initial commit

examples/Educational/PAsystem/ReadMe.txt

@@ -0,0 +1,71 @@
+When I deliver seminars, I usually travel with
+a sound system instead of relying on renting
+one from the conference facility.  I carried a
+mixer console, power amp and speaker in one
+wheeled piece of luggage.  There was a rack-
+mounted wireless mic system in another rolling
+bag.  I daintily carried the microphone, the
+same model as Britney Spears uses, in my pocket
+because it was expensive and didn't want it
+banged up by all the heavy sound gear I was
+dragging along.  Navigating unfamiliar airports
+solo with a two rolling pieces of luggage and
+a heavy brief case around my neck is difficult.
+
+So I decided to make my own PA system.  First of
+all I really didn't need a wireless mic. Handsfree
+is the right paradigm.  HandsFreePreamp.asc is the
+schematic of a microphone that works well with the
+electret mics that are intended for handsfree cell
+phone operation.  Basing the system on such a mic
+also meant that I could replace the most important
+part of the system from a roadside convenience
+store if required.  The points to the design are
+(i) a higher-valued resistor is used for reading
+out the mic current(10K instead of 2K as is more
+common).  This gives this preamp an edge in S/N
+because the signal grows like the resistance but
+the noise goes like the square root of the
+resistance.  (ii) The FET input gives low noise at
+this high impedance and (iii) even though it's
+class A, feedback is used to reduce distortion and
+reduce the output impedance.  The larger than
+normal-valued output capacitor that you see is
+used to short the cable to this low impedance.
+
+PowerAmp.asc is the schematic of the power amp
+I use.  The idea behind the design is to have
+a bullet-proof circuit that runs off of the same
+voltage as my notebook power brick.  That gave
+me international power capability and a spare
+notebook supply, which is also replaceable at
+most field conference sites.  I used discreet
+components over IC's because they were more
+available, better documented, and more forgiving
+of the input voltage range of my notebook
+powerbrick, than the automotive power amp IC's
+that first caught my attention.
+
+The circuits where build on plated-through hole
+perfboard.  The layout is shown in
+HandsFreeLayout.asc and PowerAmpLayout.asc.
+
+For speakers, I was planning on being able to
+use a 5 1/2" woofer from my un-used home
+theater center speaker.  But that "woofer" made
+voice sound like a public address from the movie
+M.A.S.H.  So, instead I used a pair of 4 Ohm car
+audio speakers in series.  This worked out well
+because (i) they are thin(to mount in car door
+panels) and (ii) have polypropylene cones to
+handle adverse environments, like the frozen
+luggage compartments of commercial aviation.
+The box was vented with a 1/4-20 nut that serves
+as a tripod mount and equalized the sealed speaker
+cabinet for the air pressure changes of flight.
+ 
+Now I have a PA system that fits in a suit case.
+I guess my next project should be a compact
+teleprompter.
+
+--Mike