[August, 2009] JFET phono stage prototype

Transistors. I know. This is a major divergence from my tube-o-philia. The truth is, I only got into tubes because they were easier to figure out than solid state stuff. My whole thing was that I wanted to learn to build my own stereo system. I looked at a bunch of solid state amp designs and got totally put off by the complexity, and turned to tubes. Fortunately tubes sound really great.

But lately I have been trying to figure out simpler, cheaper, and lower-voltage versions of the tube circuits I love. It turns out not to be so easy. Eventually I stumbled on a bunch of info on using JFETs in audio circuits (Nelson Pass, Borbely) decided to give solid state a try.

The result is pictured above and below.

This is a super simple phono stage built with 2SK170 JFETs for the gain devices. It is just 2 common-source amplifier stages with a passive RIAA EQ section between them. This is a topology that is very popular with tube circuits, and is about as simple as it gets.

There is a little voice in my head that asks whether old tube circuits sound good because of the simple and elegant circuits, or because the tubes themselves are superior devices. I think probably both. Based on the few times I have listened to relatively complex Williamson amplifiers and whatnot, I think that the topology has a lot to do with it too. This project was a way to try a tube-like circuit using solid state devices.

Having listened to it for only 2 sides of an album so far as I write this, I think the results are pretty dang good. This is much more transparent than my Radio Shack phono stage, and also seems to have better bass. And more gain. And is just as quiet. Now I need to compare it to some tube phono stages.

The other thing I'm excited about is the ground-plane construction style. I have seen a lot of RF circuits built this way and have been wanting to try it. It worked out really well. I first built the circuit to look just like the schematic, then folded it up a little so that all of the leads that needed to be grounded would face down. Then just soldered all the grounds to the circuit board. It is surprisingly sturdy, and seems to be very quiet. Definitely a cool way to prototype a circuit.