A recent discussion on diyaudio.com reminded me of the opamp-based RIAA idea I shared last summer. It turns out that someone else has done something similar and reports very good results. Koifarm was after a more integrated build with phono, streaming, and line level all in a box, but the basic idea of using an opamp to perform the RIAA corrections and a tube to provide some/all of the voltage gain is the same. We differ just in how it’s integrated: I’m after a simple RIAA module with outboard tubes while Koi was going for an all-in-one.
Here’s where I’ve landed so far on a board to contain the opamp bits and bobs:
The opamp runs from the 6.3Vac heater winding that would be included on any tube-centric power transformer, meaning there are no special windings or an extra transformer to power the solid state section. The output of the RIAA module would be fed to a simple tube gain stage of your choice. Here’s a grounded cathode application, but keeping the tube off the board means there’s tons of flexibility.
So, will it work? Koifarm thinks so and he’s a pretty prolific phono preamp builder. I’ve also already tested the same RIAA correction scheme in the battery powered phono project. I’m saving up a few designs to place a board order, but I’m hoping this RIAA module would make for a relatively simple and fun build this year.
Bob Katz has been writing a series of articles over at InnerFidelity for several years and they’ve recently taken a turn down a more experimental path. His most recent article details a device that mixes a transparent solid state signal and a Nuvistor signal biased to provide a distortion spectrum with just a small percentage of second harmonic. Check out his write up here!
A Nuvistor is a small metal and ceramic tube released by RCA just as transistors began supplanting vacuum tube technology in most electronics. They are a true vacuum tube with familiar triode operation and characteristics and an indirectly heated cathode. The most common Nuvistor in consumer electronics was the 6CW4 (high Mu) though there are several triode flavors and even a couple of tetrodes.
Because they were originally intended for radio and TV usage, Nuvistors enjoy very good bandwidth, low noise, and high gain (high Mu variants). The metal envelope is integrated with the basing and the tube plugs into what RCA dubbed the Twelvar base. You can probably guess how many pins that had. With the Nuvistor, RCA also introduced the RCA Dark Heater, a lower temperature filament that guaranteed higher stability and less AC leakage. Despite this innovation, most Nuvistor heaters require around 1W to light (e.g. 150mA @ 6.3V).
The 8056 used in Bob Katz’s project has the following characteristics:
And the following very respectable plate curves:
With a modest Mu, low plate resistance, and very low B+, it’s no wonder Bob decided to marry this interesting tube to a solid state partner for his Blender. The 8056 heater requires 6.3V at 135mA. At this voltage and heater requirement, it’s close to being practical for modern portable devices. In their heyday Nuvistors were used in battery-powered and efficiency-critical applications like the US Space Program and military radios and communications equipment.
Would I ever build something with Nuvistors? It’s tough to say. I’ve been on a casual hunt for tubes that might be suitable in a portable battery-powered application. Other candidates are the Korg Nutube or the sub-mini 6088. Like all things in this hobby, there are trade-offs. The Nuvistor 8056 heaters are hungry relative to these other options, but the other characteristics are very attractive. In all likelihood, I’ll try them all eventually. This is why I DIY.