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  1. audioguru2

    Purpose of NPN transistor

    Nope. All transistors in an amplifier amplify all frequencies. Frequencies don't have polarity. If the amplifier has a complimentary class AB push-pull output and has a positive and a negative supply, then the NPN is an emitter-follower for the positive output voltage swing and the PNP is an...
  2. audioguru2

    Power Supply Project needed

    The digital voltmeter project cannot measure its own supply. It needs a separate supply. Its IC has a max reading of 0.1999V and measures higher voltages with a voltage divider at its input.
  3. audioguru2

    Power Supply Project needed

    Look at the datasheet or at a tutorial for the LM317 to see how to calculate its resistors. A 240 ohm resistor will have 1.25V across it which makes 5.2mA. 5.2mA times 5k= 26V. The output is 26V + 1.25V= about 27.25V. If you use a 120 ohm resistor then its current is 10.4mA and the max output...
  4. audioguru2

    Power Supply Project needed

    Your pots are set to half. Therefore their max voltage setting is nearly double. Your 28VAC transformer's voltage is too high, it produces an unregulated voltage of 38VDC. A 24VAC transformer will produce 32VDC. I also noticed that multisim has LM317AH, H, K, LM, LZ. I tried the LM317LZ and...
  5. audioguru2

    Use a speaker as a microphone while useing it as a speaker

    Q1 can be any little NPN transistor. The project uses a little bridged amp IC but an LM386 amp would work fine (different pinout). The project is here: http://www.redcircuits.com/Page78.htm
  6. audioguru2

    CM8870, DTMF Receiver

    Hi Walid, To reduce hiss, FM stations and TV stations use pre-emphasis for their audio transmission, and FM receivers have a corresponding de-emphasis. Pre-emphasis boosts high audio frequencies and the de-emphasis cuts them back to normal and also cuts hiss. DTMF needs a frequency response that...
  7. audioguru2

    Very long RC time contant!

    A diagram of response vs phase-shift can be shown with phasors, but I couldn't find one. All tutorials show that the response of an RC lowpass filter is down 3dB (times 0.707) at the cutoff frequency where the reactance of the capacitor equals the resistance of the resistor:
  8. audioguru2

    Use a speaker as a microphone while useing it as a speaker

    If the speakers and/or microphones at each end were in rooms without any echoes and the speakers don't have any resonances, then switching back and forth at a high audio frequency will stop acoustic feedback howling. But in our world, rooms and speakers have echoes and resonances. Distance adds...
  9. audioguru2

    Very long RC time contant!

    Of course a voltage divider made with a resistor in series with a capacitor to ground have a phase shift.  At the frequency that the reactance of the capacitor is the same as the resistor's value, the level is 0.707 times, not half. It is called a lowpass filter. 0.707 times is 3dB down.
  10. audioguru2

    Power Supply Project needed

    The 2200uF electrolytic capacitor is a good filter and a low impedance at low frequencies, but since it is large and is made with its layers wound in a circle, it has inductance which causes its impedance to rise at high frequencies. The 0.1uF ceramic disc capacitor is small and flat so has low...
  11. audioguru2

    Power Supply Project needed

    It does not boost the input voltage. It won't regulate the output voltage if its input voltage is less than about 2V more than its output voltage. If the resistance of the pot is too high then you will not have regulation at the highest setting. Definately not, it is not a generator! The input...
  12. audioguru2

    Power Supply Project needed

    Make a variable voltage regulated power supply from an LM317 IC. Its voltage varies from 1.25V to a max of 35V but a 24V transformer will allow it to have a max output of about 30V and a current of 1.0A. Below 15V output the current will be less. Our project has a few problems so read the topic...
  13. audioguru2

    Transistor datasheets with collector curves

    A transistor operates completely differently than a vacuum tube. A common emitter stage has a voltage gain equal to the ratio of its collector resistor to its emitter resistor. If there is not an emitter resistor then the transistor will be sensitive to temperature, have very high distortion at...
  14. audioguru2

    CM8870, DTMF Receiver

    Hi Walid, It is just a DTMF tones decoder. It has nothing to do with a radio. Get a good datasheet from www.datasheetarchive.com . If you use an ordinary FM broadcast band radio then it has de-emphasis and the transmitter that you use must have a matching pre-emphasis.
  15. audioguru2

    Transistor datasheets with collector curves

    Hi Blacksheep, You don't need collector curves for silicon transistors that are used well within their ratings. The current gain vs current is an absolute straight line! A curve of a typical base-emitter voltage is usually shown in datasheets, but doesn't mean much since each transistor with the...
  16. audioguru2

    thermometer

    Hi Indulis, Don't you think that a starter motor would make a high reverse EMF across itself, when it is disconnected from the battery and this thermometer circuit.
  17. audioguru2

    FM Transmitter

    Solder wires to the variable capacitor and plug the wires into the holes in the breadboard.
  18. audioguru2

    thermometer

    You also need to know the minimum operating voltage from the lawnmower. Then you can calculate the value of the series current-limiting resistor so that the thermometer gets its 3mA and the zener diode gets a couple of mA. Assume the minimum operating voltage is 14V. Then the resistor will have...
  19. audioguru2

    Help with transformer

    Impossible. A transformer is supposed to pass power, not use it. It would smoke if it dissipated 1kW. A little 9V/200mA wall-wart adapter is sometimes 18V without a load, but with a 200mA load it dissipates only 1.8W.
  20. audioguru2

    FM Transmitter

    Correct. Then the radio can tune in the entire AM broadcast band.
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