Need Help, using an op-amp for the first time.

Discussion in 'The Projects Forum' started by Kevin H, Mar 5, 2013.

  1. Kevin H

    Thread Starter New Member

    Jan 29, 2013
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    I've made a small guitar amplifier for fun using discrete electronics only. I though I would try my hand at using an op-amp but not having much luck getting things to work. I want to use a 9 volt battery for power and use the op-amp I have for the preamp instead of transistors like I did before.

    The chip(s) I have are RC4560ip IC OP AMP DUAL AUDIO

    www.ti.com/lit/ds/symlink/rc4560.pdf

    I've tried quite a few different ways of hooking this up without ANY luck. I am considering a dead chip may be at fault which I may have caused. I've got two others and would like some input from you veterans before I continue. Sorry for no schematics of my attempts. Thanks for you time.
     
  2. Kevin H

    Thread Starter New Member

    Jan 29, 2013
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    [​IMG]

    Since I can't get a schematic to you here's the rudimentary way I have tried to hook this up

    pin 1 - capacitor - then to head phones then to ground
    pin 2 - ground
    pin 3 - capacitor - then guitar then too ground
    pin 4 - ground
    pin 8 - +9V

    I get dead silence. If I unhook the ground on guitar and and pin 2 from ground I get lots of noise so I know the chip is ok.
     
  3. patricktoday

    Member

    Feb 12, 2013
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    This is probably your most basic op amp circuit for a single 9V battery (attached). The (+) input resistors set the bias or center voltage at the output to 4.5V; the gain will be R2/R1 so it would be 10 in this example. If you replace R2 for a pot, you could adjust the gain accordingly (that's just one way). Guitar pickups will generate a voltage in the 50-200mV range peak to peak when you hit the strings fairly hard.

    A few caveats: op amps are low power so you can't really drive a speaker directly off them well; you probably need a transistor stage afterward if you want to get any volume. The most basic transistor can do 10X the current of an op amp. The second is that it's pretty much mandatory that you decouple the power pins of each op amp chip; some op amps will go crazy without it :D This means that you should attach a single 100nF cap across the two power pins as close to the chip as possible (on a breadboard, the next hole out should work).
     
  4. Kevin H

    Thread Starter New Member

    Jan 29, 2013
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    I understand now how this works without the bipolar power supply. I'd imagine the r1/r2 is a negative feedback network to control gain. I've decoupled the power inputs like you said and the circuit is working great now.

    Since this is a dual amp can I use the same 4.5v for the 2nd +input? and continue with another r1/r2 on the output? I'll couple the output and input with a cap though.
     
  5. patricktoday

    Member

    Feb 12, 2013
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    Yes, definitely. Since the (+) input draws almost zero current you can use the same voltage reference on other op amps, too.
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    If you want even better performance use a Jfet input op-amp like the TLO74 in patricktoday's circuit instead of the 4560. And then increase r1 and r2 by a factor of 5x-10x

    Some guitar's are high impedance (if they don't contain active electronics), This requires a preamp with a very high input impedance for the best performance. If the input impedance is too low it will 'load down' the signal which means loss in volume and poor frequency response.
     
  7. Kevin H

    Thread Starter New Member

    Jan 29, 2013
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    The source impedance from the guitar is definitely high but I can deal with that. I didn't realize how noisy this chip was compared to using transistors as I did before and wow is it ever prone to oscillation! One last question about op-amps then is what are some of the ways to help tame the oscillations when using very high voltage gains?

    Was the comment, earlier on, for decoupling the power one of the ways?
     
  8. patricktoday

    Member

    Feb 12, 2013
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    Well, now that you have hooked it up and gotten it basically working, that's when the hard part begins :) And questions are always fine. Yes, the decoupling helps tame random oscillations.

    A few things:
    A) Impedance. As mentioned, 10k is really too low for a guitar input. The pickups will be more loaded down resulting in poorer frequency response. A more typical input impedance into a guitar amp would be 100k or more. In inverting configuration, the input impedance is completely determined by the input resistor leading to (-). Google "Virtual Ground" and "Inverting Op amp" for more info on what that is so.

    Here's some guitar-specific impedance info:
    http://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp

    B) Overload. The op amp itself can probably only deliver 20-40mA at full performance. I don't know how you attached your speaker/headphones but if it's connected to the op amp output via a cap, that's definitely too much current and will cause a lot of distortion. If your output signal is, say, 2V RMS and your speaker is 8 ohms that's 2/8A of current being supplied by the op amp. The op amp can amplify your voltage but can't provide much current so you'll want to use an output stage to mirror the voltage but at much higher current. Look into a Class A or Class AB output stage (or tubes! Oh wait, maybe not on a 9V battery ;)

    C) High frequency oscillation. Op amps can pick up on very high frequencies from various interference sources. It will happily amplify them even though they're way beyond the audible range. They can cause instability/problems in the op amp. Putting a very small capacitor parallel to the feedback resistor (R2) is helpful. It will negatively feed back signals above a certain frequency to the input and therefore _not_ amplify them. Look up "Cdom capacitor" or "Dominant pole capacitor" or, to do it by ear, put in a too-large cap at say 100nF and you'll hear the high frequencies audibly drop off. Continue lowering the value until you get a good high frequency response. It will still do its job with the supersonic frequencies. The value may end up around 100pF.
     
  9. Kevin H

    Thread Starter New Member

    Jan 29, 2013
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    This has given me a great starting point to which to work from and learn. Thank you for all your efforts!
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    If you are attempting to build a small guitar amplifier to drive a speaker or headphones, think in terms of two stages.

    1) You need an amplifier with high input impedance to amplify the very low signals from the guitar pickup. An op amp like the TL071 is good for this.

    2) You need a power amp with low output impedance to drive the speaker or headphones. An LM386 is a popular chip for this though a bit under powered.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    All ICs, and especially op amps, need a 100nF ceramic power supply decoupling cap from each power supply to ground, Keep the leads short. If you are using a single supply, then you don't need a cap on the grounded pin (duh).
    Simple-minded explanation: The wires or traces that provide power to a pin have inductance and resistance (i.e., impedance). Signal currents that flow through the supply pins from within the chip generate voltages when they encounter these impedances. These voltages feed back to input circuitry within the chip, leading to oscillations. Decoupling capacitors lower the impedances at the supply pins, shunting the AC currents to ground.
     
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