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  1. audioguru2

    LOAD RESISTOR IN A MOSFET AMPLIFIER

    Hi Shaiq, Why are you using very high power IRF250 Mosfets in your very low power circuit? They are rated at 200V and 30A. Your circuit uses a max of only 20V and 1.47mA. Their current rating is more than 20,000 times what is needed in your circuit! :o The IRF250 datasheet shows its performance...
  2. audioguru2

    telephone related question #2

    Hi Walid, I have made many telephone DTMF tone dialing projects, but I have never made one do rotary dial pulses. I don't remember what is the standard for M/B ratio in the USA and I don't know if it will work if it is incorrect. Yes, in tone mode there isn't a break. Sometimes tone mode has a...
  3. audioguru2

    Keys finder circuit

    Hi Lapsap, Thanks for the zipped intermittent beep. It should stop buzzing a couple seconds after connecting the battery, if the room is quiet. Yes. Then that is the key to your problem. D4 is shorted or backwards, or C7 is open. D4 is supposed to rectify the oscillator's output, charge C7 and...
  4. audioguru2

    LOAD RESISTOR IN A MOSFET AMPLIFIER

    If you want a basic lousy amplifier with high distortion, you can replace the two current sources with simple resistors, the two differential transistors with a single one and remove the bias-current setting pot and its bypass capacitor and replace them with a piece of wire. Then it will be...
  5. audioguru2

    telephone related question

    Hi Walid, Your sketch clearly shows that the phone's current continues, forward-biasing D1 and D4 continuously, but the current is modulated a little by the audio. The other two diodes don't do anything. The full-wave bridge rectifier is used only when electronics in the phone needs a certain...
  6. audioguru2

    Turn the lamp On & Off gradually...

    Sorry Prateek, A lightbulb turns-on at 100Hz on a 50Hz mains. It is trying to reach full brightness at each of the two peaks of the AC voltage, for each cycle. An ordinary lamp dimmer changes the firing angle of a full-wave thyristor (triac) in each half cycle not each second. Early for full...
  7. audioguru2

    telephone related question #2

    Hi Walid, The make/break ratio is the ratio of the time that rotary dial pulsing makes the phone's connection to the time it breaks the connection. Europe has a different standard than North America. Setting pin 10 either high or low selects the ratio, 50:50 or 67:33. I don't know what is...
  8. audioguru2

    LOAD RESISTOR IN A MOSFET AMPLIFIER

    Hi Prateek, RF power amplifiers use impedance matching to reduce standing waves in the cable connecting the transmitter to its antenna, and to reduce resulting over-voltage or over-current in the output devices. Audio power amplifiers don't have standing waves in the connecting cable so they use...
  9. audioguru2

    LOAD RESISTOR IN A MOSFET AMPLIFIER

    Hi Shaiq, Your amplifier doesn't apply AC across the speaker that it is designed for. It applies a fluctuating DC voltage and current that causes its coil to be offset in the magnetic gap causing distortion and possibly overheating it. Your ouput Mosfet doesn't have negative feedback to keep its...
  10. audioguru2

    audio amp

    Hi Fafa, Most audio power amplifier IC's use a class AB output stage and darlington configured output transistors. Very high quality IC's are the LM3875 and LM3886 by National Semiconductor. Philips also has many audio power amplifier IC's...
  11. audioguru2

    Keys finder circuit

  12. audioguru2

    telephone related question

    Hi Walid, For a phone to be detected as being off-hook, and to remain off-hook, it must draw a certain amount of current DC continuously. When a phone receives or transmits audio, the audio modulates the off-hook current only a little, so that the minimum amount of current flows and the diodes...
  13. audioguru2

    LOAD RESISTOR IN A MOSFET AMPLIFIER

    Hi Prateek, Using a power amp with an input impedance of about 100k ohms, a transformer isn't necessary. Impedance matching is never used in audio circuits, usually the load is a much higher impedance than the source to obtain the most voltage transfer. The output impedance of most power amps is...
  14. audioguru2

    LOAD RESISTOR IN A MOSFET AMPLIFIER

    Hey Prateek, Calculate how little the power will be: 1) 0.33V squared divided by the 10k load is only 10.8uW. If you held the 8 ohm speaker directly to your ear you might hear something faintly. The transformer matches the 8 ohm speaker to the 10k circuit, but the power isn't increased. ;D
  15. audioguru2

    Keys finder circuit

    Hi Lapsap, The output of N6 isn't very strong and won't directly drive the piezo very well. The circuit uses T2 as a emitter-follower current-booster so that the piezo produces an amplified  strong 4kHz whistle, not just a little buzz. The piezo is used as a speaker and T2 is its ampliier/driver...
  16. audioguru2

    Need help repairing and setting up this old computer.

    Hi Dazza, I installed Win98 1st edition on my 486. First I had to replace its Bios that limited its hard drive to about only 560MB, then installed a 2G hard drive. Win98 1st ed used nearly 1G of space. :o When I had AOL dial-up, I found lots of tricks to fool its install program that my 486 was...
  17. audioguru2

    LIGHTENING STRIKES

    I heard the thunder from positive lighning that zapped a kid on his bike near my home. He is very lucky to still be alive but is badly burned. It was sunny and the storm was far away. :'( A car battery and everything near it would be vapourized with 300,000A and a billion volts! ;D
  18. audioguru2

    tachometer

    Hi Ante, I don't know how Google gets around Silicon Chip's internet re-direct to their "buy now" page, since we can't with the same link. Since all their stuff is copyrighted up to and including this year, I think Mixos would get in trouble if an entire project was hosted here. :o ;D
  19. audioguru2

    C_B transistor impedance matching

    Hi Walid, A common base transistor circuit uses the emitter as a low impedance input and the collector as a medium impedance output. The emitter has a low impedance because it is a forward-biased diode which has a small voltage change with a high current change. Also, the emitter resistor to...
  20. audioguru2

    transistor voltage regulator

    Hi Walid, You can't make a voltage regulator without a voltage reference like a zener diode or a voltage divided from a stable reference voltage. Since you are using a PNP transistor, input and output voltages in my descriptions are negative: 1) If the 10V is stable, then the 2N5401 can be used...
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