Questyle CMA800R headphone amplifier

Headphone amps and amp/DACs
Questyle Audio CMA800R
Questyle CMA800R headphone amplifier

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.

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