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== Amplification ==The Realistic MPA-20 is a 20 watt Solid State PA Amplifier sold by Radio Shack at one time. This is a good choice as the unit can run constantly powered on for years without problem. A less reliable amp or standard home audio receiver would burn up after such extended use without being powered off. This unit must remain constantly powered on and ready to amplify whatever line level signal it receives.[[Image:realisticmpa20.png]]Originally there were only two speakers in the speaker system. These could be wired either series or parallel, thus doubling or dividing the total resistance rating.The back of the MPA-20 supports a single connection to the "com" common ground and any one of the positive posts. :*16 Ohms:*8 Ohms:*4 OhmsThere is a 70 Volt connector for using with line transformers.Originally there were only two speakers in the speaker system. These could be wired either series or parallel, thus doubling or dividing the total resistance rating. Two 8 Ohm speakers wired in Series can be connected to the 16 Ohm post on the back of the MPA-20 amp. Two 8 Ohm speakers conencted in Paralle thus would have to be connected to the 4 Ohm post on the back of the amp. A resistance value too low can cause damage to the amp. A resistance value too high will result in low volume audio.{||[[Image:mulspeakpubaddr2spor1.png]]| Series and Parallel Illustrated|[[Image:mulspeakpubaddr2spor2.png]]|}*Power handling: **'''30 watts''' "normal power handling"**'''40 watts''' peak*'''8 ohm''' impedance*Frequency response: '''450Hz-10kHz'''*30 watts nominal power handling*40 Watts peak*450-10,000 hz freq. range*8 ohm ImpedanceDual LU43PW 3-Way 100 Watt Dynamic Indoor/Outdoor Speakers*Power handling: 50 watts rms, 100 watts peak*Power handling: **'''50 watts''' rms**'''100 watts''' peak*'''4-6 ohms''' impedance*Frequency response: '''100Hz-20kHz'''*4-6 ohms impedance*Frequency response: 100Hz-20kHz 1/2 WATT LINEAR TAPER POT 1/2 WATT LINEAR TAPER POT W/SPST SWITCH== Amplification ====== Linear and Audio Taper ==== Be sure to use an "'''audio taper pot'''" for volume control. This is a special kind of [[Potentiometers are Variable Resistors|Potentiometer]] designed for audio. The volume control is nothing more than an audio taper pot connected as a voltage divider. The taper of a pot refers to the way its resistance changes as the shaft is rotated. A linear taper pot is exactly what you would guess it to be, a graph of resistance (from one end to center) versus shaft rotation would be a straight line. If you tried to use a linear taper pot as a volume control you would be most unhappy with its performance. The sound level would go from nothing to almost full volume in about the first ten degrees of rotation and the rest would have almost no audible effect. This is because the ear responds to sound logarithmically rather than linearly. That's why sound engineers use the decibel which is a logarithmic function of power. An audio taper pot has a function which approximates a logarithmic function. The way to identify an audio taper pot is to set it to half rotation and measure from center to each end with an ohmmeter. If the resistances are approximately equal you have a linear pot. If the resistance from the counter clockwise end to center is about 10% of the total and the resistance from the clockwise end to the center is about 90% then you are holding an audio taper pot. ==== Some popular audio Attenuators ==== *ALPS RK27 (alias, "ALPS Blue") *Chinese made ALPS (clone) Stepped Attenuator *Panasonc EVJ *Radio Shack 271-1732 *DACT CT2 ==== key points which may be worth considering ==== *Simply inserting a stepped attenuator or potentiometer in the signal path without an adequate buffer can lead to serious frequency response deviations due to the RC low-pass filter effect of the passive's output impedance and any cable capacitance. The lower you go with the volume setting, the narrower the bandwidth becomes right in the audio band. *If you want a cheap solution for your potentiometer in your sound system, then use carbon pots. They are more neutral than the expensive plastic ones and of course much cheaper. *A conventional 100 KOhms potentiometer, when set at medium position, places a 50 kOhms resistor in series with the signal. With a 300pF cable or input capacitance, this situation leads to an attenuation of - 3 dB at 11 Khz. The Realistic MPA-20 is a 20 watt Solid State PA Amplifier sold by Radio Shack at one time. *Putting resistors in series with the sound signal always results in dynamic constriction and loss of details.[[Category:Intercom Systems]]