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The Federal AM864:

After building the STA-Level I was interested in building a 2-channel compressor and came across the schematics and design ideas for the Federal AM864 on the wonderful website Preservation Sound. It's a simple circuit and it seemed like it would be a good contender for 2-channels.

I decided to try built it into a cute blue box that I lifted from a collection of old tube radios I'd bought at an estate sale in the countryside nearby. It was a tight fit and I had to put the power supply in a separate box and feed to it through an umbilical chord, which was fine for me cause it meant less worrying about induced noise from the power supply.

In this compressor the main signal only goes through two tubes: the preamp tube and the output tube. Normally the sidechain signal used for the compressor is taken from the preamp tube, but I found it more interesting when I took it from the output tube. Coming from there it's a hotter signal and it allows for a greater range of ratios. Since the "ratio" knob is really just setting the gain for the preamp before the sidechain diode you can still dial in lower ratios if you want to.

How does it sound? Great! When you push the input gain it has quite an interesting kind of crunch to it, a very useable distortion, and very different than the kind of crunch I get from my other tube stuff. The compression can go from quite subtle to extreme. It's a lot faster than the STA-level but still not the quickest compressor. I don't really believe the meters but they seem to represent a rough approximation of what's going on. I didn't put any meter trim pot in cause they move around quite a bit as the ratio and threshold pots are changed I don't want to bother trying to reset the needle to 0 all the time.

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