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Basic Q: Simple OpAmp Circuit

  1. Jun 7, 2012 #1
    A little while ago I built myself a passive mixer (some resistors, some switches and a bit of solder). Now I've decided to take this further and build a proper mixer using op-amps.

    So I have got myself some NE5532 opamps to use.

    I am trying to build a very simple test circuit using just one op-amp to see if I can get the gain I expect. My first problem was not realising the opamp needed ±12V, so I have now taken an old ATX power supply unit and have connected the +12V and -12V rails to the appropriate Vcc connections. Now I can turn on the power supply and can read ±12V (well, 11.6) across each rail.

    My problem is that when I power on the 'complete' circuit, the power immediately shorts out. I tried to lower the current by putting a resistor in series to each of the +12V and -12V op-amp supply lines, which stops the supply cutting out, but there's nothing detectable coming out of the opamp output either.

    My circuit is basically this: hobbyprojects(dot)com/operational_amplifier/setting_opamp_gain.html

    R1 = 47k
    R2 = 470k
    therefore gain should be ~10
    R3 = 4.7k

    and I tried 10k resistors on the input rails.

    I've also tried a pot on the output so I can adjust the output impedence, but still get nothing.

    Other relevant bits: input is single channel from headphone jack on my phone, output is to an old laptop speaker - they work when connected together!

    Attached Files:

  2. jcsd
  3. Jun 7, 2012 #2


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    Science Advisor

    Your circuit should not be shorting out the power supply, so you need to find out why this is happening.

    Remove the power supply wires and the IC and measure the resistance between the connectors where the power supply was connected.

    If it measures low resistance, then check with your multimeter until you find why this is happening.

    If it only happens when the IC is plugged back in, it is possible the IC is connected the wrong way around or it may be faulty (or both).

    Be very sure the circuit is right before you apply power again with the IC (or a new one) in circuit.

    One check is to apply power without the IC and measure the voltages on the socket where the IC goes.

    The computer power supply will also give +/- 5 volts and this may be worth trying before you give the circuit full voltage.
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