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I am attempting to debug the following circuit. Looking at the schematic, starting at Vin on the left of the diagram, I want to figure out if the 10uF capacitor is the issue with the circuit.

This capacitor has no indication for how the positive or negative sides need to be oriented in the circuit. How cna I determine which way the capacitor is supposed to be oriented in the circuit?

enter image description here

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  • \$\begingroup\$ have a look at Figure 12, Section 9.2.2 LM386 with Gain = 200 of the TI document ti.com/lit/ds/symlink/lm386.pdf \$\endgroup\$ – V.V.T Jun 18 at 2:58
  • \$\begingroup\$ Same schematic, however the capacitor was removed from the pin 3 line, and now there is a capacitor on the 7 bypass pin. The symbol does not show a capacitance value. Is this what you are referring to? \$\endgroup\$ – McWayWeb Jun 18 at 4:09
  • \$\begingroup\$ Pin 7 bypass capacitor is optional and you are free to not use it; still, 100u capacitor is good to significantly reduce noise. In your circuit, 10u cap is a blocking capacitor; it does not alter the operation in other way than filtering out DC component from input signal. Yes, I refer you to this doc and recommend it to guide your design. \$\endgroup\$ – V.V.T Jun 18 at 4:52
  • \$\begingroup\$ If you have a DC bias in your input signal, you definitely need a capacitor. So you can't just copy that figure 12. \$\endgroup\$ – Abdullah Baig Jun 18 at 4:52
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There could be many reasons why polarity is not indicated. When thinking about this, you must remember that the capacitor must pass audio which is an AC signal, and it must block any DC bias if present.

  1. Not indicated because it defines you have to use a non-polar capacitor. Bipolar electrolytics have existed and have been used for audio. The LM386 is extremely old device and 10uF value was reasonable size for bipolar electrolytic technology back then. These days 10uF ceramics exist, but whether or not they should be used for audio due to non-linearities is another thing. A bipolar electrolytic is basically nothing more than two polarized electrolytics in series so you can build one from two 22uF capacitors.

  2. Not indicated because it depends on the circuit. The input audio could have DC bias from previous stage, and LM386 input has approximately 0V DC bias, so polarity would depend on if the previous stage had positive or negative bias compared to 0V.

  3. Not indicated because it can't be defined. If you have an audio signal with 0V bias, the capacitor can be put either way, as the bias will be 0V at LM386 input too. It will actually not be exactly 0V but near 10mV so close enough 0V. Since there is virtually no DC bias the capacitor could be removed from the system as it is not necessary.

  4. Not indicated because it does not matter. Assuming the amplifed audio bias is almost 0V and the LM386 input can take amplitude of +/- 0.4V audio signal as input, the capacitor won't have voltage above 0.4V over it in any direction even for long steps, so a standard polarized electrolytic capacitor will function in the circuit just fine.

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Could be calling for a non-polarized capacitor, like a ceramic type. There are unambiguous symbols for polarized capacitors after all. That being said, I would be inclined to put the negative end toward ground (i.e. away from V_in).

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  • \$\begingroup\$ Schematic instructions just say "capacitor", however on the diagram it shows the ceramic type. Are the two types not interchangable? When you say you would be inclined to put the "negative end toward ground", is this a rule that always applies or does it vary on a case by case basis? \$\endgroup\$ – McWayWeb Jun 18 at 3:28
  • \$\begingroup\$ A 10 uF non polarized capacitor would be about 5-10 times larger than all the rest of the components in this circuit combined. A non polarized capacitor is definitely not intended here. \$\endgroup\$ – Abdullah Baig Jun 18 at 4:50
  • \$\begingroup\$ @AbdullahBaig is that a serious comment? You can buy 0805 10uF ceramic capacitors... \$\endgroup\$ – vicatcu Jun 18 at 18:34
  • \$\begingroup\$ oh sorry, my bad \$\endgroup\$ – Abdullah Baig Jun 28 at 1:51
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Polarized capacitors are only rated for voltage potentials in one direction. They like to collect charge in one polarity on their plates. A non-polarized capacitor such as generic ceramic types are capable of collecting charge in both positive and negative polarity (you can use them in circuits that have voltages that swing both above and below your zero/GND reference).

In this circuit you show, it is likely you are passing an A/C audio signal into the input. In this case, you are going to be feeding in a signal that swings to both positive and negative with respect to your GND points. You need a non-polar capacitor for this.

You'll damage a polar capacitor if you reverse the charge on it's plates. Fun fact! This is how capacitors explode, especially when used in circuits capable of sourcing high current.

Now you may ask "If I am passing in an AC audio signal, how come I can use a polarized capacitor on the output (the 250uF one)?". This is because the output of the LM386 can only swing between the rails of the power supply you provide it, in this case 0 volts to Vs. If you power this circuit with a 9v battery, or another DC source, you will not get a negative voltage on the output, allowing your high pass filter capacitor (250uf)to be polarized.

Fun fact: they list a 250uF polar cap symbol in the output because high value electrolytic capacitors are cheap and mass produced at such high capacitance values, whereas a 250uF non-polar ceramic or film capacitor is not something you can find very easily, if at all. Certainly not at a reasonable price. Hope that helps!

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If the input signal is coming from a Mic potential divider, like in the attached figure, you need a capacitor here. Its positive terminal should be towards the Mic and negative towards LM386.

If the signal is coming from an audio source, like an MP3 player, or a mobile phone, you don't need a capacitor here.

enter image description here

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