hen the courier left me a package from ACUHORN, I thought that Wojtek Unterschuetz, its owner and designer, sent me remotes for the system that we agreed for a review, maybe some manuals, and possibly even two bottles of some nice wine. I had his speakers in mind - such as model 15 that are large, so I automatically assumed that other components had to match them in size somehow.

I realized that they would not be the kind of huge boxes that the likes of Soulution or Dan D’Agostino components arrive in, but still I expected something reminiscent of at least the S2 Stereo, reviewed in January this year by Marek Dyba, or the nowe audio mono3.5, the first commercially available amplifier built by Wojtek.

In fact, neither the R2R streamer nor the RATE power amplifier feature remotes, but they both are of the same, rather small size of 320 x 200 x 28 mm and of a lightweight. It seems that the change of Acuhorn's attitude towards the size of their devices was not sudden. The mono3.5 amplifier, a very interesting design that featured an aluminum chassis of a size of a “coffee table” book, was the first indication of this trend. The S2 and amplifiers from this series have introduced the characteristic chassis, that is also used here in a miniaturized version.

| R2R

The system I requested for the review consists of the R2R streamer and a stereo power amplifier. The amplifier features an adjustable input sensitivity, so it is possible to dispense with an additional preamplifier, which would be a choice in Harmonizer with Wojtek Unterschuetz philosophy, that aims to minimize the signal's path length. In corporate materials he writes: "Acuhorn R2R, a tiny modern device, is a twin brother of the RATE amplifier." Both have solid aluminum enclosures with large openings for cooling purposed and the laser cut logos and they look - well, that's probably the best term - cute, despite such an obvious industrial aesthetics.

R2R in the company materials is called a "streamer" or "headless player". So one can say that this is an audio files player that is controlled using an external device such as a smartphone, tablet or a computer.
Sort of a complication in its name is introduced by the R2R term that might suggest it is a digital-to-analog converter. R2R is a shortened version of the name R-2R Ladder, i.e. a discrete DAC, with separately soldered resistors. A common consensus is that it is the “noblest” form of the DAC; for example CEC uses in its top D/A converter DA0 3.0, and so do companies such as MSB, dCS, Chord, and the French company totaldac in all their products. In Acuhorn's R2R surface mounted resistors are controlled by the mighty FPGA Spartan 6 chip. The converter module was designed by the Danish company Soekris Engineering and features a real resolution of 28 bits.

The heart of the device is the Raspberry Pi 3 microcomputer. The signal is delivered either via LAN or from hard drives and the pendrives connected to one of the four USB ports located on the back of the device. The player accepts PCM signals up to 24 bits and 384 kHz and DSD up to DSD128. It is compatible with MP3, FLAC, WAV, AAC, ALAC, and DSD audio files. Digital music signals can also be delivered to it from external transport via RCA or TOSLink inputs. There are two analogue outputs featuring two pairs of solid, rhodium-plated RCA jacks.

Two pairs? The connections between components are a true bottleneck. This can solved in an elegant way - see Krell and Current Mode, and currently DartZeel and 50-ohm BNC current links. They all used current links. The problem with these type of solutions is that it is not compatible with other products. Acuhorn offers a similar solution, but based on classic RCA sockets. Named 2-R2 Direct output number one has a high output impedance of 625 Ω and a lower than standard voltage of 1.4 V.

If you want to use the player in other than the Acuhorn system, use the output number 2, delivering 2 V signal with 10 ohm impedance. A small knob placed nearby allows user to adjust volume within a range from -90 to +15 dB. It is a digital volume control, so if you want to use a high-end preamplifier just set it to 0 dB. When using the R2R with the RATE amplifier, you can use an analog attenuator at the input of the former.

| RATE

The Acuhorn RATE is a class D power amplifier featuring an SMPS. The combination of these two techniques allowed the whole device to be housed in a cabinet no bigger than a laptop with a small size screen. On the outside, it looks identical to the R2R, with the only difference being the connectors on the rear side. There are two pairs of the solid, copper, rhodium-plated Chinese GaoFei ("Red Copper Rhodium") connectors, and two pairs of analog inputs. They are identical, and we choose between them using a small switch.

Since there is also a volume knob that works with a discrete, stepped attenuator, one can use two sources with the RATE. This attenuator was built using surface-mounted, high quality Nichrome resistors. Previously I mentioned "attenuator", and "adjusting sensitivity", because that's what this element does in this device. However, if we take into account that right after connectors there are Lars Lundahl's matching transformers, it can also be said that this is almost a full-fledged passive preamplifier.

The amplifier offers up to 150 watts output per channel. Acuhorn used the Texas Instruments modules and powered it with Vicor. Recently, the use of high-end SMPSs has become increasingly common, as it seems that many of the most important problems associated with them had been finally solved, such as high RF noise and insufficient "headroom" needed to properly convey transients. In addition to the companies that have been using this solution for years, such as Linn and Chord, also Soulution use it in their amplifiers, and even Jarek Waszczyszyn is experimenting with it planning to use them for the new version of his top-of-the-line Silver Grand Mono amps.

WOJCIECH UNTERSCHUETZ
Owner, designer

The first commercially available Class D amplifiers were introduced in the 1960s by the British company Sinclair Radionics. The next attempt was in the 1970s and these were produced by Infinity Systems. Ten years later, the so-called "digital energy conversion amplifiers" by Peavey Electronics Corp. came to market. These projects were based on the available components available at the time that were not good enough for high quality sound. Their biggest problem was the too slow switching speed of the output transistors.

Attempts to shape sound waves switching signal hundreds of thousands of times per second were successful only in the 1990s, thanks to reliable and fast enough MOSFET transistors. The second wave of pioneers of this technology appeared in the late 1990s and came from Denmark. In 1998, the TacT Millennium integrated amplifier was introduced, and B&O released their ICEpower modules and power amplifiers. It was then that the first TA-class amplifier, the TA1101 by Tripath Technology in San Jose, California was made available for designers. Today monolithic chips from Texas Instruments, ST Microelectronics, NXP Semiconductors, Infineon/IR are available. The advantages of Class D amplification will be better and better used in the future, especially thanks to the advancement of technology. For example, let's mention a very promising fact - recently IBM presented 100 GHz transistors made of graphene.

In 2002 the German magazine "Stereoplay" prepared a comparison of three amplifiers. The AB class solid-state was represented by the Pass X 600 integrated, the tube amplifier by KR Audio Enterprise Double Kronzilla MK2, and the digital amplifier was the Audio Physic Strada. It turned out that the best were the large KR tubes, behind them there were 48 MOSFETs, and the cold, colorless, low resolution class D came last. The tube amplifier's summary was "Super-powerful and wonderful sound. Substantial bass and more lively in the treble. That's how a power amplifier should sound like. "

But do you really think that since then nothing has really changed? Has the technological progress made in the last 15 years brought nothing to the audio? Let's look at the extraordinary popularity of the French Devialet Expert 130 PRO amp, which receives signal through a digital input, think of Bel Canto Design Black, that according to American "Stereophile" magazine is a breakthrough product, primarily due to sound quality. I can honestly say that it is able to play better than my non-class D reference system. " And there are also Wadia amplifiers, such as the a315, and many more.

With the Acuhorn RATE amplifier project, I wanted to use analog components and solutions from my previous MOSFET amplifiers and tube OTLs. It turned out that the most versatile solution was the Texas Instruments monolithic chip, according to manufacturer featuring the best parameters available on the market. All components used in the audio signal's path were listened to in the comparative sessions. And the ultra-fast, precise Vicor Power power supply is a true foundation of this device. Apart from the crazy music lovers who opt for the vacuum tube technology, for all the others the bar has been raised again by engineers. These are no longer dry, true only on paper, technical parameters, these do offer the best listening experience. Welcome to the “re-invented” Class D Amplifiers era!

I also wanted to ass the right audio source to Acuhorn's RATE. As with tube amplifiers, vinyl is the best source of sound, especially if you are listening to analogue recordings from before 1980s period. I do realize however that it is not the perfect medium in all respects and not in the whole sound picture. For a long time, the LP was winning, right from the moment the stylus hit the groove with all other mediums, and even the first R-2R Ladder Philips TDA1543 converter and the PCM1704 did not change that.

However, the high resolution files with the use of technology, i.e. FIFO signal buffering and re-clocking in high performance FPGAs, have changed this. Delighted by progress and achievements that have been getting closer and closer to the analog sound, of market leaders such as dCS, Chord Electronics, MSB, I decided to use this type of DAC in my source. During my search I came to the conclusion that the best choice would be the module of the Danish company Soekris.

To make operation of my audio files player simply, I used the MoOde player wirelessly controlled. With this solution Acuhorn R2R plays all types of files in high quality. I designed for it a Vicor super power supply for the player almost identical to the one I created for RATE. I am absolutely sure that power supply is one of key elements of any player. The high frequency SMPS, similar to the ones used by Linn in the Dynamik series, are almost perfect.

The player and amplifier are both small and one might be tempted to place one on top of the other. However, I suggest you don't do that because both devices generate a lot of high frequency noise, and so they can interfere with each other. Therefore, during the test, I built a kind of "sandwich" of two Acuhorn components and three passive RFI / EMI Verictum X Block filters. For all connections I used Acoustic Revive Triple-C cables, except for the digital interconnect - Acrolink 7N-DA-6100II Mexcel - which I used to connect the transport section of the Ancient Audio CD player. I listened to the power amplifier with the Ayon Audio Spheris III preamplifier, the DAC with the CD player as a source, and to the complete system.

The system is very easy to operate, and the web application to operate the player works well enough to let you play music without constantly digging into settings. The only problematic issue is an idea to skip the preamplifier. I mean understand the idea behind this but its implementation reduces the functionality of the system. Let me explain:

One adjusts volume either in the player or in the amplifier. The volume control in the player is performed in the digital domain, which means a loss of signal's resolution. I am concerned with the fact that this regulation also works with DSD files, and as you know the DSD signal is not processed. This means that it must somewhere be converted to PCM. So I would suggest, if possible, to resign from this function, i.e. to disable it in the DAC.

The volume control in the amplifier is not very precise because it is stepped and does not cover the whole volume range. I.e. it starts at a certain level, not from complete silence. Furthermore, the input sensitivity is too small to play loudly with loudspeakers of comparable sensitivity to my Harbeth M40.1, which was a particular issue with classical music and high definition files, that generally carry a lower average signal level. It would be better if amplifier's volume controlled cover the whole volume range and for the knob to be on the front or side wall. The current solution is really annoying.

Every technology leaves its mark in the sound. If it is repetitive then called a “signature” or “inherent” sound to this particular technology. When such a view is widespread and generally adopted, these qualities become a stereotype. And stereotypes are both useful and dangerous because apart from the original function of being used a useful as a short for a more elaborate issue, they also tend to freeze the point of view on what the stereotype really describes. And that hinders any discussion, closing us in within certain routines. And yet the value of the audio in the form in which we do it, i.e. a perfectionist audio (audiophile) lies in constant search. That is why we constantly replace components, cables, racks, music releases with different ones - we strive for something better than what we already have.

Interestingly, there is no universal stereotype of amplifiers working in class D. For some it's a technology that offers a cold, impersonal sound, and for others it's a sound that lacks details, treble extension and closed-in. Bearing in mind that the stereotype does not come out of nowhere, is an essence of some general view, it must be said that each of the above quoted versions has its own origin. If you were watching what was going on in the last several, let's say twenty, years, you should realize that there were amplifiers on the market that justify both above described stereotypes. However, they have been outdated for a few years now.

The breakthrough, at least in my understanding, came with the amplifiers of the Japanese company SPEC. Their founder and CEO, Shirokazu Yazaki, prepared a device that at first glance was little different from other products of its species. But this was only the first impression, because under the "skin" this was a completely different beast. I think that the same can be said about the Acuhorn amplifier. Although in this case it is obvious from the start that we are dealing with something completely different.

It offers a sound that could be very easily classified as "a classic SET sound". And not even 300B-like, and offering even lesser output but extremely musical triodes such as 45 and 2A3. The RATE performs in a relaxed way, creating a huge space. Not even the sound stage, because imaging is of a smaller scale, but about space. This is a feature characteristic for power triodes, which is extremely difficult to replicate using other solutions. Maybe it is an effect of the minimal complication of such tube circuits and ultra-short signal path? If so, then repeating this topology, but using a PWM modulator and transistors can be an elegant solution to the problem of a small output power.

Wojtek Unterschuetz's amplifier also builds very reliable instruments' bodies. It does not cut them from the background, but shows them in a “rounded” way. Yet, the bodies are clearly marked. Instruments amazingly interact with each other, primarily through spacial and tonal differentiation. Because it is tonal presentation that is the key here in my opinion, even if it is the spacial aspect of the presentation that will force most listeners to re-consider their beliefs or accepted stereotypes. Both, the files player and DAC share it with the RATE – the whole system delivers an amazingly colorful, tuneful performance, but using color in a way of Dutch masters rather than pop-art's ones. The point is that the upper treble is not “stingy”, and yet there is nothing missing in it. The whole is slightly withdrawn.

So we get beautifully three-dimensional and highly energetic performance. The latter feature is an Achilles heel of each of the solutions used, here, i.e. of files players and discrete converters, and class D amps. Manufacturers handle this in a variety of ways, either hardening the attack, or lightening the tone – there are many solutions. The Acuhorn system appears to be "untouched" by such solutions. The fact that it is able to present the magic of Kurylewicz's band when playing Go Right from both, the Master CD-R and the 24/88.2 master file is truly noteworthy.

But you should also realize that it is not a system focused on presentation of a strong rhythm, or pushing forward. I would say that it behaves a bit like a warm sounding turntable, except it is not an artificially warmed-up sound. But the lack of a harsh attack and an annoying treble evokes the stereotype of "analogue sound." And the system copes with it very well, because this was a constant impression through all the listening.

The player is a good example of a "do no harm" school. Music played through the S/PDIF input from the transport section of the Ancient Audio The AIR V-edition CD player had more power, better definition and more was happening in it, there was a clear “drama”. The DAC in R2R presents a similar characteristics, i.e. it retracts the phantom images behind the line connecting the speakers and builds a credible space in our room – it does not transfer the instruments to us, but us to that space. At the same time it is extremely resolving and it accentuates dynamic changes.

Each of these elements tries to reach the core of music, i.e., the emotions, encouraging listener to reflect on what he listens to. The system does not attack us with information, it rather feeds us with them subcutaneously, it smuggles it in larger packages, so that we do not perceive the sound as highly detailed. I would say that R2R together with RATE draw bigger assumptions and based the whole presentation on them, pushing in-depth analysis of them, so to speak, behind the horizon. It's all there, but we have less access to it than with more unambiguous components.

Summary

The Acuhorn system plays music in an extraordinary way. There are many elements of the sound associated with turntables and SET amplifiers, but without pretense, without mimicry. It's a different technology as the very well controlled, low bass and some kind of calm indicate. The devices encourage to listening to the music in a discreet - nomen omen - way. They build a huge space – deep and surrounding listener, if the recording allows, but with the foreground situated behind the line between speakers.

The amplifier sounds soft, but it's incredibly resolving. This is, apart from the SPEC amplifiers, the only device of this type, which I can sincerely count among a sophisticated high-end class devices. Did I say already that it is a similar sonic aesthetics as that presented by Kondo Overture II? No? So let me say that.

The DAC follows the same direction, but with emphasis on softness. It's not as dynamic and energetic as with the power amp. Also the player has a similar sonic signature, but it's not as resolving as the amplifier and the DAC. All three create together something special. I have no doubt that I was right from the very beginning - Wojtek Unterschuetz is a true artisan in his profession. Although from the outside and from the inside, both new products look like some industrial machinery, they re-produce music in a way that should speak to the fans of analogue sound. The music always comes first, closely followed by emotions, and thirdly is sound differently from most similar devices. RED Fingerprint.

The devices share an identical housings, made of two aluminum parts. They are very low and compact, which together with thick walls created an extremely rigid shell. They function not only as shielding but also as a heatsinks. There are no LED indicators on the front walls or any other ones. Manipulators – volume control and in the amplifier additionally also input selector - are located on the rear walls, which is not particularly convenient for users. An automatic switching of DAC inputs helps a little.

2R2

I already mentioned that Wojtek Unterschuetz is an artist. But I did not realize that he was bold enough to completely emancipate himself from the schemes imposed by the audio industry. One look into inside of the "R2R" and one can see something that one most likely hasn't seen ever before, and certainly not in any commercial products.

At the heart of the device one finds a small Raspberry Pi 3 microcomputer model B. This system is designed as a low cost computer for everyone. It features a 1.2 GHz Quad Core Broadcom BCM2837 64-bit processor, 1 GB RAM and BCM43438 chip for wireless LAN and Bluetooth. In this case a wired connection is used. The PCM or DSD digital audio signal is sent to an external D/A converter.

This one was designed and manufactured by Soekris Engineering. Almost always the DAC is a small chip, an integrated circuit with miniature transistors and laser-corrected tiny resistors. The converter board in R2R is 80 x 150 mm and is controlled by the extremely efficient Xilinx Spartan 6 FGPA. It handles digital signal processing as well as the integrated switching circuits and those control nearly 100 surface mounted resistors. The output signal is sent in one of two ways - either in a direct mode, i.e. directly from the output, or is buffered in a transistor circuit that increases the output voltage and reduces the impedance.

However, the most space is occupied by the power supply. It is built around the SMPS module made by the US company Vicor Industries, the same as used for the power amplifier. Here, however, the DC-DC converters modules work together to stabilize voltage - separately for the computer and separately for the DAC. Similar modules are used in the amplifier. Internal wiring is the same as in the amplifier - see below. Let me add that the RCA digital input is buffered with an impedance matching transformer.

RATE

The amplifier's interior looks different than the usual one of Class D amplifiers. Almost always, the manufacturer of the device uses some third-party modules as the base for their design. Here the manufacturer's own contribution is much larger. At the heart of the RATE operate small Texas Instruments amplifier modules. They are powered by Vicor Industries (FARM1C21 and V375C24C150AL) modules, screwed to the bottom of the device. Interestingly, they have been developed not for audio, and for power supply systems and industrial automation solutions. The power supply is extremely efficient (750 W) and delivers high voltage (375 V). The circuit is protected against overload.

The input was built around the Lundahl LL1521 B transformers, matching impedance and gain. Before them there is a hermetic switch with miniature Nichrome SMD resistors, which determines the sensitivity of the circuit. In the output there is a large board with elements that shape the sound of amplifiers of this type, i.e. with the modulator and the transistors, but also with coils and capacitors forming the reconstruction filter (output). All connections are made using expensive, high quality solid-core PCOCC copper wire. The RCA and speaker connectors are not soldered but clamped - this is one of the best ways to connect components.

Specifications (according to manufacturer)

R2R
input voltage mains autoranging 90-264Vac
maximum input power 20W
discrete R-2R sign magnitude DAC type
digital volume control
3 signal input I2S by RPi, SPDIF Coax and Toslink
2 audio output R-2R direct and buffered
internal cable OCC copper
terminal rhodium
weight 1.5kg
dimensions W 320 x D 200 x H 28mm

RATE
input voltage mains autoranging 90-264Vac
maximum input power 340W
monolithic class D amplifier type
stepped attenuator volume control
line high impedance input transformer
input sensitivity 2V
output power 2x150W
internal cable OCC copper
terminal rhodium
weight 1.8kg
dimensions W 320 x D 200 x H 28mm