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I'm trying to build a very simple circuit, that will amplify an audio signal coming from a 3.5mm jack and play it back on a small 32ohm speaker.

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I can see that the amplifier is actually amplifying, if I compare the strength of the input voltage versus the output voltage. But the problem is, it's amplification level appears to be only a factor of 6x. Based on the R2 / R1 division, the amplification factor should be around 100x, correct? if I crank up the voltage to 15v, then the amplification factor increases, but not significantly. Why is the amplification factor so low, even through the datasheet states the the amplification factor is calculated based on: R2/R1?

Another problem is that the speakers output is as quiet as an output from headphones. Even if I crank up the power supply of LM318 to 20v - the speakers volume doesn't increase.

As far as I understood, I will get very limited current supply out from the op-amp and won't be able to drive the speaker directly, correct? Do I need to drive the speaker with a transistor to make it louder or what would be the proper method? Thank you for you time.

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  • \$\begingroup\$ check the datasheet and see what kind of current the opamp is capable of delivering. \$\endgroup\$
    – dannyf
    Mar 17, 2017 at 21:53
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    \$\begingroup\$ Disconnect the speaker and re-measure gain. An LM318 is not an audio power amplifier. \$\endgroup\$
    – user16324
    Mar 17, 2017 at 22:23
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    \$\begingroup\$ Why is the white lead connected to the - input? \$\endgroup\$
    – WhatRoughBeast
    Mar 17, 2017 at 23:13
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    \$\begingroup\$ Also, is that really a stereo 3.5 mm jack? You show three wires there. \$\endgroup\$
    – jonk
    Mar 18, 2017 at 0:20
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    \$\begingroup\$ LM318 is a typical BJT Op Amp with 33 Ohm output impedance and current limited to 20mA with internal Vbe drop but reducing gain from 1k to 1 also lowers output impedance but it is still current limited to about 0.6Vp into 32 Ohms. Complementary emitter followers inside the feedback loop will work. \$\endgroup\$
    – Tony Stewart EE75
    Mar 18, 2017 at 0:29

2 Answers 2

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The LM318 is not designed to drive the speaker directly .You could make a NPN / PNP follower that has a voltage gain of almost unity .The proposed follower would have a high current gain .The actual current gain is roughly the HFE of your transistors .This would raise the impedance that the LM318 opamp sees to several thousand ohms depending on transistor gain.Now the opamp will perform as predicted and your gain equation will be valid .If you biased the follower well into class AB you should have something that will sound better than a chip at the expense of increased power drain .

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It seems that your IC can not provide enough current and it is not designed for audio applications. The ratio of R1/R2 is the gain of voltage not the power.

I suggest using LM386 it is a power amplifier (not only a voltage amplifier) and it can provide gain up to 200. It is designed for audio applications. It is a very powerful IC, and easy to use and requires few components. It's just great.

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