Balanced tube amplification with matched paired KORG Nutube 6P1

Our Vacuum Tube/Solid State Dual Timber feature shook the world back in May 2018. It was considered a miracle to the portable audio industry when we managed to solve the microphonic problem with innovative techniques and for the first time, facilitated the choice of vacuum tube and solid-state timbre in a portable DAP. Unfortunately, we can only install one Nutube 6P1 in N8, so the vacuum tube is only available for Single-end headphone output. We received loads of comments and requests to make the vacuum tube timbre available to balanced output as well. Naturally, when we start our N8ii project, one of the priority requirements is to implement a fully balanced Tube Amplification circuit in N8ii.

Circuit Design

To be frank, with their experience and track record of Cayin on the design and manufacturing of vacuum tube amplifiers, circuit design with Nutube in a balanced configuration is hardly a real challenge. Ever since we laid our hands on KORG Nutube 6P1, we have conducted a very thorough study to apply the “flat” miniature Double Triode in different circuit designs. We have gone through over 30 rounds of PCB with Nutube already, which roughly tells how exhaustive we are on this subject.

Based on the feedback from dealers and users on N8 and C9, and our observations of market response in the past 10 years (Cayin entered the Personal Audio market in 2013), we adopted the Zero Gain Negative Feedback Loop circuit in the N8ii design. This will deliver strong current and impressive handling at a difficult load. We have adjusted the feedback loop carefully to enhance the transparency, transient and extension that can rival the best solid-state circuit but yet remain warm and mellow.

To a certain extent, Zero Gain Negative Feedback Loop is not a design that is preferred by typical tube enthusiasts because this offers less tube flavor relatively. However, we believe this is more in line with the market preference as portable DAP users tend to appreciate transparent, crispy and well-defined presentation with faster transient. In our opinion, there is no right and wrong on these issues. The vacuum tube is the mean, not the end, therefore maximizing tube flavour is not necessarily the most important objective. It’s our duty, as a vendor, to understand the market we serve and to provide options different needs of the bigger crowd.

Reinventing the Nutube installation.

The Dual Nutube implementation in N8ii is even more challenging than C9 because of the limited space and form factor. There is not enough room to lay the two Nutube flats N8 and C9 because we need to accommodate a bigger screen for Android implementation. We can’t lay it flat on the bottom panel either because that will reduce the space available for the battery. Finally, we started the process all over again and developed a mechanical shock absorption and damping system to install the Nutube vertically. The new installation system involves:

• Compartmentalized CNC Aluminum chassis to create an isolated, fully damped chamber for Nutube installation
• Flexible Printed Circuit (FPC) to minimize shock transmission
• Custom Built Shock-absorption Silicon Housing to hold and damp the Nutube
• Vertical-hanged Suspension Pin to mount the Nutube and Silicon Housing into the designated chamber securely

Having a dedicated chamber to isolate the Nutube from the main PCB is a head start in dealing with microphonic and anti-interference. We are surprised by the final result of this new design, it works like a charm and we are confident that the N8ii can cover our daily activities effectively.