[2006-12-28]

I need a preamp for my TV system. The push-pull spud amp has no volume control, and I have more than one input. And I'm sick of swapping cables. I like the idea of a simple, transformer output preamp. A lot of preamps use a cathode follower to get a low output impedance, but I'm not sure I like the way cathode followers sound. And I sure don't need all the gain of a 6sn7 gain stage. So a transformer output seems like a good solution. I'm calling this transformer output because the novel thing is to have a transformer in a linestage, but it could also be considered a parallel feed or parafeed linestage since that is how the transformer is configured. But that seems more common than a series fed linestage by a mile. Whatever.

I originally planned on using the Edcor XSM 10k:600 ohm line transformer for the output, but it is unshielded and in thie tiny chasis, it hummed like a bastard. So I went with my favorite little red nugget of copper, nickel (I think it is a nickel core) and mu-metal - the Altec 15095A.

Unfortunately at the time of building this thing, I only had one 15095A. So imagine both sockets filled with transformers. Or cover one eye. Or something.

3 inputs and 1 output. The switch is a ground-lift switch. I wanted to take advantage of the transformer output somehow, and I didn't need balanced outputs. You can do balanced outputs by connecting the top of the transformer to the hot, the bottom to whatever they call the not-hot-but-not-ground connection of a balanced interconnect, and the ground to ground.

After it was already done, I thought it might be cool to make the output a TRS connector. It would work as a balanced output, and if you want to use RCA connectors, you could plug in a TRS-to-RCA adapter.

As you can see, the chassis is pretty stuffed. It was even more so with the Edcor transformers stuffed in there. You can see how an unshielded transformer would pick up hum from the power transformer. It had over 5mV of hum with the Edcors. Don't get me wrong - the Edcors are supposed to sound great, and certainly did sound great above the hum. They just need to be at least 6 inches or more from the power transformer. It's easy to see how far they need to be. Just turn on the amp, or at least the power transformer, and hook up a meter to the secondary of the transformer. Since the hum is purely inductive, the transformer doesn't have to be in the circuit. You can move it around wherever you want and watch the meter. When the meter reads 0mV, you have found a good location for the transformer. If you want to make it even more quiet, hook the meter to the primary (more windings) and orient it so that you can't read any voltage.

Upgrades: Try connecting the bottom of the transformer primary to the cathode. Better plate resistors, or ideally plate chokes.