It’s no secret that I admire Nelson Pass both for his design skills and for what he gives to the DIY audio hobby. Unfortunately, us vacuum tube enthusiasts are mostly left out in the cold when Pass flexes his design muscles. That is until Burning Amp 2017 when he presented a pre-amp using the Korg NuTube.
Ok, so it isn’t the first “tube” that comes to mind when we think thermionic emission, but hey, it’s got a vacuum at least!
The Korg NuTube is a twin “triode” made by adapting vacuum florescent display technology to audio applications. Just like a DHT, it has an anode, a grid, and a directly heated cathode. The principals of operation (i.e. emission from cathode to anode modulated by a grid voltage within a vacuum envelope) are essentially identical to the little glass bottles we all know and love.
However, the low-voltage miniaturized technology requires certain compromises. With a max dissipation of only 1.7mW, the NuTube is limited in the maximum anode voltage and this forces positive grid operation, requiring a buffer to drive the inevitable low impedance. Like wise, the high plate impedance also necessitates a buffer on the output for most applications.
This is exactly what we see in Pass’s design, using his signature CCS-loaded JFET follower style buffer stage at the input and output. The result is a very compact and relatively low-voltage tube preamp with around 16dB of gain (as designed). Best of all, you can pick up a board and parts at the DIY Audio Store here!
I had the opportunity to hear this preamp not long ago in a nice system (Magnepan speakers, tube and solid state amplifiers) and compared it with an Alex Cavalli designed tube buffer and a MOSFET-based preamp. The Pass preamp sounded fantastic in this good company.