Building a Mic Pre

From pjk on Mar 12, 2007

I'm starting a project build of a tube mic pre.

I've been playing guitar through a tube amp for awhile (67 super reverb), and am very fond of the tube sound. I'm also building a studio in my basement, and started thinking that my pre's on my yamaha board are a little too sterile and clinical for my taste, at least for vocals and acoustic.

So I went looking for a tube preamp schematic. Building a few guitar amps, the preamp section seemed pretty simple, so I figured I should be able to find a mic preamp that was similar, but built for mic usage. It was not so easy to find a simple build.

Over on the reaper board I was posting my dilemma regarding hard-to-find tube pre schems, and someone over there posted this:

http://www7.taosnet.com/f10/micpre.html

This is a preamp with parallel triodes, feeding a mosfet source follower. The writeup on the page seems pretty stable, and the builder seems pretty knowledgable, and was very impressed with the preamp, so that was my choice. Plus, it's not overly complex, like some of the other tube preamps I've found.

Now I had a choice picked out, I need to get a build design. The builder used a PCB, home-made, I'd assume. I haven't done home-made PCB yet, so I decided to pass on that idea. I'm more inclined to persue the point-to-point layout, or an eyelet board. Seeing as how I already have an eyelet board from a previous not-built-yet project, I decided to use that.

Here's a layout of my plans so far:

* image removed due to layout changes. new one coming soon *

This isn't totally proofed, so don't think this is a perfect layout design. This is just what I've put together so far. I'm pretty confident with it, but I have to go over it point by point, to make sure I don't have any logical mistakes.

There's no power supply on the layout, or the schem. I'm creating one on my own, using duncanamps.com power supply designer II.

I've designed one using a hammond 270x power transformer, which provides 240vac per side, as it has a center tap. This xfrmer should give around 310vdc rectified with a bridge, full wave, or tube full wave rectifier. I've not decided which I'm going to use, but I'm leaning towards a solid state rectifier, but not firm yet.

The input transformer is a jensen, which I purchased directly from jensen, for 73$. This is the highest one piece price of the preamp. The power transformer I purchased from stf-electronices, for 39$, i think. The components (res, caps, etc) I purchased from mouser.com, as they have a nice project manager function, which you can build up the parts of a project, save it, and come back later. I purchased 2 of all the components, so I can easily build up a second preamp, if the first one goes well. Then I'll be set for stereo. The power supply can power both sides, as preamp tubes take very little current, around 5ma or so, so I'm set there. I'll just have to purchase another input xfrmr, and I'll be ready to go.

I don't have a case / chassis figured out yet, but I will pretty quick, as most of my parts have shown up, so I'll be starting to populate my board pretty soon.

More later.

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