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Building the A12 Microphone Preamp
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October 15, 2008

Building the A12 Microphone Preamp

Lately I've really been in the mood to build things. So I was very happy when I finally got some cash lined up and plunked down for the really nice-looking Seventh Circle Audio A12 preamp kit. My audio/recording friends will know why this is such a big deal, but for the others, it's a thingy that makes mics sound better. And its way cheaper than many of the alternatives. But the real beauty of this particular kit is that you can mix and match eight different channels of several different "sounds" in one box. So it's got lots of room to grow. Check out their full line of preamps at

Here's a basic walkthrough of what I did to get my preamp running.

When you get the kit and break through the really great job they did packing the thing, you find yourself with a chassis & power supply prebuilt, and then a circuit board, some bags of parts, and a schematic.

So you have this blank circuit board in front of you and it's a little daunting. There are a lot of holes and many of them are tiny and close together. Luckily, you have just bought a nice soldering iron and some helping hands, so you shouldn't worry too much. Download the assembly instructions pdf and get going.

Be sure your work area is well-let, and start stuffing the components. Follow all directions carefully, look up what you don't fully understand, and double-check each resistor with a DMM. I just have a cheapy $10 DMM i got at Sears, but it was enough to complete the project with relative ease.

Remember to trim the leads on the back a little. It will make working with the board easier. I used a nail clippers to do this, I'm not sure what you're supposed to use...

Before long, it will start looking like something:

The kit uses high-quality Cinemag transformers, which should make EVERYONE happy. Audio guys sometimes underestimate the amount of coloration that comes from big iron transformers, and just call everything that's colored "tube."

Before long, the board is fully-populated.

Before you go plugging anything in, be sure to follow the instructions carefully and test as much as you can. I did fully re-check all the resistor color codes and cap values, as it said. It took a while, but you can smoke components if something is wrong. And that's a hassle, I'd imagine.

At about this time you realize that there's a few things you need to do with the power supply too, like attaching the ground wire (VERY IMPORTANT), and soldering up the lamp and power switch. My only gripe out of the whole project came here, because the PDF of instructions for the power supply appears to be for a previous revision where you had to do more of the wiring yourself. Now, they ship with a wiring harness. This normally would make your life a lot easier (and probably still does), but the instructions are vague on what you need to do and what you can skip. Also, the freakin' little metal things you crimp on the wires to get into the white connectors are REALLY hard to get on solid. I suggest soldering them all after you crimp them, to be safe.

Now all that's left to do is power up, do more testing with your DMM, and then calibrate the voltages with your DMM. Note that at no point should the bottom of the board or any of its electrical contacts touch the case or anything. This could create a short and I think bad things would happen. Note the bubble wrap:

You're supposed to connect the leads to the op amp at one point here, and it is a little tricky. Some notes: You NEED both alligator clips and probes for your DMM. Mine just had probes, so I bought a cheap bag of alligator clip jumpers to attach, and that worked fine. It just looked funny. Also, you might wonder how you're supposed to get the clips on the op amp leads while the op amp is still in the socket - well, the simple answer (for me) was to clip them on to the back of the board, attaching them to the big metal post things the op amp pins go into. Hope that makes sense. You'll figure it out either way, I'm sure.

Wahoo! Stick it in its case, plug a mic, power it up, and YOU'VE GOT YOURSELF AN AWESOME PREAMP!

Hope that's been enlightening. I hope to do more of these photoblog type things for more projects in the near future.

Posted by pedalboy at 10:44 PM | Comments (2) | TrackBack