The successor to the genre-defining C9 throws in no-feedback operation, local feedback adjustment around dual 6P1 Nutubes, and anode voltage adjustment to reinvent the rule book, once again. In its rawest form – with no global feedback and the barest minimum around its Nutube voltage gain stage, where the majority of an amplifier’s signature is shaped – C9II is an exercise in circuit purity. It’s a simple circuit that outperforms any complex one, especially in portable form factor. With Hyper mode, where operating voltage and bias get pushed to the linear open-loop maximum, and fully balanced differential operation, Cayin equipped C9II with the means to operate at its maximum in every possible mode of operation. A listener’s ability to adjust every parameter including tube anode voltage and the feedback loop around Nutubes means up to five potential sonic signatures are possible. The operation of C9II’s Gen 5 Nutube circuit in Classic mode eliminates the use of global negative feedback, increasing immersion with a simpler and faster circuit that avoids time lag and transient intermodulation distortion. Distortion figures also increase on paper but are of a low-order variety that is less jarring to the ear, compared to the complex intermodulation associated with unabated feedback levels Here is where a good old linear open-loop circuit based on immaculate parts gain matching raises the victory flag. And that’s before Class A bias and high operating voltages come into play, setting up C9II for success. You can even play with Nutube anode voltage for very audible sonic outcomes. Throughout, C9II allows linearity specs to remain respectable even without error correction, because there are fewer errors to correct in the first place. Cayin’s biggest asset in that regard comes from a differential, balanced operation, where common-mode rejection enters the picture to amplify the signal and block off non-linear distortion and noise. While 6P1 Nutubes are always going to be the start of the show, and adaptable to sonic preferences and tastes, don’t ignore C9II’s solid-state input either. Four channels of single-ended Class A Toshiba JFETs work in a quasi-complementary fashion. Just NPNs, without PNP mismatches, plus even-order harmonics and square law make for a totally unique differential circuit in a portable setup. Once you find your ideal match from five possible permutations, the input signal passes through JRC’s MUSES 72320 resistor ladder volume controls – kept far away from the input and implemented only after the driver stage to preserve fragile small-signal integrity and excellent signal-to-noise ratio. This preserved resolution is handled all the way by a discrete transistor signal path. C9II’s design is more than up to the task of preserving the signal – mere op-amp ICs need not apply. Instead of wasting their input stage advancements by entrusting the analogy job to commercial ICs, Cayin’s labour of love continued into its discrete, differential, analogy stage for common-mode rejection. C9II’s driver section counts on all the benefits that come with a discrete FET & bipolar design – low open-loop gain and the resultantly lower negative feedback necessary, high slew rates, current-handling and unity gain stability. Cayin’s headphone power amplifier section is inherently stable without going into oscillation caused by too much gain product. The utmost attention is paid throughout. Differentiating itself from even other amplifiers that cling to op-amp layouts seeking THX-certification but with inordinate amounts of gain and feedforward/feedback, C9II’s linearity remains excellent already in the open-loop, owing to its high operating voltages and bias. Those ensure individual transistors are working in optimum condition. Distortion and noise are kept low, without resorting to the excessive use of negative feedback. Cayin designed a true-balanced differential amplifier to further improve C9II’s linearity, and you can count on common-mode rejection at its best whenever you plug in via 4.4mm preserving its resolution. It’s an output stage that’s direct-coupled to improve low-frequency performance, and C9II audibly plumbs the lowest bass depths. With the extension down low, phase reproduction in the audible human hearing range for taut, hard-hitting and life-like dynamics is excellent. At the tip of the power stage’s spear, large bipolar devices work in parallel emitter-follower configuration to ensure excellent beta and current handling for a good reason, reaping the full benefit of Cayin’s quad of 18650 batteries outputting direct DC and working without voltage boost conversion. These power transistors are linear enough in the open loop, running fully biased and at high operating voltages, that they can be removed from the global feedback loop with the push ...