Audiophonics HPA-S400ET stereo amplifier subjective review!

The Audiophonics HPA-S400ET stereo Amplifier(Class D Stereo – Purifi ) whose design was implemented by Bruno Putzeys, is fully evaluated by Howard Milstein.

When I heard that this relatively new amplifier contained two PURIFI 1ET400A modules and a HYPEX SMPS1200A400 power supply of Bruno Putzeys design for Audiophonics I immediately contacted David Aubert, CEO of Audiophonics to see if I could get a sample to hear. Anyone familiar with Bruno Putzeys’ previous works will know that he is a leading designer and engineer within the world of Class D high-end “Purifi” audiophile products. 

Mr. Putzeys most notable design ventures have been the renowned Purifi, Hypex, Kii, and Mola Mola DAC and monoblock amplifiers which have been praised by virtually every critic in the high-end of the consumer electronics industry.

Bruno Putzeys Designer
Bruno Putzeys

The blueprint of the Audiophonics HPA-S400 ET Amplifier Class D Stereo Purifi is quite impressive, to say the least. This compact beast of an amplifier produces 200 watts of power into an 8-ohm load and a whopping 400 watts into 4 ohms. Needless to say, it will drive just about any type of loudspeaker currently on the market with the power to spare. Enormous SPLs were obtained with the test loudspeakers, in this case, the Harbeth C7es/XD as well as the recently acquired QLN ONE  which has proven to be a sensational speaker at its relatively modest price point. (review forthcoming).

Here are some specifications for the technical purists among us that you should find impressive and rather interesting:

A Dual PURIFI 1ET400A module

Audiophonics HPA-S400ET stereo amplifier
  • Both channels of the amplification circuit use their PURIFI 1ET400A module. These high-performance Class D amplifier circuits each offer a very comfortable power of 200W (@ 8Ω) while making the most of the Class D amplifier’s high-efficiency rating.
  • The 1ET400A modules have a >75dB high gain loop over the entire frequency band, providing high performance across the whole spectrum and surpassing any amplifier regardless of its class.
  • Thanks to hyper-low intermodulation distortion (IMD) the HPA-S400ET achieves incredible precision on its stereophonic image over the whole spectrum, arguably unmatched by audio amplifiers of almost any technology or operating class. (Comparisons below).

THD remains extremely low at any frequency and any power level right until clipping. (testing has proved this amplifier to be quite untouchable in this respect!) This translates into a total lack of sonic signature, and an ability to reproduce any type of music without preference for genre or production style.

Listening tests have proved The HPA-S400ET can and does achieve exceptional precision in imaging as well as its magnetic audio reproduction qualities thanks to its hyper-low intermodulation distortion, which many audio amplifiers of whatever technology or operating class may find difficult to match.

The Power Module is the Hypex SMPS1200A400

The SMPS1200 switching power supply is a model with a very high (theoretical) maximum output power of 1200 Watts. It has been specifically designed and developed by Hypex to provide a 2 x 64V voltage required for the operation of the most demanding amplifier modules.

This low noise high quality Power supply meets all the energy needs of the amplification modules. The HPA-S400ET has a balanced stereo (XLR) and unbalanced (RCA) input. Bipolar selectors make it easy to change the input source and even allow independent channel control.

Audiophonics HPA-S400ET stereo amplifier open top
Back panel shows extremely high quality speaker connections:


I have been an admirer of Bruno Putzys and his designs for many years and consider him a pioneer of Class D amplifier applications. Hypex and N-core have been widely adopted by so many high-end manufacturers such as NAD, Bel Canto, etc. to name just a few. Fundamentally, It certainly was an eye-opener when Purifi announced its introduction of Eigentakt.  Even more so, Audiophonics packaged this unit so attractively and affordably while using the well-proven Hypex SMPS power supply.

Its Component-style chassis fits perfectly with many other components with an elegant look and ample heatsinks. The fine front-panel power button is tastefully simple and precise at your fingertips. But how does this amplifier fare sonically against other Class D models available to the audiophile, as well as other Class A and A/B contenders, within its formative price point or above?

Initial listening tests showed it to be a blatantly superior amplifier that can take on many of its rivals, regardless of their design implementation, and/or price and that is a mighty high compliment indeed!

The first thing one notices when good program sources are employed is a truly breathtaking soundstage spread in depth and image width while just unleashing itself from the speaker enclosures themselves. (assuming your loudspeakers are of the highest quality of course.) The best esoteric amplifiers can do this easily, but I was not expecting this unit to show itself as it did.

The smiles on my face came fast and easy as both good vinyl, digital files, and CD programs were tried with the HPA-S400ET. The music had fine detail, and a supremely natural balance with excellent ambiance while the stereo images were stable and cohesive. The instrumental tonality and audible precision were always memorable and it consistently offered that huge sound field while displaying a relaxed and subtle aural quality.

Numerous digital and vinyl sources were employed in the listening tests (3 listed below) of both vocal and orchestral music with all of them displaying consistent and substantial musicality throughout the whole frequency range. Its Bass response was firm, taught, and immensely impactful while the mid/treble balance was quite close to ideal. After about a few weeks of complete enjoyment with the Audiophonics, it was about time to make a few subjective comparisons with some other Class D and/or AB units that I had on hand. The most obvious initial candidate was the PS Audio M1200 monoblocks currently under review.

In this instance, the M1200 (with a hybrid tube input design) was consistently comparable to the Audiophonics musically as far as total loudness capabilities (although the former is definitely more expensive with a far greater 8ohm power capability). However, both amplifiers can handle any load and any loudspeaker you could ever think of throwing into the ring. The key here was some subtle differences in the basic audible cleanliness, musical tonality, harmonic structure, soundstage, and other areas that are of more importance to the discerning listener, overall.

Nonetheless, the similarities here were very, very much closer than one might expect! In all fairness, the M1200 was a flicker more detailed as it offered a tiny bit more luster and transparency in its soundstage reproduction than the Audiophonics, yet in almost every other respect, they both implemented audible properties that were exceedingly close to each other in just about every way.

A more interesting (and different) comparison (though not necessarily better) would involve the Pass Labs XA30.8 whose design elements offer 30 watts of pure Class A power. Understandably, this may be the only area where the HPA-S400ET inevitably fell a touch short, overall. It ultimately took a few changes back and forth between The Pass Labs and the Audiophonics to hear a ‘’cinch’’ more sound clarity of the former amplifier. (There are good reasons why the best Class A amps have earned their highest reputation among sound enthusiasts!)

However, at this amazingly competitive price, there is no way you can truly fault the Audioophonics HPA-S400ET in any way possible. This is an amplifier that can give close to stunning results in any high-end system it may be asked to perform in and that is saying a lot!


For someone moving from traditional Class A/Class AB to Class D, the  HPA-S400ET should be number one on your shortlist of amplifiers to audition. It is a great-sounding amp showcasing neutrality as the keyword here while also offering an immaculate and effortless sound quality. It is designed with some great modules that many other Class D amps should be envious of.

The excellent build, in and out is quite obvious and unless you spend another 5k or more it can not be matched! Undoubtedly, it builds on the superb platform created by Purifi, yet they were able to add value without impairing the platform’s functionality. Close to an epiphany in transparency, liquidity, and detail, it is up there with the very best Class D amplifiers available for the discriminating listener and offers oodles of power for truly any loudspeaker.

Only when compared to the finest Class A designs could an enthusiast maybe notice a minuscule decrease in overall sound “translucency”. All in all, The Audiophonics HPA-S400ET stereo Amplifier is a super audiophile piece of equipment that produces a gorgeous sound at bargain basement prices. Therefore, it deserves an excellent rating and is certainly a “best buy” for everything that it brings to the table; a true winner!

Sources for this review include;

Ashkenazy: Beethoven Piano Concerto # 4 / Solti – Decca

Journey LIVE

Journey Live in Houston – Escape Tour

Mozart: Sacred Arias with Barbara Hendricks

**outstanding vocal performance and reference sound quality


Output power (max)8Ω @1%THD : 227W
4Ω @1%THD : 425W
2Ω @1%THD : 450W
THD+NPO=1W, f=1kHz 0.0007 %
PO=10W, f=1kHz 0.00026 %
PO=100W, f=1kHz 0.00015 %
PO=1W, f=20-20kHz 0.0007 %
PO=10W, f=20-20kHz 0.00029 %
PO=100W, f=20-20kHz 0.00017 %
Dynamic rangeA-weighted 4Ω 131 dB
SNRA-weighted 4Ω 131 dB
Frequency responseRL= 8Ω, f= 20-20kHz ±0.01 dB
RL= 4Ω, f= 20-20kHz ±0.01 dB
RL= 2Ω, f= 20-20kHz ±0.01 dB
Output impedance1kHz, Iout=1A 0.07 mΩ
20-20kHz, Iout=1A <0.65 mΩ
Input impedanceRCA: 47kΩ / XLR: 94kΩ
Bypass: 2.2kΩ / 4.4 kΩ
Power supply230V (110V on request)
Dimensions413 x 322 x 60mm
Weight5.63 kg

The Review system:  Loudspeakers: QLN ONE Loudspeakers~ Harbeth Compact 7ES-3 XD loudspeaker ~ Quad ESL-63, ~ Digital: Mojo Audio ‘Mystique’ X DAC ~ Border Patrol DAC SE-I ~ Innuos Zenith Mk.3 server/streamer ~ Audio Note (UK) CD3.1x/2  Amplification: ~Pass Labs XP-12 preamp, Pass Labs XA30.8 power amp ~PS Audio M1200 / ~ Cables/ Conditioners: Argent/Pur Silver stranded loudspeaker cables, Audience Studio One loudspeaker cables / Inakustik AC-3500p power station &, AC-2404 reference Air Power Cord ~ Audience Studio 1 interconnects,~ Clarus “Crimson” 75-ohm digital spdif / Audio Art 1 e” AC Power Cord

PRICE :       Audiophonics HPA-S400ET stereo purify amplifier ~ $1,241.67   € (1490 with 20% French VAT)

CONTACT  Att: Dave Aubert

ADDRESS  Audiophonics
                   10, Rue Marcelin Berthelot 33270 Floirac, France


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