Cannery row. "Current-mode drive has another big boon for headfi. Most headphone drivers exhibit an almost flat impedance across the audio spectrum. That results in more stability and better response. As the HPA-21 [next to AMP-21R at right] drives headphones with current, impedance doesn't factor. We can expect far lower distortion. Such has been studied by Dr. Yoshiaki Muda of the Department of Electrical Technology at the Nara University of Education in a 1996 paper.

"During his experiment he found that when a speaker drove from a current source, he achieved up to 1/10th its distortion. The HPA21's current output achieves the same. The major drawback of current drive for speakers is that despite their continuous impedance changes across all frequencies, the current-driving force remains fixed. Therefore the unit's impedance curve becomes the frequency response of the driver.

"Because of this it's not appropriate to use current drive for most loudspeakers whose impedance varies too greatly. With headphones using one driver of mostly constant impedance however, we can safely adopt current drive with great results both measured and subjectively. Damping factor with a headphone amplifier is of less significance especially in current drive. The formula for damping factor is D=R/Z (damping factor = load Ω/amp's output Ω). Since the HPA-21's voltage output is about 1Ω, a 10Ω headphone sees a damping factor of 10. A 100Ω headphone sees 100. Changing the load just varied damping x 10. The HPA21's current output is about 2.5MΩ. This renders damping factor effectively zero. Now the music signal itself becomes the drive current. That is unaffected by any other variables to generate much higher and more precise output drive."

To remind us what the HPA-21's batteries were back when, "our supply doesn't incorporate the BPS-02 but EQA-11R circuit. That runs a dual-voltage supply on higher-voltage batteries. We use four cells in series to generate 16.8V. Our battery control circuit is as complex as the BPS-02's. Once again in the analog domain it must control the likely dissimilar behavior of two batteries simultaneously. To make this circuit non-stop would require a total of four Li-ion batteries plus the complex power-cycling control circuit of the BPS-02. This would overreach the HPA-21's size and budget. During prototyping we already achieved up to 7 hours of playtime despite rich class A bias. The automated recharging process cuts off during playback but starts as soon as the power switch is depressed."

HPA-23RM photo shoot.

Now we've walked Memory Lane back into the HPA-23RM's mid 2022 present. For portability, there's 116 x 164.5 x 22mm WxDxH and 730g. My first guess was close but outsized. The real deal is just two stacked jewel cases thick, slightly narrower than their short end and a tad deeper than their long side. Forget an oversized parka. This will fit into the side pocket of any gent's sports coat. Ladies won't need a Birkin tote bag either. M not for murder but mobile? It's deadly serious. Flaunt designer cans in the great outdoors at your own risk. I'll venture as far as Ivette's greenhouse in the backyard. It's all glass without utility power. Come rain or gusty winds, it's a place to chill surrounded by plants and swallows doing aerial acrobatics. Dog fights without any dogs. It's an ace bubble of seclusion. Completing my off-grid test headfi would be a Shanling M3 Ultra and vintage M1 Pro Esther DAP. Unlike Questyle's infernal capacitive click wheel on the QP1, Soundaware's seven hard front controls have aged beautifully. Reliability trumps flash.