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Project #1 - Ambisonic Microphone - Exp2 DOCUMENTATION - Part 1 |
I had 2 new circuit designs, one with an opamp, the other
with descrete transistors. Each had its advantages.
The availability of off-the-shelf electronic components
continues to be a problem in the high tech town of Austin.
To design the printed circuit board, I had to have the
parts (and be able to continue to get them) so that I knew
their exact dimensions. The time and money costs of making
2 sets of pc boards was too high since one set would not be
used except for prototyping and testing.
I chose the opamp version due to the quick availability of the IC and the slow response of a transistor manufacturer's customer service representative. And, the system gain structure was easier to lay out on the printed circuit board for the IC version. I used schematic and pc board layout software from ExpressPCB, and sent the board files to them over the Internet (along with a credit card number, of course!), and the completed bare boards arrived a few days later. |
Even with parts and boards in hand there was still some amount
of tweaking, and I built a single test pc board. Initially, there
was an annoying hiss in the output when I powered the board from
phantom power, so I decided to revert to a couple of 9-volt
batteries. I could still build a microphone with the boards and
power it from batteries while working out the problems with
phantom power. It will still be a smaller package than my first
design.
I settled on a preamp gain structure with 4 fixed gains which could be set with a switch or jumpers. That should cover the range from ambience recording in the woods to loud thunderstorms, aircraft, fireworks, and drums. I found an extruded aluminum enclosure at Fry's which has slots for all 4 printed circuit boards; a second box will house the batteries. The 4-capsule tetrahedral will mount to one end of the box, and the microphone stand adapter will be attached to the other end. A quad-pair Mogami cable will replace the four 1/4-inch TRS jacks in the original design. |