Cayin C9II

The vast majority of an amplifier's tuning comes from its initial input and voltage gain stages. Here, we see how far Cayin have come since C9 was released in 2021.

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 reduces negative feedback to a bare minimum, 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.

READ ALSO The full blow-by-blow story of C9II's development

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 comes 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 less errors to correct in the first place.

Cayin's biggest asset in that regard comes from differential, balanced operation, where common-mode rejection enters the picture to amplify 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 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 set-up.

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 analog job to commercial ICs, Cayin's labour of love continued into its discrete, differential, output 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 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 feedback loop with the push of a button.

The reduction in feedback to simple local quantities innocuously just controlling small-signal gain means sound becomes all the more natural. A faster, simpler and more responsive circuit is just able to present musical information in a more effortless way. 

Capable of following the impedances of the Empire Ears Raven, Odin and Legend Evo down low, Cayin's tradition building amplifiers that provide actual power as proper voltage sources into halving resistances continues with C9II.

This is Cayin's latest exercise in old-school linear amplification brought away from the shackles of heavy hardware and into a portable future – with nary a compromise in sound, and adapted to manage the increasingly complex crossovers flagship IEMs represent.

The versatility C9II yields in tuning options is only acknowledgement by Cayin for how much the portable audio market has grown. This Nutube hybrid promises to adapt to and improve your ever-changing transducers and portable sources, making you the chef that is in full control of the flavour of your audiophile nourishment.

For much, much more detail into the portable technological advances C9II heralds, check out Cayin's full development story here plus the rest of their gear here, and all our other portable amplifiers here.