# What is a good capacitance to use for a low pass and high pass filter?

First, I am using the minimum components circuit for the LM386

I am going to use the first circuit on this page. I am going to use a potentiometer in series with the circuit to give me variable cutoff frequency, so what should I make the capacitor value?

Similarly, what inductor value should I use for the low pass filter here?

In the minimum parts circuit, there is already a capacitor in series with the output, so do I get rid of it?

You haven't specified what frequencies you want, nor the values of the other components, so it is impossible to state what the capacitor values should be. Once you have constraints, you can solve for cutoff frequencies with the following set of formulas:

$$f_{(RC)} = \frac{1}{2\pi RC} \\ f_{(LR)} = \frac{R}{2\pi L}$$

Or, when you're feeling lazy, use this wonderful site.

In the minimum parts circuit, there is already a capacitor in series with the output, so do I get rid of it?

Typically, you don't place filters on the output of an amplifier because they dissipate a lot of energy (which you need all you can get to drive your speaker), and the impedance of the speaker interacts with the filter. Keep the filters on the input instead.

Regardless, you need a capacitor on the output of that amplifier to block DC from the amplifier from the speaker. Without a series capacitor, a significant DC current flows through the speaker, which is not good for the speaker and not good for the amplifier.

• I am actually not sure what frequency should be used - what would you recommend as good cutoff frequencies? – electricviolin Mar 29 '16 at 22:46
• So then the filers should be put before even the potentiometer in the minimum parts circuit? – electricviolin Mar 29 '16 at 22:48
• What are you doing with the circuit? And why do you think you need the filters? That will determine what frequencies should be used. – uint128_t Mar 29 '16 at 23:07
• I am trying to run two speakers (one is 8ohm 1.5 W and the other is 4 ohm 4 watt). I know the LM386 isn't good for this application, but its just for educational purposes. The larger 4W speaker would use the low pass and the smaller 1.5W would use the high pass. – electricviolin Mar 29 '16 at 23:28
• So you are trying to make a crude crossover between two speakers? – uint128_t Mar 29 '16 at 23:31

The value of the capacitor will depend entirely on the range of cuttoff frequencies that you want to achieve. Also, I assume that you will be driving a speaker (8 ohm possibly) with this circuit. To maintain low power dissipation in the POT, the POT will need to be very low resistance, which will make your approach very impractical. Remember that all the current that flows through the speaker will also have to flow through the variable resistor. There are at least two solutions, one is to implement a variable capacitor, the other is to implement the variable filter on the signal (input) rather than trying to impose it on the output. By the way, the capacitor (250uF) together with the 8 ohm speaker forms an 80Hz High-pass. The 0.05uF capacitor together with the 10 ohm resistor diverts the drive current (bypasses the speaker) for frequencies above about 320kHz.