were immediate and illuminating. I’m getting a little ahead of myself.

For those of you who are technically challenged or computer phobic or just want to have an expert consultation, DEQX offers their ‘DEQXpert’ service to hold your hand and take you step by step through the installation process or to consult with at any stage of the process.

I must also note that after going through setting up the system, your ears are the final arbiters of the sound. And after all the measuring and calibrating is complete, it’s wonderful to have the ability to make final tweaks to the sound with the remote, with or without the computer connected to the unit, to create what you want the sound to be. It is also, a great convenience, to create different performance characteristics and store them and remotely switch from one to another and you become the master of your sound.

The DEQX HDP-4™ • Preamp processor

In my set-up, I was able to take full advantage of the HDP-4 by using the steep slope active crossovers in the unit. My speakers, the Levy Acoustics FRM-1’s (previously known as the Wasatch Acoustics MUSINA), were designed specifically to be used with the DEQX and contain no internal crossovers. Thus, each driver of the two-way speaker requires its own amplifier, 4 channels of amplification in a two-way system and the crossovers provided by the HDP-4. The HDP-4 contains a third set of outputs for each channel, with accompanying crossovers, for either tri-amping a pair of three-way speakers or adding a pair of subwoofers to a two-way system.

The reasoning for using the DEQX is quite simple. Speakers are electro-mechanical devices that create the sound you hear. They commonly have distortion figures magnitudes greater than electronics. Yet, the speaker, an important piece of equipment in the listening chain, is also the greatest distorter of the signal. DEQX fixes this problem by reducing distortion to minimal levels and lets you hear more of the music with greater realism. Since your speakers already have their own internal crossover networks, you will not need the internal crossover networks in the DEQX HDP-4. As such, a less expensive DEQX model may be more suitable.

Audiophile engineering.

Audiophile engineering.


As an HDP-3 owner I had come to expect superior sound quality. The HDP-4 exceeded my expectations. Staying in the digital domain, I ran several digital interconnects from my MSB Transport to the HDP-4 and immediately heard improved levels of transparency over the HPD-3. The quality and natural detail were a joy to experience. The soundstage became larger and grew beyond the room boundaries. Images were further defined and more three dimensional. I spent many hours re-listening to the same selections as stated in my HDP-3 review and in every case the improvements were clearly noticeable. Standard CDs were handled so well that I wondered about the validity of the higher resolution formats — I played a few discs of the 24/96 variety and they simply sparkled. It was clear that the implementation of the DAC in the HDP-4 was quite superior and would make for an excellent stand-alone product. Perhaps, something for the future?

I invited several writers from Audiophilia to drop by and hear what I’d been excited about. At the time I was using a recently acquired, wonderful preamp, the MUSICFIRST Classic v2 preamplifier (a passive preamplifier with superb transparency) for my analogue sources and then running the signal into the HDP-4’s analogue inputs. We compared that set-up vs. running the analogue outputs from my phonostage directly into the analogue inputs of the HDP-4, completely bypassing the MUSICFIRST preamp. The transparency and musicality of this preamp are well known. Would the transparency, which I loved so much, suffer? Did the v2 add something beneficial to the sound or would removing it from the audio chain provide an improvement or be detrimental to the sound? How good was the HDP-4 for analogue sources and how good was its analogue to digital convertor (ADC)?

After listening to various LPs with and without the v2 in the chain several things became clear. At first the results were very close but it became apparent as we listened further, that the sound without the preamp in the system had more body and was more three dimensional while the soundstage became larger. As good as the v2 was, not having it in the chain proved the maxim ‘less is more’.

Another interesting capability the DEQX provides is the ability to create a subsonic filter tailored to your system, the lack of which makes playback of any even slightly warped records problematic. With the HDP-4 it is easy to change the response curve creating a perfect sub-sonic filter without any negative effects on the sound. In my case, I set it to quickly drop off below 15Hz which proved to be ideal.

Next, still using vinyl, we compared the v2 directly to the amplifiers vs. the HDP-4, in by-pass mode, directly to the amplifiers. This produced some quite wonderful and unexpected results. The expectation was that the v2, would give the clearest, most transparent and musical presentation of the two. After all, how could a preamp that takes the music signal, converts it to digital and then back to analogue, match the sound quality of the pure analogue v2? Interestingly, it was almost impossible to determine which set-up we were listening to or to choose one over the other.

After many hours of listening, a few of us could hear, or thought they could hear, a slightly improved clarity through the HDP-4, others preferred the v2. That debate continues. Was it expectation that colored perception or were we actually hearing minute differences. All I can say, definitively, is that the DEQX HDP-4 showed itself to be a superb sounding preamp for both analogue and digital sources. I would place the HDP-4 in any system and save boatloads of cash by not needing a preamp and gain all those additional capabilities that the DEQX provides making it an incredible performer.

To sum up, the HDP-4 is a significant improvement on an already excellent product. Before I give my full blessing, I have one wish — a model with at least three, possibly four, analogue inputs. It would be greatly appreciated. That said, its performance, flexibility and capabilities make it an indispensable component and receives my highest recommendation.

[It is with great pleasure that we award The Audiophilia Star Component Award to the DEQX HDP-4™ • Preamp processor. Congratulations! – Ed]

Reference System

Amplifiers: Hephaestus HMA-1000 Monoblocks
Speakers: Levy Acoustics FRM-1
Phono Preamplifier: AVID Pulsare updated to II
Digital: MSB DATA CD IV Transport w/ DEQX HDP-4 Preamplifier/Processor
Analogue: Avid SEQUEL SP/SME IV arm/Cartridge: Shelter 7000
Speaker Cables: Acoustic Zen’s Absolute, Wasatch’s Ultama
Power Cords: Kaplan GS Mk II
Interconnects: Antipodes Audio KOKIRI, KOMAKO and REFERENCE, Acoustic Zen’s Absolute, Morrow Audio MA-7
Accessories: Redpoint BLAKHOLE’s, Herbie’s Audio Lab Tenderfeet, Soundcare products, Acme Audio Labs wall outlets
Power Conditioning: Kaplan Alchemist

The DEQX HDP-4™ • Preamp processor

Manufactured by DEQX PTY Limited
Unit 1, 1 Roger Street, Brookvale, Sydney NSW 2100, AUSTRALIA
Tel: +61 2 9905 6277
Fax: +61 2 9905 8066

Price: US$4,950
Source: Manufacturer loan


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