Holo Audio Azure - Headphone Amplifier/Pre-Amp - Review

Review written by Ian Dunmore (@Torq)

Introduction

Holo Audio are probably best known, in the USA at least, for their DACs; in particular their extremely well regarded Spring DAC, and perhaps more recently for their all-in-one Cyan DAC/amp unit, which I have previously reviewed. Holo Audio’s product line-up is not limited to digital products, however, and their latest offering is a compact, fully-balanced, solid-state analog headphone amplifier and pre-amp called “Azure”.

This review was made possible by the kind loan of the Azure unit from Kitsune HiFi, who are also “Holo Audio USA”. Some of this review was conducted specifically with the matching Holo Audio Cyan DAC/amp, which was also on loan for this process.

Features

The Holo Audio Azure is a fully-discrete, fully-balanced/differential headphone amplifier and pre-amp and is equipped wit full remote control. The amplifier elements are modular, with four for each channel; specifically there is one buffer and one gain stage for each phase of each channel. This means four modules per channel and eight modules total. Each module features about 50 transistors, in a configuration designed to eliminate/cancel-out any non-linearities while maintaining high slew-rate and bandwidth.

Volume control is achieved via a 64-step relay-switched analog attenuator, operating in 1 dB increments. This uses multiple, individual, precisely matched resistors to ensure there is no channel imbalanced even on the lowest volume settings.

Headphone outputs are the usual 1/4” (6.35mm) TRS single-ended socket and a 4-pin XLR balanced connection. Power output in low-impedance mode power output is 400mw into 32 ohms single-ended and 1600mw from the balanced output. In high-impedance mode you’ll see 180mw into 300 ohms single-ended and 720mw balanced.

Headphone impedance matching is provided, allowing for both high and low impedance cans to be driven without skewing their frequency response and maintaining appropriate damping levels.

The simple, and neat, rear panel has both single-ended (RCA) and balanced (3-pin XLR) inputs (locking) and pre-amp outputs. A standard IEC AC input receptacle allows the use of your choice of power cable.

Review Equipment & Material

Headphones used for this evaluation include: Focal Utopia and Clear, JPS Labs Abyss AB-1266 Phi CC, Audeze LCD-4, Sennheiser HD820, HD800S, HD660S & HD650, ZMF Eikon (Padauk).

Sources (DACs) used in this review include the matching Holo Audio Cyan, Chord DAVE, Schiit Audio Yggdrasil Analog 2 and RME ADI-2 DAC.

Amplifiers used for comparison were the SPL Phonitor X, iFi Audio Pro iCAN and Woo WA234 Mk2 MONO.

The majority of the music I use in my evaluations is in “Red Book” CD format (16 bit, 44.1 kHz), most of which comes from CD rips; an initial playlist for my audition listening can be found here. Where appropriate/referenced I utilize a number of high-quality, high-resolution, albums, needle-drops, and also some native DSD content.

Build

As seems to be typical for Holo Audio, the build of the Azure is substantial and exudes luxury - using a heavy, all-metal, black-aluminum case/chassis with solid copper side-cheeks. I find the aesthetic to be one of the best in the audio world at the moment, being both understated, classy and yet still distinctive.

Internal construction has similar attention to detail with a, heavily-shielded, separate internal enclosure for the high-quality toroidal transformer - which is the foundation of the linear PSU implementation in the Azure. Speciality Mundorf capacitors and other appropriately high-end components and connectors are featured throughout.

Even the full-function remote is a full metal, aluminum and copper, build. Which while a consistent thing for Holo Audio’s products, is often not the case for other manufacturers - even those on the extreme high-end.

Operation

In use the Azure is very straightforward; the unit has balanced (4-pin XLR) and single-ended (1/4” or 6.35mm TRS) headphone outputs on the front panel, along with the display and indicators show volume level, selected input, and output mode, and the main dial that controls both volume (w/ instant mute) and input/output selection.

In normal use, turning the dial raises and lowers the volume as you would expect, with each 1 dB step being accompanied by the audible, and solid-sounding, click of relays switching the resistor attenuator network. As mentioned, there are 64 steps for the full volume range, beginning at a displayed value of “36” (-64 dB) and a maximum of “100” (0 dB attenuation).

The main display indicates the currently selected volume level, and can be set to multiple brightness levels - including fully off.

You can either use the remote to directly switch inputs, outputs, volume level and muting functionality, or the main dial can also accomplish this - pushing and holding the knob changes which setting it affects, and turning it alters the value or setting in question.

The RCA (single-ended) and XLR (balanced) inputs are treated as separate inputs, so you can connect two different sources to the Azure and easily switch between them. Both of the outputs are driven in parallel and are both active when “Line Out” mode.

There are three output modes on the CYAN, which are cycled through using the “output” button on the remote (or by pressing and holding in the main dial). These modes are “LINE” out which, as expected, sends the output to the RCA and XLR connections on the rear of the unit and “HP LO”/“HP HI” for low and high-impedances headphones respectively.

Niggles

I have one minor issue (and it IS minor) with the operation of the Azure, and it’s the same one I had with the Cyan - namely that the volume level changes too slowly for my tastes. The granularity of control is nice, but having the rate of change scale with how fast the volume dial is turned would be a nice addition - as it is each “detent” on the dial always results in a 1 dB change.

Sound

Due to the Azure being an aesthetic, and form-factor, match with the Holo Audio Cyan, and something that is likely to be considered alongside/stacked with that unit, I am going to tackle my thoughts on the sound in two specific manners. The first will be a direct comparison to the built-in amplification of the PCM version of the Cyan, and the second in its use as a general purpose headphone amplifier/pre-amp (which is the appropriate application if you have a DSD-module equipped version of the Cyan).

Azure & Cyan

The Holo Audio CYAN in PCM form has a built-in headphone amplifier (and pre-amp output), and a cursory look at the specifications will show that it has the same basic output rating (power-wise, at least) as the Azure. This raises obvious questions:

• “Is it worth adding an Azure if I already have the PCM Cyan?”

... and …

• “What do I gain if I do?”

… and perhaps less obviously …

• “What if I only care about the pre-amp?”

The last question, “What if I only care about the pre-amp?”, is the easiest to answer: There is no reason to buy the Azure as a pre-amp to pair with the Cyan PCM, unless you have an additional source you need to switch. The line-output of the Cyan PCM will not be improved by the Azure - even a perfect amp/pre-amp could only avoid degrading the signal! So … that, as they say, is that.

If you need a pre-amp or amp to go with the DSD version of the Cyan, then you’d be served very well with the Azure - with the added benefit that it is a perfect aesthetic match - but more details can be found in the “General & Pure” section.

The first question, “Is it worth adding an Azure if I already have the PCM Cyan?”, is harder to answer than the second, and depends a great deal on both what you’re listening to, and what you’re listening with. There is no question that the Azure offers significantly better performance than the built-in headphone output of Cyan PCM. However, the headphone amplifier on the Cyan is the best I’ve heard in an all-in-one unit to date and is more than sufficient to get the most out of the majority of mid-range headphones (and a good number of top-of-the-line/flagship units) and from any non-lossless source.

If you’re running Sennheiser HD800(S), Focal Clear, ZMF Eikon, Audeze LCD-3/X (and above) and similar level, or better, headphones, then the Azure is something definitely worth considering. Otherwise, invest your money in better headphones first and then come back to an amplifier upgrade (the Cyan’s built-in amp is really that good).

So … to question two! Assuming a suitable source … and appropriate headphones, what do you get from adding the Azure to the Cyan PCM? Increased resolution, both in terms of raw detail and micro-dynamic resolve, better transient response, purer tone, and despite the identical power-ratings somewhat more drive authority with difficult/insensitive loads.

The drive-authority part, particularly with the LCD-4 and Abyss AB-1266 Phi CC, was surprising - since the raw output from the Cyan PCM is supposed to be at the same level (i.e. the same power delivery). Regardless, both of those cans sounded closer to their absolute capabilities via the Azure than when driven directly from the Cyan. Bass-slam was more visceral, and higher peak-volumes were achievable without encountering any sense of strain.

General & Pure

The Azure can act as both a pre-amp and a headphone amplifier. I am going to address the pre-amp functionality first, simply because it’s the easiest to do and it feeds nicely into the evaluation of the unit as an amplifier.

Pre-Amp

Here, the Azure distinguished itself as being both entirely neutral, imparting no audible tonal shift to what it passed on, as well as exhibiting excellent transparency. In comparative terms, tonal purity was on the same plane as the iFi Pro iCAN (in solid-state mode) and SPL Phonitor X. Transparency was on par with the iFi unit and just slightly behind the Phonitor X. We’re splitting hairs, here, between the transparency innate to good passive attenuation and an actively buffered interface, and well into the realm of things that would only be apparent in an already super-resolving configuration, unusually high-quality source material and paying significant attention to tiny details and nuances.

Headphone Amplifier

It does not take a lot of listening time with the Azure to determine that it is a very clean, fast, highly-resolving, neutral, dynamic and transparent amplifier, which plies its trade against an inky-black background - something that enhances both low-level dynamic contrast and seems to add to the vividity of its musical delivery.

There is no tonal emphasis or shift that I can hear. If neutral was what Holo Audio was shooting for (and it should be), then they scored. Glissandos are as smooth as one could want, and one end of the piano is a clean and even as the other. If you’re looking for an amplifier to correct tonal issues elsewhere in your chain, or to add some kind of euphony or coloration, then this is the wrong place to look! Compared to the direct output of a Chord DAVE instrumental timbre was similarly unmolested with instruments sounding natural and authentic with no change in apparent weight or body.

Azure adds no stridency or sibilance to difficult female vocals or otherwise shrill compositional elements. And at the opposite end of the spectrum, driving, tuneful, baselines are kept whole. No matter what is going on at the extremes the mid-range remains composed, detailed, lucid and balanced.

Transient response is extremely fast. The ultra-incisive nature of the Cyan PCM DAC in NOS mode, coupled with music employing lots of electronic percussion or, in particular, plucked-string work, illustrates this nicely. Staccato elements are perfectly punctuated with instant start/stop when called for, without impinging on the proper attack or decay of notes. Coupled with the impressive dynamic performance of the Azure, dramatic music, with big level swings, massive crescendo, contrasted with delicate, solo, pieces are rendered with impressive scale and believability.

Micro-dynamics are also excellent. Not necessarily state-of-the-art, but well above the norm all the same. Minor vocal inflections are easily discerned. This reveals the depth and character to voices, as well as making it easier to hear (and indeed feel) the emotion in a performer’s delivery. In a few cases, other (admittedly more expensive) amplifiers were also able to expose very subtle details, such as the inconsistent draw of a bow over strings or differences in how a brush was being applied to a cymbal, which the Azure did not resolve as clearly. And to be fair, these are the sorts of details that you both have to be aware of and generally have to specifically listen for, anyway.

To assess transparency and raw resolution, I used my Chord DAVE and compared how it sounds feeding the SPL Phonitor X, iFi Pro iCAN and the Woo WA234 MkII Mono to the output of the Azure, as well as to the direct output form the DAVE (which is, of course, the most transparent way possible to listen to its output). And in this regard the Azure fares very well indeed. A simplistic ranking would place the Azure ahead of the Pro iCAN and behind the SPL Phonitor X in this regard, but reality - especially in audio - is rarely that simple. With the right combination of source material, source and headphones and a lot of listening-focus it is possible to discern that the Azure just out-resolves the Pro iCAN, and that both are similarly out resolved - in details and micro-dynamics - by the Phonitor X and the big Woo. Still … that’s some pretty heady company to be keeping and speaks very well of the Azure’s capabilities.

On a more purposeful, but less granular/technical level, sitting back and listening to music via the Azure was a lovely experience. I frittered away far too many hours that I could have been doing something productive just sitting and listening.

vs. Pro iCAN

The Azure out resolves the Pro iCAN (in all three of the iFi amplifier’s operating modes) in terms of raw detail and microdynamic resolution, and does not have the sense of added tonal density that the iFi unit imparts on the music. As result it sounds a bit faster and more open than the Pro iCAN, with somewhat more air around the upper registers. Additionally the Holo Audio amplifier presents a more tangible stage, with a better sense of space around instruments - sounding more like an accomplished tube-amplifier in this regard.

The Pro iCAN pulls ahead when it comes to raw drive authority, flexibility and does a better job with really demanding cans - keeping them sounding tighter and more coherent at higher listening levels.

If you need the added power, or the features/functionality of the Pro iCAN, then that might be a more appropriate choice for you, but on raw technicalities and in reference to an amplifier being “a wire with gain”, the Azure is technically ahead.

vs. SPL Phonitor X

This is really where you have to set your sights if you want something that can deliver more power and/or better resolution/transparency than the Azure. While the Phonitor X remains the best solid-state amplifier I’ve heard, the Azure is no slouch in comparison. These units offer similar neutrality, but the Phonitor X is a lot more powerful, and is discernibly more transparent and resolving, especially with regards to micro-dynamics - at least if you have a suitably resolving source.

The Azure has a better native presentation of stage, though the Matrix functions on the Phonitor X will put it on-par, or maybe ahead, when properly employed.

If I was pairing with high-end sources and more difficult to drive headphones, the Phonitor X would be my pick. Otherwise there isn’t enough in it to make me choose on more than aesthetic or feature grounds - and those are always going to be a personal thing. For example, back to back with the Cyan feeding the Phonitor X and the Azure, it was only the power difference that the Phonitor X exhibited … as the Azure was fully resolving the output of the Cyan and definitely does that unit justice.

Summary

This is a natural, and excellent, pairing with the Holo Audio Cyan in DSD form. With the PCM version, which already has a built-in headphone output, it’ll kick overall performance up to the next level, exposing more of the detail and more nuanced dynamics that the Cyan is capable of and is definitely worthwhile if you have headphones that can deliver at that level.

Simply put, if I was going to put an amplifier with either flavor of the Cyan, then it would be the Azure. There’s no need to look further. And you get a perfect aesthetic match to go with the high-end sonic performance.

In more general terms the Azure still gives a decidedly excellent account of itself. Pair the Azure with most flagship headphones (like the Focal Utopia, above) and it delivers a resolutely excellent performance and will drive most such headphones with real authority well past the point where you’d be wanting for more. And unless you’re running with the highest-of-high-end sources, on the level of the Chord DAVE or Schiit Yggdrasil you’re not going to miss out on anything your DAC can deliver.

The only real caveat I have here is, simply, that the most difficult headphone loads are not the best match for the Azure. To be sure, it does a creditable job with the likes of the LCD-4 and Abyss AB-1266 Phi CC, and is usefully better than the native output from the Cyan, but doesn’t have quite enough raw grunt to show them at their absolute best - particularly if you like to listen loud (though it is unexpectedly better with both than the native output of the Cyan PCM). Something like the HiFiMan HE6 or HE6SE would be a non-starter.

Overall technical and audible performance is excellent. And the musical enjoyment on offer is also superb. You have to step up to markedly more expensive solutions to find something as resolving, fast, transparent and neutral while retaining the Azure’s ability to engage you and portray the raw emotion of the music quite as vividly.

This is an easy unit to recommend and a slam-dunk if you’re looking for something to go with other Holo Audio units.

All in all I think the Azure is an excellent amplifier and definitely something one should be considering if you’re lucky enough to be shopping for solid-state gear in its price range.

-Ian Dunmore (@Torq)

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Join the discussion about the Holo Audio Azure at "The HEADPHONE Community".