If you’ve invested in a pair of high‑end headphones, you’ll want a high‑quality headphone amp to power them. And headphone amps don’t get much better than this...
British pro‑audio electronics manufacturers DACS have built an enviable reputation for high‑quality studio headphone amplifiers over more than two decades. Mostly, these have been marketed under the ‘DACS HeadLite’ banner, and I’ve reviewed several models over the years, all of which have been highly capable and versatile, with the latest version (the HeadLite 3+, reviewedin SOS August 2021) having been upgraded to provide sufficient power for even the most demanding of low‑impedance headphones which dominate the market today.
Today, the very best headphones tend to be relatively power‑hungry ‘planar magnetic’ models, which only achieve their true potential when paired with very well‑designed and capable headphone amplifiers. To that end, the boffins at DACS decided they’d set out to build the ultimate headphone amplifier — one that could realise every nuance in the audio signal with total fidelity through the world’s most demanding headphones. The design took two years of careful development, and the result is this: the Purity stereo headphone amp.
Black Box
Shipped in a surprisingly large, velvet‑feel cardboard case and well‑protected in fairly dense, moulded, black polystyrene, the Purity itself is a pleasingly simple and understated black box, with few connections and even fewer controls. A brief user manual describes some of the technology and design choices, but it really isn’t needed to get up and running. Also included with the review unit was a hand‑made, heavy‑duty IEC mains cable (using oxygen‑free copper wire). I can’t say I noticed any audible difference between that and a cheap, standard moulded IEC cable but it’s a nice inclusion, nonetheless!
The sturdy steel chassis measures 210 x 100 x 300mm (WHD, including feet, knobs and rear connectors), and it weighs a little under 5kg, which is lighter than it looks. The rear panel carries an IEC mains inlet with integrated fuse and separate 120/240 V AC mains voltage selector, while the audio connections are a pair of RCA phono sockets for unbalanced inputs and a pair of combi XLRs for balanced inputs. It’s worth noting that as these inputs feed physically separate circuit boards for the left and right channels, the left and right connectors are spaced 55mm apart. While that won’t be an issue for XLR or TRS cables, it might be a challenge for some exotic hi‑fi RCA phono cables built with the expectation of closely adjacent sockets.
Connection posts on the rear panel provide external access to the chassis/mains safety earth (black) and the audio ground (green), and the two are kept separate internally.
A pair of spring‑loaded connection posts is also provided on the rear panel, for external access to the chassis/mains safety earth (black) and the audio ground (green); these two ground references are kept separate internally. I expected to find a suitable rod or wire link in the box to couple these terminals, but none is supplied and I operated the unit without linking the audio/chassis grounds in a variety of systems without any issues at all. As the headphone amplifier is biased into Class‑A mode all the time (see ‘Technology’ box) the output drivers get quite hot, and so a large cooling fan is built into the lid of the unit. However, this is genuinely completely silent in operation.
Moving around to the front, a very large volume knob dominates the centre of the panel, with an embedded light to indicate the Volume setting. Initially glowing red, the light gently fades and pulses like a heartbeat as the unit warms up, gradually moving through purples and on to blue as the unit reaches its optimal operating condition — an effective and elegant feature. To the right of the huge volume control is a miniature toggle switch selecting the unbalanced RCA phono or balanced XLR/TRS inputs (via sealed relays), and a circular mains on/off rocker switch is also present.
Over to the left are two separate headphone outputs in the form of a standard quarter‑inch stereo TRS socket and a 4‑pin XLR. The manual doesn’t specify the XLR wiring configuration, but it appears to conform to the ‘standard’ (left channel positive on 1, negative on 2; right channel positive on 3 and negative on 4). Both outputs can be used simultaneously if needed, albeit without individual level control.
The XLR output is described as ‘differential’, which I think is actually a misnomer (see ‘Balanced Headphones’ box). Its key advantage is that with suitably wired headphones the signal currents flowing through the two drivers are kept completely separate and returned directly to the power supply in a star‑earth arrangement (with near‑zero‑Ohm impedance). The benefit of this approach is zero crosstalk between channels; something that can’t be guaranteed with more conventionally‑wired headphones connected via the quarter‑inch socket.
Bench Testing
The balanced input can easily accept +26dBu (the limit of my Audio Precision test set) without overloading the circuitry, while the power output stage starts to clip at an output level of +23dBu (0.5% THD, with a 600Ω load). That equates to about 31V peak‑to‑peak, or about 1.2W (RMS) into 100Ω headphones. In other words, there’s more volume here than anyone is ever likely to need! Frequency response measures ruler‑flat between 10Hz and 20kHz, reducing ‑0.5dB at 5Hz and 30kHz, while THD measures below 0.002% at all sensible listening levels with an even balance of odd and even harmonics. Amplifiers just don’t get any cleaner or more transparent than that.
My overriding impression with all these headphones was of effortless power and stunning clarity, with superb dynamics and a 3D‑like separation between individual elements of complex mixes.
I auditioned the Purity with my full gamut of headphones and in‑ears (headline models: AKG K702 and K812, Sony MDR7509, Sennheiser IE800 in‑ears, HD25‑II and HD600). Most of these have impedances between 16Ω and 80Ω, although the HD600s are 300Ω. All were connected via the quarter‑inch TRS socket, except the Sennheiser HD600s which I modified to connect via the XLR output.
My overriding impression with all these headphones was of effortless power and stunning clarity, with superb dynamics and a 3D‑like separation between individual elements of complex mixes. I’ve never heard the K812s sound so revealing, with outstanding control and definition across the full bandwidth. It mattered not that the Sennheiser IE800s presented a lowly 16Ω impedance, or the Sony MDR7509 just 24Ω — the Purity delivered oceans of transparent power and vice‑like dynamic control, dealing easily with these low‑impedance, current‑hungry headphones. The amplifier’s high voltage swing deals equally well with high‑impedance headphones, and the HD600s performed superbly well with fantastically crisp transients.
I also compared the sound of the HD600s using both XLR and TRS connections, but I couldn’t hear a reliable difference. There was no audible crosstalk when driving a single channel with either format, but this isn’t a fair test as even the standard TRS cable maintains four wires back to the plug, avoiding the common ground return issue. In contrast, though, driving a single channel of either the AKGs or the Sony headphones (through the TRS socket) revealed surprisingly audible crosstalk on the undriven channel, which certainly proves the technical benefit of a four‑wire connection. In practical terms, I was occasionally aware of low‑level crosstalk on carefully chosen tracks with all of my TRS‑connected headphones, but it was totally absent using the HD600s with either connection format.
Heading Out
The DACS Purity is a mighty headphone amp. Its understated styling and simplicity belies its impressive sonic credentials and outstanding capabilities. If you want your headphones to deliver their full potential the Purity can certainly ensure that, and although it is far from an impulse purchase I doubt you’ll find better at any price. Simply stunning!
Balanced Headphones
Headphones are traditionally connected using three‑pole plugs (either quarter‑inch or mini‑jack) and a connecting cable with three wires. The plug tip carries the left channel, the ring carries the right channel, and the sleeve carries a common ground connection from the negative sides of both earpiece drivers. However, although cheap and convenient, the inherent resistance of the cable itself can cause unwanted crosstalk between channels.
Current passing through both earpieces returns to the amplifier via the shared ground wire, but this has a small but finite resistance. Consequently, a small signal voltage develops across this wire (an attenuated mono sum of the left and right channels) which gets added to the wanted channel signal voltages feeding each earpiece. Consequently, some left channel signal inherently crosstalks into the right channel, and vice versa, and although quiet it can be surprisingly audible, which is clearly undesirable!
Some headphone manufacturers obviate this problem of a shared ground return by cabling each earpiece separately back to the jack plug using a four‑wire cable. In theory, the headphone socket’s sleeve connection at the amplifier should be solidly grounded, preventing currents from the two channels mixing and hence preventing the crosstalk. Many Sennheiser headphones are wired in this way, for example.
However, it has become increasingly popular to avoid the shared ground connection altogether by employing a four‑pin connector instead of the traditional TRS plug and socket. Most manufacturers have adopted the four‑pin XLR for this purpose — as on the Purity — although I’ve seen some bizarrely choose a pair of three‑pin XLRs instead!
So that’s the connection format... and it is often associated with the idea that the headphone amp uses ‘balanced’ or ‘differential’ connections. However, the vast majority of mains‑powered headphones amps actually use standard single‑ended amplifier configurations driving only the positive terminals of each earpiece, returning the signal current directly to ground in the usual way (just via separate ground connections).
A true ‘balanced’ or ‘differential’ headphone amplifier requires two output amplifiers for each channel instead of one, with the headphone earpiece connected between these amps (and no ground connection at all). This is the same configuration as the ‘bridged’ mode offered in some stage power amplifiers, where the speaker is strapped between two amplifier channels configured so that one ‘pushes’ while the other ‘pulls’!
In theory, a bridged amplifier configuration applies double the signal voltage swing of a single‑ended setup, making it significantly louder and more dynamic, and also more able to cope with low‑impedance headphones. It’s a useful solution in battery‑powered devices where the power supply rails are restricted, or where low‑power amp modules can be doubled up to provide greater power. However, for a high‑quality design doubling the number of amplifiers makes it a more costly and complex solution, and the same peak voltage swing benefit can be achieved simply by providing a single‑ended amplifier with a higher‑voltage power supply — which is exactly what DACS have done in the Purity!
Purity Technology
The cooling fan, which you can just about make out here in the top‑right quarter of the unit behind the grille, is a technical necessity — but it’s also slow-spinning and inaudible.Technology‑wise, the Purity employs an internal linear power supply installed along the right‑hand side of the unit with a compact toroidal mains transformer feeding sophisticated balanced ±16.5V regulation circuitry on an adjacent circuit board. Two tall heatsinks cool the voltage regulator chips.
A pair of independent headphone power amplifiers are built on two more circuit boards running the full length of the unit, front‑to‑back, down the centre and left‑hand side. Each of these cards also employs heatsinks to cool the discrete output transistors, which are complementary Darlington transistors (MOSPEC TIP115 and TIP110). Although using a push‑pull output configuration, the devices are biased at around 200mA to operate fully in Class A, inevitably generating unwanted heat, which is why an ultra‑low‑noise PWM‑controlled fan is fitted to the case lid directly above the heatsinks. Convection cooling alone was never going to cut it with practically sized heatsinks!
The rest of the active audio circuitry on each card comprises a pair of Analog Devices AD823 dual‑JFET op‑amps used to provide an instrumentation‑style differential input amplifier and to drive the output devices. Sealed relays are used to select inputs, and audio‑grade JFX polypropylene input capacitors are in evidence. A trio of multi‑turn cermet trimmers on each board ensure accurate factory calibration of offset and bias voltages. The front‑panel volume control is an ALPS Blue Velvet, which has a smooth and nicely weighted feel.
Pros
- Simple and elegant styling.
- Effortless power and control regardless of headphone impedance or volume setting.
- Four‑wire connection via XLR to avoid channel crosstalk.
- Multiple input connection options.
- Excellent build quality.
Cons
- You’ll need demanding headphones or ears to justify the cost!
Summary
An impressively capable headphone amp with remarkable transparency, control, and effortless power. Both XLR and TRS output connections are provided, along with XLR/TRS and RCA phono inputs.