A Valve Phono-stage Cube


Radu RIAA top

How I’ve got to build this phono stage? My first phono stage build was Kevin’s “The Muscovite” using a nice sounding HF triode 6S3P. I was delighted with the sound of it, but never got beyond breadboard stage. The second build was Salas’s “Valve Itch” phono stage. I had a pleasant sound and dead quiet.

Few years later, I pushed myself to build a good phono stage for my Dual turntable and few records that I own. I had few items that determined the final shape of the preamp: many 6S3Ps bought when trying the Muscovite, a complete RIAA filter components, a readymade cherry box from a different project, and not only. I almost could say that I used most of the components from other projects leftovers.

The design is mostly influenced by Merlin Blencowe. I used the 6FY5/EC97 for the first stage and 6S3P for the second stage. The output is low impedance due to a Mosfet source follower DC coupled to the second stage.

Having a single tall box, imposed to a toroidal mains transformer and SiC rectifiers followed by a CLC filter. To ensure a very low ripple and low output impedance of the power supply, I’ve used an shunt regulator SSHV1. ¬†As a must, I’ve used a regulated supply for the 6.3V filaments.

I’ve also built a Lipshitz-Jung precision Inverse RIAA to test the phono stage. It showed a perfect flat frequency response and a 41-42dB total gain. The THD measured 0.04% at 1Vrms output.

Despite the simplicity of the circuit I’m very pleased with the sound result.