[2007-07-16]

My friend Mike Vaughn has a neat little voice booth, and wanted a BHL mic preamp for it. As is my usual style, I got really excited about the project for about 3 weeks, and then got a crippling mental block about working on it, so this project sat half-finished on the workbench for months. Finally I managed to get excited about it again, and finished it. Thank you Mike for your patience :)

I used basically the same circuit as in my previous Aikido Mic Preamp. This is a great circuit for mic preamps because it is so dang quiet. It's pretty complicated, but it's worth it. In fact, I can't measure any power supply noise at all at the output. Less than 0.01mV. That seems impossible, but I guess the meter doesn't lie.

Octals and Altecs.

I went all out on the panels and had them made by Front Panel Express. The epoxy-filled lettering is really amazing. +48V is phantom power, phase is so that you can make it sound right when recording and monitoring yourself. There is phasing between the headphones and the sound generated by your vocal chords and so it is nice to have a phase switch because one phase will sound better than the other. I made the switch side to side so that there wouldn't be an implied preference for one phasing over the other.

I think the layout turned out really good. Definitely embodies the principle of "tight is right". I am really enjoying the bus bar method of grounding. The Aikido circuit lends itself very well to point to point layout. Many of the connections are between tube pins, so the layout is nice and tight without having to do too much on the terminal strips. The sockets on the far left and right are for octal mount Altec transformers

Vintage Ampex VU meter salvaged from an Ampex PR-10 tape module.

Surplus Russian paper in oil cap for the output. I love the way paper in oil caps sound.

Here is the eyelet board I used. Note the zener diode stack for the phantom power, and the DC filament supply on the left. The far right is a snubber circuit to keep the diode noise out of the power transformer, but I think it is probably redundant since the filaments are filtered DC. Not sure about that. I also forgot to add bleeder resistors to discharge the circuit when power is removed, so those were added after the fact. The phantom power circuit neede it's own bleeders too.

Here is the preamp in Mike's studio. I love the mix of DAW and old school tube mojo.