With UK importer Guy Sergeant dispatching my loaner, he confirmed required listening maturity. "I've sold a few but it's been difficult, mostly because I don't think that headphone-based listening is anywhere near as evolved in the UK as it is in parts of Asia; particularly those parts where floor-space costs and the required speakers might cause friction with nearby neighbours. The other interesting feature—which I've noticed some other Japanese manufacturers also are adopting—is not to pursue maximum damping factor. They seem to have realized that the hoops that would need to be jumped through appear to compromise sound quality. Guaranteed to upset the measurists!" Maximum DF means maximally low output impedance. This usually means copious amounts of negative feedback which means more stages or output devices to make up for the loss of gain caused by NFB. From this we get at four obvious questions for Shinobu Karaki. How many gain stages does he use; is there any negative feedback (and if so, is it tied to the gain switch to be variable); what is the output impedance; and why go after bridged mode?

These very softly suspended vertical gain cards sport fat gold-plated traces on their backs.

"Total gain is 19.6dB in high gain mode. This equals an amplification factor of ~x 9.5. We do use negative feedback but it does not tie to the gain switch. Low-gain mode attenuates the signal after the input buffer amp and before the volume control. Hence the NFB value remains constant and so does the output impedance. But we don't disclose its value. As you know, the damping factor should be low for best speaker drive across all frequencies. In modern audio engineering, that's common sense except for non-NFB tube amps. For headphones, the same theory works less well. High damping factor is good for speakers but makes for a hard dry sound with headphones. That's why some headfi amps have variable impedance to tune the damping factor for particular loads. This function looks good and adds value but is not good for THD and S/NR. For Heada, I designed a DF adjustment circuit for best headphone sound. This is a fixed value but works well for many loads. I evaluated almost 50 different headphones to arrive at this.

One of two socketed BurrBrown OPA604 op-amps in the foreground. TKD pot in the back.

"For headphones, bridged mode drives each channel completely separately by not sharing a common ground. This improves separation for better 3D staging and deeper ambient recovery. It also increases high-power drive to suit planar-type loads which need high power. It also creates wider dynamic range with good signal-to-noise ratio so one can enjoy very low playback levels. And you're correct, the XLR input is not true balanced and only takes the hot signal. The internal circuitry is single-ended up to and including the TKD volume pot. It then becomes balanced for the headphone output. There are only two gain stages: input signal --> input stage buffer --> volume control --> current buffer --> output signal. This keeps constant impedance for both input and output at any volume level and the pot can be a low-impedance type to have good THD and lower gang error."

Max power transfer for the Heada happens into low-impedance headphones. It drives 2wpc into 40Ω in balanced mode, half that into 6.3mm. These figures shrink to 800mW/240mW @ 600Ω. Total harmonic distortion at 1kHz is 0.004% regardless. Input sensitivity is 1Vrms and input impedance a standard 47kΩ. Minus 1.5dB bandwidth is a broad 10Hz to 120kHz, making this a fast circuit with low phase shift. Half size is 26 x 25 x 10cm WxDxH, i.e. of nearly square footprint. Weight is 3.5kg.

To do the XLR-rated deed, I had Audeze LCD2/XC, HifiMan HE 1000 and MrSpeakers AlphaPrime. To throw in a universal reference, I'd also use Sennheiser's HD800 in 6.3mm mode. Except for the HifiMan's stock cord, all other leashes would be Polish aftermarket issue from Forza Audio Works' Noir range (hybrid for the planars, pure copper for the dynamics). For amp comparators, I had two Questyle CMA800R amps and the Bakoon AMP-12.