Getting started with Op Amps (and failing) - audio mixer

Discussion in 'The Projects Forum' started by JazzyGM, Jun 7, 2012.

  1. JazzyGM

    Thread Starter New Member

    Jun 7, 2012
    Hello, any help is appreciated, I'm fairly new to electronics/circuits but I've stumbled much earlier than I expected...

    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. All it needs to do is take AC signal in and produce a boosted signal - the intention will be to use a pot to vary the gain.

    I am trying to build a very simple test circuit using just one op-amp to see if I can get the results (i.e. gain) I expect. My first problem was not realising the opamp needed + and - rails, so I have now taken an old ATX power supply unit and have connected the +12V and -12V rails to the appropriate Vcc connections. The power supply works and I 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 and I don't know what I've done wrong.

    I tried to lower the current going through the opamp by putting a resistor in series to each of the +12V and -12V op-amp supply lines. This stops the supply cutting out, but there's nothing detectable coming out of the opamp output either, at least not out of the attached speaker.

    My circuit is basically as shown in the attached diagram.

    R1 = 47k
    R2 = 470k
    therefore gain should be ~10
    R3 = 4.7k (calculated to limit drift - a concept I'm not familiar with)

    and I tried it with (and without) 10k resistors on the input rails.

    I've also tried a pot in series with the speaker so I can adjust the output impedence, but still get nothing.

    So - what's the correct way to power an op-amp, do I need to put resistors in the supply circuit or should the op-amp have enough internal resistance that it only draws what current it needs? If so, any suggestions on why I'm getting a short circuit?

    Am I missing something obvious from my circuit (probably) an extra resistor here, or a capacitor there for example?

    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 so the problem is with my circuit!
  2. crutschow


    Mar 14, 2008
    That op amp should work fine with those voltages. Are you sure you applied the proper polarity to the op amp power pins? Reversing the polarity will cause the op amp to look like a short and likely zap the amp. From your picture I can't tell the orientation of the op amp.
  3. absf

    Senior Member

    Dec 29, 2010
    Your breadboard circuit doesnt tally with your schematics. I see that there's a resistor between pin 5 and 6 which was not in your schematic. You pot was in series with a resistor going to pin 7 and the end goes to perhaps pin 5, right. I see that the wire is too long.

    One more thing, you should put a 0.1uF each between the pin 4 to 0V and pin 8 to 0V to prevent interference on the DC supply. Have you accidentally reverse the 5532? That might kill the 5532 too.:D

  4. MrChips


    Oct 2, 2009
    You are not using that breadboard correctly.
    The IC is plugged into the wrong holes.
    The holes ABCDEF are all connected together making a dead short.
    The IC must be plugged in between columns L and A or between columns F and G
  5. JazzyGM

    Thread Starter New Member

    Jun 7, 2012
    Unbelievable. At some point I reseated the IC and completely forgot that those breadboard rows are connected.

    Will go back to the breadboard and update.

  6. wayneh


    Sep 9, 2010
    There's a valuable lesson here. "Stupid" mistakes are common even for experts and can be hard to detect. Over time we learn to approach a failure with a broader mind, to consider even the silliest possibilities. Assume nothing.

    In your case, a few probes with a DMM would have quickly uncovered your problem. Data is your friend.
  7. Audioguru


    Dec 20, 2007
    Another problem is that an opamp can drive a minimum of 2k ohms load, not a speaker. You need a power amplifier to drive a speaker with plenty of current. The opamp can drive the high input impedance of the power amplifier.
  8. #12


    Nov 30, 2010
    I call this a "forest for the trees" problem or, cranio-rectal inversion. That's when you call in a friend or post a pic on your favorite electronics forum.
  9. JazzyGM

    Thread Starter New Member

    Jun 7, 2012
    Thanks all, updated picture - of a working circuit - attached.

    absf - not sure I followed your post, i think because the IC is the other way round to how you think it is (1OUT & Vcc+ are at the 'top' near the banana posts). The part about the 1μF cap's on the DC supply is interesting though - being as I'll eventually need this to run off a single supply I'll see how things are looking once I've made that modification.

    Audioguru - this will be plugged in to an amp, the attached speaker (ripped out of an old laptop) is only for testing purposes.

    Next question: Why do some schematics of a mixer include a 1μF capacitor on each of the AC (audio signal) input lines? What does this achieve?
  10. Audioguru


    Dec 20, 2007
    Audio signal lines are usually at 0V DC. But the inputs of an opamp that has a single-polarity supply are at a DC voltage that must be maintained. Then an input coupling capacitor is used to pass the signal but block the DC.