T.H.E Show Newport 2015: Questyle QP1 and QP1R Digital Audio Players
I'm pretty sure the world doesn't need another digital audio player unless it's something unusual and different. Fortunately, Questyle is indeed "different" when it comes to their electronics design philosophy.
In a typical solid-state audio electronic circuit, the audio signal is a voltage going up and down in a rather jagged and complex manner. To ensure the stability of the circuit, negative feedback is used. A bit of the signal at the output is fed back in reverse polarity to nul the input signal. The problem is, because of the capacitive nature of solid-state components interacting with the voltage signal, it takes a small amount of time for the signal to travers the circuit before it gets fed back to the input. This propagation delay causes the negative feedback to not quite line up with the incoming audio, and produces transient intermodulation distortiona particularly dissonant and unpleasant sounding type of distortion.
Many high-end audio designers choose to design amplifiers with zero feedback to get around this problem. Unfortunately, that produces problems of its own in that the output impedance of the amplifier can't be forced to a very low number, and zero feedback designs may become unstable and oscillate or make odd chips and tweets.
Questyle takes a completely different approach by converting the audio from a voltage signal to a current signal within their productsa technology they call current mode amplification. Current signals act quite differently and don't suffer the propagation delay problems of voltage based circuit designs. As a result, Questyle claims almost unmeasurable amounts of transient intermodulation distortion in their amps. The current signal is converted back to a voltage signal at the output of their ampsthese are not transconductance amplifiers.
Current mode signals have been used, primarily for signal transmission, in high-end audio products before. Krell, for example, had CAST (Current Audio Signal Transmission) technology for audio transmission between components. But using current mode active amplification is rare as it is quite difficult to achieve the level of parts tolerance and matching needed to pull it off. And therein lies Questyle unique value proposition: they've figured out how to efficiently manufacture electronic circuits that require extremely tight tolerances.
And now, in the QP1 digital audio player, they've produced an all discrete component, class-A, current mode circuit small enough to hold in your hand. The QP1R uses the same design, but increases the quality of various critical components within. Output impedance is claimed at a remarkably low 0.19 Ohms.
I had a fun time playing with the QP1R for about ten minutes in the booth. The font size on the display seemed a little small and hard to read for me, but otherwise I found the user interface to work well and was rather more intuitive than many rotary wheel user interfaces I've experience previously. I'll certainly be hunting down a review sample for John Grandberg to include in his DAP round-ups.
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