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Questyle CMA800R headphone amplifier

Questyle CMA800R headphone amplifier

OK, I have to admit this review of the Questyle CMA800R had a painfully long gestation period, due to me getting it colossally wrong at the outset. You see, my ‘go to’ place for running headphone amplifiers in involves connecting them to a decent DAC and using my pair of ever-reliable Sennheiser HD-25-1 II ENG/Studio cans. This is because the HD-25 design is sufficiently sensitive to be driven by anything. And in the CMA800R, that wasn’t like putting the headphone amp in low gear; it was like switching it off!

So, when the CMA800R returned the sound of the Sennheiser HD-25-1 II, I made a rookie error: I expected ‘magic’ and benched the Questyle. And it stayed benched. It was only reading just how much the company’s QP1R digital audio player took our Publisher Chris Martins that I remembered this was lurking in the back of our storeroom. In fact, there were two of them, because one of the great joys of this headphone amp is it lends itself very easily to monoblock use. In fact, the CMA800R now forms just one part of the company’s top four-box desktop stack, comprising DSD-ready DAC, preamp, and mono headphone amps (the preamp, originally commissioned by Stax and designed to complement the Stax SR-009 Earspeaker/SRM 727MkII, can act as a useful controller in its own right, both improving overall performance and bringing one volume to rule them all). There is also a special Golden version, which uses higher specified components, as well as the gold finish. We’ll look to the full stack in a later issue – first it’s time to catch up with the CMA800R as a one-box headphone amplifier.

This is a pure headphone amplifier, without a DAC. The CMA800R has one stereo balanced and one stereo single-ended input (with a small toggle switch on the front panel, it’s almost best to think this a one-input amplifier), and a pair of single-ended outputs to a preamplifier. There is also a single full-balanced XLR input, should you decide to use the CMA800R as a mono pair of headphone amps. There are two single-ended ¼” TRS jack sockets for single-ended headphones, and a balanced headphone output XLR for one half of a true balanced headphone output.

The ‘CMA’ in the title stands for ‘Current Mode Amplifier’. This is the defining aspect of the headphone amplifier, and in many ways shows why Questyle chief designer Wang Fengshuo (Jason Wang) is so highly respected in the audio field that a company like Stax would approach him to build an amplifier. And it owes its development at least in part to a failed experiment. While still at university in 2004, Wang was debugging a failed current mode circuit that should have been processing communications signals, but was instead acting as a high-speed amplifier with vanishingly low intermodulation distortion. Wang, already a music loving audiophile, hit the books to see if this circuit had been used in audio amplification, and fortunately his teachers saw the innovator rather than the failed circuit, and let him run with the concept.

 

Wang and his classmates ultimately designed an exceptionally high performance audio circuit, and his search for other designs only partly bore fruit. He recognised that companies like Krell were beginning to explore the advantages of current mode, in the company’s CAST (current audio signal transmission) system, but where CAST uses current mode a means whereby signals can pass from device to device with the minimum of noise and distortion, Wang applied the technology across the entire amplifier.

Essentially, current mode acts as it sounds like it acts: the signal is amplified by modulating the current rather than the voltage. Current and voltage are not exactly strange bedfellows, their relationship is forged in Ohm’s law, and the resultant current mode amplifier creates an inherently low distortion and wide bandwidth design. Current mode is a very common amp design in high-speed communications and video processing. The CMA800R features an additional voltage-controlled current source and a current amplifier in front of a more traditional Class A output stage, but creating those two amplifier stages requires a low-impedance negative feedback circuit that reacts a couple of orders of magnitude faster than conventional voltage mode amp designs. In addition, the amplifier’s slew rate achieves a linear increase as input signal amplitude increases, in direct proportion to the input signal amplitude. When receiving a high amplitude signal, a current mode’s amplifier’s slew rate is much faster than traditional voltage mode devices, eliminating intermodulation distortion and ensuring a high amplitude signal, with an extremely wide linear bandwidth and an almost distortion free realistic playback.

The other big advantage here for Questyle is Wang is not simply an electronics designer, but a keen listener, and spent four years, 22 model iterations, and eight complete back-to-the-drawing-board circuit redesigns in order to make a circuit that is notionally a world-beater, into something that sounds a true world-beater, too. Having developed the CMA800R circuit, Wang Fengshuo then stacked the amp full of some of the best components you can get (Nichicon and WIMA capacitors, mil-spec DALE resistors, Shottky rectifiers, and a custom Piltron toroidal transformer), designed into an elegant, all-business milled aluminium chassis, and handed the manufacture over to electronics experts Foxconn. Well, if it’s good enough for Apple…

Going after a complete rethink in amplifier design is all well and good, but the more pivotal questions are ‘why?’ and ‘what does it do for the sound?’ In fact both questions are answered in one: using current mode delivers and amplifier that reacts to real-world dynamic signals we listen to (as well as steady-state signals we measure to) better than other designs. My rookie error with the Sennheisers actually masked what the CMA800R does so well – deliver the sound of your headphones, without grace or favour. With a pair of headphones designed for use with the output of any passing video camera, mixing desk, or smartphone, that’s no big deal. They are designed to deliver detail at this grade, but lack the nuance and finesse to go deeper into the music beyond a basic analytical level: that’s not a criticism of the HD-25-1 II, more a statement of design intent.

 

Push the headphone envelope and the CMA800R just keeps telling you what those headphones are capable of and what the DAC is capable of, too. Not in a revealing way (although if you use a DAC or a set of headphones that is lacking in some manner, the Questyle CMA800R will expose that limitation – it just isn’t so edgy that it sounds like its parading the limitations of other devices), but in a way that highlights everything about the up and downstream components.

The CMA800R is a phenomenally dynamic amplifier, too. In a way, it sounds ‘free’ in the way some of the best single-ended triode amps can sound with efficient loudspeakers, but without the lush midrange and lack of high-frequency extension. It’s extraordinarily detailed and transparent, too, and there’s one last feather in its cap: the other part of the name – that ‘800’ part comes from the fact the amplifier was designed as a result of Wang listening to the Sennheiser HD800 and thinking it was a great headphone in search of an amplifier. If you own a pair of HD800, this is your amplifier. Stop looking – this is it! And if you don’t own a pair of HD800, but something in the same vein, this is probably your amplifier too! In fact, the only limitation to the CMA800R is that some of the more difficult headphone loads would need more amplifier lifting power to drive them. Like, maybe, a second CMA800R…

I think the CMA800R is the headphone amp that grows with you. Good headphones require a great amplifier, and that’s where the Questyle CMA800R comes in. It’s so good, you might start to look at the matching CAS192D DAC, possibly even driven by the Questyle DAP. At which point the weak spot is your good pair of headphones, and you change the cabling for balanced operation. Rather than have to give up your great headphone amp, you just add another CMA800R. A few months later when you are done with using two volume controls, you’ll buy a CMA800P preamp. To someone who hasn’t experienced the CMA800R that sounds like hyperbole, but to someone who has, it’s the next steps in their headphone enjoyment plan.

It’s not in a reviewer’s interests to stop looking for the next big thing, but I can’t help feeling that when it comes to headphone amps, the Questyle CMA800R is all I’ll ever need. And if I need more, there’s always the second CMA800R! Very highly recommended.

Technical Specifcations

  • Type: balanced and single-ended headphone amplifier
  • Input:
One pair XLR stereo, one pair RCA stereo, and single XLR mono balanced
  • Output:
Dual 6.35mm stereo headphone jacks, three‑pin XLR mono balanced output, one pair RCA stereo pre-amp output
  • Gain: 15.5dB
  • Frequency Response:
DC–200kHz (+0, –0.3 dB); DC‑650kHz(+0, -3 dB)
  • Max Output Power: 180mW (7.5Vrms) @300Ω (stereo),
710mW (15Vrms) @300Ω (mono)
  • Sensitivity: 1.2Vrms
  • Impedance: 47KΩ
  • THD+N:
0.00038%@1kHz, 300Ω (stereo).
0.00026%@1kHz, 300Ω (mono)
  • SNR: 114 dB (stereo), 118dB (mono)
  • Dimensions (W×D×H): 33×33×5.5cm
  • Price: £1,599

Manufactured by: Questyle

URL: www.questyleaudio.com

Distributed in the UK by: SCV Distribution

URL: www.scvdistribution.co.uk

Tel: +44(0)3301 222500

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