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I have a circuit that implements a guitar amplifier (I ordered a couple LM386 but in the meantime I used an OP484). The following figure represents my schematic: (cap C2=10uF as well) enter image description here

The signal from the guitar seems to amplify perfectly and without much noise when the speaker is not connected: Input signal vs Output signal

but as soon as I connect the speaker to the output of the amplifier the amplified voltage goes way down and the signal gets very distorted which sounds like a lot of humming noise, the clipped signal can be observed in the following image: enter image description here

Any ideas why this might be happening? The only thing I can think of is maybe an impedance mismatch and if that is the case, how would I solve it? (the speaker data sheet says impedance is 8 Ohms)

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  • \$\begingroup\$ Is the power supply able to provide adequate power? why does that "humming" occur? \$\endgroup\$
    – analogsystemsrf
    Apr 22, 2019 at 13:29
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    \$\begingroup\$ Your very weak and severely overloaded opamp is not powered and is not biased correctly. It has positive feedback instead of normal negative feedback. An LM386 opamp is designed to produce only 0.5W into an 8 ohms speaker when powered from a 9VDC/300mA power supply (not a little 9V battery). it will be as loud as a cheap clock radio. Who is the manufacturer and what is the diameter your speaker? \$\endgroup\$
    – Audioguru
    Jan 25 at 0:56

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The OP484 is an operational amplifier, not a power amplifier. Your speaker is simply too much of a load for that IC to drive.

You could get around this by including a push/pull transistor stage at the output of the opamp (inside the feedback loop.) But, since you have already ordered an LM386, you could just wait until you get your power amplifier.

The LM386 is (despite its schematic symbol) not an opamp. The circuit you put together for the OP484 won't work for the LM386. The suggested minimum circuit for the LM386 is very different. Follow the suggestions in the datasheet. Pay special attention to the gain setting - it is nothing like how you set the gain on an opamp.

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Here's a link for schematics and explanations about an electret, a speaker and a OP484 quad op-amp (as of 2021-01-28): https://wiki.analog.com/university/courses/electronics/electronics-lab-electret_microphone .

« The objective of this lab activity is to design and build an audio amplifier that takes the small output voltage from an electret microphone and amplifies it such that it can drive a small loudspeaker...».

Breadboards are also shown to illustrate the schematic and to use an oscilloscope to check out signals and amplifier gain.

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The OP484 is an op-amp and can't drive a loudspeaker. Use one of the LM386 chips.

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