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  1. BobK

    Constant Current Source

    With a constant current source, you cannot make it constant current and constant voltage, if the load changes and the current does not, the voltage must change. This is due to Ohms law: V = I R If I is to remain constant, and you change R then V must change in the same direction. I.e. if...
  2. BobK

    Comments on this minimal amp?

    Did you turn up the input? The darlingtons will not increase the voltage gain, so with the same input I would expect the same output. Also, how are you measuring p-p voltage? When I said 10V p-p I was actually using it wrongly to mean single sided, i.e. from ground to + or -. p-p actaully...
  3. BobK

    Why do crystal sets use diodes instead of transistors?

    Because the regen receiver needs a transistor (or tube) to amplify. It just happens to do the demodulation as well, so there is no need for a diode. . Bob
  4. BobK

    Demodulation

    If this is a one-off transmitter receiver project, you don't need a variable capacitor, you could tune it to a fixed frequency with a few caps in parallel. Bob
  5. BobK

    Old motor from old walkman (how many volts does it handle?)

    The voltage does not divide up among the various components. If you turn on two lamps at home, does each one get half as bright? Bob
  6. BobK

    Old motor from old walkman (how many volts does it handle?)

    Most likely the motor is designed for 3V. Bob
  7. BobK

    Power supply help...

    Here is a line from another listing for the same board: I Think that says it all. You DO need the heatsink, as I said before, unless you want to run <10W total power, and that might even be too much. And a new transformer is going to cost you more than you want. Your best bet is to find a...
  8. BobK

    Power supply help...

    Oh, and by the way, check out the heatsink that is missing from your amp, here: http://www.ebay.com/itm/TDA7294-2-1ch-amplifier-board-80W-2-160W-SUB-heatsink-/110669287527#vi-content Without this, you will be limited to a few Watts total output. Bob
  9. BobK

    Power supply help...

    3A at 24V will give you only 72W of power. How can you power two 80W and 1 160W amp from 72W? And you need a dual power supply. i.e. +24V and -24V, each at about 7A. If you don't intend to use all that power, you can probably get away with 3A each, but your power supply might not last long...
  10. BobK

    Power supply help...

    Now you know why you can get 320W of audio power so cheaply. The transformer is going to cost you at least 3 times the price of the board. Look for toroidal transformers. I can find them on EBay U.S. for about $100., though not exactly the one you are looking for. Your best bet may be to...
  11. BobK

    Charging a 0.8AH 12 V battery

    The 12V supply does you no good at all. It cannot push current through a 12V battery. The 15V supply might work. What is the battery chemistry? The purpose of the resistor (which might or might not be 100 Ohms) is to limit the current going to the battery. That has to be calculated...
  12. BobK

    Default Wiring Multiple Lamps in Parallel to the Mains

    You would want 100 of those in series to connect to the mains. And each time one went out... Use a transformer to lower the voltage and place them in parallel. Is there any specific reason you have chosen the lowest voltage one? Bob
  13. BobK

    Can you build your own TV aerial?

    Yes. http://www.diytvantennas.com/ Bob
  14. BobK

    Charging a 0.8AH 12 V battery

    Actually that would be a trickle charger. Bob
  15. BobK

    Comments on this minimal amp?

    The circuit you linked to had negative feedback. If you use darlingtons, you would want to use 3 diodes instead of 1 between the bases, because you now have 4 diode drops between them. Did you try measuing the RMS output of the amp with a constant sin wave in? You could calculate the power...
  16. BobK

    Dictaphone with WiFi capabilities

    I think you will get some support, but you won't find anyone who has done this project before. Bob
  17. BobK

    Dictaphone with WiFi capabilities

    That would be an ambitious project for a novice. bob
  18. BobK

    Comments on this minimal amp?

    The main flaw I see with that circuit is that the TIP41/42 have their base current driven directly by the opamp. The opamp can typically supply no more than 40ma (min 10), and the typical beta of the transistor is 30 (min 15). So if we use the typical values, the collector current is limited...
  19. BobK

    Dictaphone with WiFi capabilities

    There's gotta be an app for that. Bob
  20. BobK

    Inputs to regulators of a PSU

    That is not what your scope is showing, it is showing a steady output voltage. Which is not surprising since you are drawing no current from the supply. Bob
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