71 lines
2.9 KiB
Plaintext
71 lines
2.9 KiB
Plaintext
When I deliver seminars, I usually travel with
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a sound system instead of relying on renting
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one from the conference facility. I carried a
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mixer console, power amp and speaker in one
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wheeled piece of luggage. There was a rack-
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mounted wireless mic system in another rolling
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bag. I daintily carried the microphone, the
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same model as Britney Spears uses, in my pocket
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because it was expensive and didn't want it
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banged up by all the heavy sound gear I was
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dragging along. Navigating unfamiliar airports
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solo with a two rolling pieces of luggage and
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a heavy brief case around my neck is difficult.
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So I decided to make my own PA system. First of
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all I really didn't need a wireless mic. Handsfree
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is the right paradigm. HandsFreePreamp.asc is the
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schematic of a microphone that works well with the
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electret mics that are intended for handsfree cell
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phone operation. Basing the system on such a mic
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also meant that I could replace the most important
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part of the system from a roadside convenience
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store if required. The points to the design are
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(i) a higher-valued resistor is used for reading
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out the mic current(10K instead of 2K as is more
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common). This gives this preamp an edge in S/N
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because the signal grows like the resistance but
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the noise goes like the square root of the
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resistance. (ii) The FET input gives low noise at
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this high impedance and (iii) even though it's
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class A, feedback is used to reduce distortion and
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reduce the output impedance. The larger than
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normal-valued output capacitor that you see is
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used to short the cable to this low impedance.
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PowerAmp.asc is the schematic of the power amp
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I use. The idea behind the design is to have
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a bullet-proof circuit that runs off of the same
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voltage as my notebook power brick. That gave
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me international power capability and a spare
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notebook supply, which is also replaceable at
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most field conference sites. I used discreet
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components over IC's because they were more
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available, better documented, and more forgiving
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of the input voltage range of my notebook
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powerbrick, than the automotive power amp IC's
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that first caught my attention.
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The circuits where build on plated-through hole
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perfboard. The layout is shown in
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HandsFreeLayout.asc and PowerAmpLayout.asc.
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For speakers, I was planning on being able to
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use a 5 1/2" woofer from my un-used home
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theater center speaker. But that "woofer" made
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voice sound like a public address from the movie
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M.A.S.H. So, instead I used a pair of 4 Ohm car
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audio speakers in series. This worked out well
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because (i) they are thin(to mount in car door
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panels) and (ii) have polypropylene cones to
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handle adverse environments, like the frozen
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luggage compartments of commercial aviation.
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The box was vented with a 1/4-20 nut that serves
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as a tripod mount and equalized the sealed speaker
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cabinet for the air pressure changes of flight.
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Now I have a PA system that fits in a suit case.
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I guess my next project should be a compact
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teleprompter.
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--Mike |