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  1. Audioguru

    Henri Abrahams astable multivibrator

    Where does the guy in the video get 0.95V? My transistors reach a Vbe of 0.65V in that circuit. Where does he get -8V? My circuit has his 8V reduced to -6.3V to -4.3V by the LEDs. Note that the transistors probably have a maximum allowed reverse emitter-base voltage of 5V so a battery voltage...
  2. Audioguru

    SW radio using OP amplifier

    I think I wrote $0.80US but my keyboard has its own mind Then a much more powerful modern amplifier with 2 channels costs less than the old single channel low power amplifier.
  3. Audioguru

    voltage divider with load question and guess

    Yes, his "oscillator" needs something in it that is active to cause oscillation. Maybe a 555 IC.
  4. Audioguru

    voltage divider with load question and guess

    Ratstar showed one LED but talks about a red one and a blue one. Is the LED a back-to-back complementary red/blue one? The capacitor does not pass DC and with the 200 ohm total of the resistors in parallel with it, its charged voltage will probably be very low.
  5. Audioguru

    SW radio using OP amplifier

    The LM386 is fairly old and is a single mono amplifier. With a 5V supply its output is inly 0.14W with low distortion into 8 ohms or 0.2W into 4 ohms. It costs $1.02US today. it wastes battery power making heat. A modern PAM8403 is a stereo amplifier IC. With a 5V supply each channel produces...
  6. Audioguru

    Clarifications on the PWM signal

    Do you know why PWM is used to reduce the current instead of using a resistor?
  7. Audioguru

    Daisychaining possible w. Remote LED Candles?

    You Tube shows many videos produced in India faking an LED lighting when a cell phone receives a call nearby. Persons posting videos on You Tube get paid for viewers watching the ads so they make many fake videos.
  8. Audioguru

    SW radio using OP amplifier

    The lousy 52 years old 741 opamp should be kissed "goodbye" then buried. It was never made for audio because its slew rate is too low therefore it cannot produce full output at the highest audio octave from 9kHz to 20kHz and its noise is so high that it is not spec'd. The LM386 is a power...
  9. Audioguru

    How to connect piezo speakers to an amplifier

    Since the piezo tweeter is 72 ohms at 17kHz then its current is much less than if an 8 ohm speaker was used so the wire thickness can be small. The RMS voltage from the amplifier is 11.32V but the 30 ohms resistor in series with the tweeter reduces the voltage at the tweeter to about 8V RMS...
  10. Audioguru

    Class A amp with NPN BC547 transistor

    Didn't you see the voltages? The supply is 18V and the collector is 17.91V so the transistor is almost completely cutoff. Adding a capacitor bypassing the emitter for AC will not correct the DC biasing error. Change the emitter resistor to 8.2K to make it biased correctly then bypass some of the...
  11. Audioguru

    How to connect piezo speakers to an amplifier

    Hee, hee. The kids are bats and can hear the harmonics of the 17kHz fundamental squarewave. The ultrasonic 51kHz and 85kHz harmonics will be annoying. Oh, maybe you are saying that the harmonics will beat with the amplifier's class-D switching frequency producing lower frequencies? The...
  12. Audioguru

    How to connect piezo speakers to an amplifier

    2.83 RMS produces 1W and the tweeter produces 94dB at 1 meter. 11.32V RMS produces 16W into 8 ohms but at +3dB per doubling of power then the tweeter will produce 106dB and the 30 ohms resistor will reduce it to about 103dB at 1 meter. The resistor is mounted at the amplifier then the tweeter...
  13. Audioguru

    How to connect piezo speakers to an amplifier

    Since the piezo tweeter represents a 0.13uF capacitor then its impedance drops at high frequencies and some amplifiers become unstable and overloaded when driving a capacitance. The 30 ohm resistor limits the impedance drop and reduces the 17kHz output level by about -3dB. Using the 17W...
  14. Audioguru

    How to connect piezo speakers to an amplifier

    I looked and found that the kSN1001A piezo tweeter is obsolete but an unknown Chinese company makes copies. 17W into 8 ohms is 33V peak-to-peak and the Hifiberry produces it with a 18V supply because its amplifier IC is bridged (a separate amplifier for each speaker wire) to double the signal...
  15. Audioguru

    How to connect piezo speakers to an amplifier

    There is a piezo tweeter that has been sold for many years and it produces awful music but it is loud and goes up to your required 17kHz. I think its high frequency sounds are VERY directional and are loud only where it is pointing towards. 2.83V RMS produces about 94dB at a distance of 1 meter...
  16. Audioguru

    How to connect piezo speakers to an amplifier

    Ultrasonic piezos cannot be heard by humans, not even juveniles with good hearing. Young people and most people used to be able to hear frequencies to 20kHz but the people who deafened themselves with loud rock music cannot. Maybe the juveniles you need to discourage. The unit that you want to...
  17. Audioguru

    How to connect piezo speakers to an amplifier

    A piezo beeper is a piezo transducer with a transistor oscillator inside it. Some of then are louder when powered from 30V and some have their loudness measured at a distance of 30cm. They are usually used at about 4kHz which is their resonant frequency (the loudest frequency). Digikey lists...
  18. Audioguru

    How to connect piezo speakers to an amplifier

    A piezo beeper has a transistor inside it. It makes a continuous beeeep at its resonant frequency when DC is applied. A piezo transducer is a speaker that produces sounds when fed an AC signal like from the 555 IC in the video. They also have a resonant frequency. The beepers are indicators...
  19. Audioguru

    How to connect piezo speakers to an amplifier

    The video you found was made by a science student who copied this simple circuit. There is no schematic but the breadboard shows an ordinary 555 astable oscillator. I think the "DC buzzer" is actually an AC piezo transducer (speaker) not a buzzer. A piezo buzzer has an oscillator inside it...
  20. Audioguru

    SW radio using OP amplifier

    The design of a lousy 741 opamp is 52 years old. It has many problems. Use a modern opamp instead.
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