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  1. Audioguru

    Pre amps or boosts

    I worked with Pro sound systems, very high quality huge intercom systems and deluxe tele-conferencing systems. The lousy old LM324 quad and its sister the LM358 dual are noisy, have crossover distortion and have trouble with frequencies higher than only 2kHz because they were one of the first...
  2. Audioguru

    FM transmitter power

    It is not a basic FM transmitter because it has an RF amplifier so that the radio frequency does not change when something moves towards or away from the antenna which happens if there is only the RF oscillator transistor (and the antenna connects to its tuning LC). But its radio frequency will...
  3. Audioguru

    NPN transistor still working? Strange ..

    Many years ago I used the reverse-biased emitter-base junction of a transistor as a 6V zener diode. It did not fully conduct and saturate like an SCR, instead it regulated the voltage like a zener diode. A low voltage zener diode also has avalanche breakdown but it is designed to dissipate the...
  4. Audioguru

    Pre amps or boosts

    I have seen many circuits that use the LM13700 IC and I have its datasheet but I never used one. Did you know that the TL08x is a "general purpose" opamp and can be fairly noisy but a TL07x is a TL08x selected for low noise? I used many TL07x opamps in my audio career. I just looked at Digikey...
  5. Audioguru

    FM transmitter power

    It is obviously designed to operate on the FM broadcast band around 100MHz. It will sound muffled because it is missing pre-emphasis that ALL FM radio stations have. All FM radios have matching de-emphasis.
  6. Audioguru

    NPN transistor still working? Strange ..

    I was told that allowing the emitter-base junction of a transistor to perform avalanche breakdown like that damages the fragile emitter-base junction due to it overheating, causing the transistor to perform poorly with low beta if it is later used normally.
  7. Audioguru

    Loud mains hum with Jaycar short circuits Vol 3 #12

    The cable from the microphone to the preamp input is supposed to be shielded audio cable so that the shield blocks mains hum pickup. If your cable is not shielded then it is simply an antenna for mains hum to be picked up. The shield wire should connect to the ground of the preamp.
  8. Audioguru

    Pre amps or boosts

    You did not post the schematic of your filter so we are guessing that it is a bandpass resonant filter of some kind. Most active filters oscillate if their gain is a little too high. I made a bass boost circuit for my audio system. It is a Sallen-Key active highpass filter with a gain...
  9. Audioguru

    How to convert Sallen-Key Low pass filter to Signal Flow Graph

    I see the circuit as two lowpass RC networks at frequencies above cutoff like this:
  10. Audioguru

    12VDC Backup supply

    The diode can be any little diode like a 1N4148. The diode is not a battery charger. Get a battery charger IC that is made to charge the type of battery that you will use.
  11. Audioguru

    How to convert Sallen-Key Low pass filter to Signal Flow Graph

    The bandpass circuit occurs only near the cutoff frequency to boost the response so that it has flat levels at low frequencies, -3dB at the cutoff frequency instead of a droopy -6dB and a sharp 12dB per octave slope at higher frequencies. Above the cutoff frequency both RC networks are lowpass...
  12. Audioguru

    Different Pass filters for different speakers.

    The small cheap Aliexpress amplifiers are designed to be powered from 12VDC. The tiny resistors and tiny low voltage capacitors will release their "magic smoke" if you increase the supply voltage. There are many fairly old TDA amplifier ICs in through holes packages because their design is...
  13. Audioguru

    Radio transmitter, options for reducing harmonic emissions

    If the old granny in your neighbourhood is listening to a weak distant AM radio station like she does every day but your 100mW or less interference is heard and she complains then you will be in big trouble. An RC filter is simple and cannot do much filtering to the first few harmonics and they...
  14. Audioguru

    Radio transmitter, options for reducing harmonic emissions

    Don't you understand about harmonics? If the two transistors produce a perfect squarewave then all the harmonics will be high level and will be odd numbers. If the fundamental frequency is 180kHz then the first odd harmonic is the third which is at 180 x 3= 540kHz on the AM broadcast band. It is...
  15. Audioguru

    How to convert Sallen-Key Low pass filter to Signal Flow Graph

    I disagree. C1-R1 is a lowpass, not a highpass. If C1 connects to ground instead of the the opamp output then there is no boost and the output at the cutoff frequency will be -6dB and the slope will gradually be 12dB per octave.
  16. Audioguru

    Different Pass filters for different speakers.

    The datasheet for the TDA2030A says that its absolute maximum supply voltages are plus and minus 22V which is a total of 44V! Its power is spec'd at 18W with low distortion into 4 ohms when its supplies are plus and minus 16V which is a total of 32V. At 10% distortion the output into 4 ohms with...
  17. Audioguru

    Pairing a guitar pedal with new AC/DC adapter

    A 9VDC wall-wart might not have enough filtering so might might produce a hum sound from the pedal.
  18. Audioguru

    How to convert Sallen-Key Low pass filter to Signal Flow Graph

    There are two RC lowpass networks, R1 and C1 and R2 and C2. Each RC network has an output of -3dB at the cutoff frequency. Actually, the positive feedback occurs only near the cutoff frequency so that the -6dB response is boosted to be -3dB and the cutoff is sharp instead of droopy.
  19. Audioguru

    Rheostat - resistor ?

    The ECU cuts fuel when the turbo boost is too high to prevent the engine from blowing up. Why do you want to blow up your engine?
  20. Audioguru

    Component getting too hot and I dont know why

    Doesn't the ambient temperature reach 30 degrees C sometimes? Isn't the 7805 tab temperature that is higher than 60 degrees C "freaking hot"?
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