LM386 Input filter trouble

Thread Starter

fujigoata

Joined Mar 22, 2022
5
Hello. I am new to electronics and I am trying to complete an LM386 audio amplifier that includes a bandpass filter between 5 and 33 Hz. I know some of the frequencies are outside the frequency range humans can hear. I am testing using a function generator on NI Multisim to get around this. My schematic is attached below. My circuit is filtering anything below 33Hz, and amplifying anything above it. I am having a ton of trouble understanding how to properly filter frequencies in an LM386 as there is very limited information available on the subject. Any help would be appreciated in understanding how to filter.
1650394985515.png
 

Audioguru again

Joined Oct 21, 2019
4,693
You do not have a bandpass filter. Instead your filter is highpass and passes frequencies up to hundreds of kHz.
The low frequency corner of your two series passive highpass filters can cut some frequencies below 5Hz but is so simple that it also cuts all frequencies up to about 200Hz.

The datasheet of the LM386 shows that its input resistance is 50k ohms that is killing your passive filters that use resistors that have resistances that are Way Too High! You need an active Sallen-Key bandpass filter using multiple RC parts and two opamps.

The datasheet of the LM386 shows that its maximum allowed heating is about 1W. With your 12V supply and 4 ohm speaker its heating is 1.1W when its maximum output power is only 0.25W.
With a 9V supply and an 8 ohm speaker its heating is 0.6W when its maximum output power is 0.5W.
With a 12V supply and an 8 ohm speaker its heating is 0.8W when its maximum output power is also 0.5W.
 

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Thread Starter

fujigoata

Joined Mar 22, 2022
5
You do not have a bandpass filter. Instead your filter is highpass and passes frequencies up to hundreds of kHz.
The low frequency corner of your two series passive highpass filters can cut some frequencies below 5Hz but is so simple that it also cuts all frequencies up to about 200Hz.

The datasheet of the LM386 shows that its input resistance is 50k ohms that is killing your passive filters that use resistors that have resistances that are Way Too High! You need an active Sallen-Key bandpass filter using multiple RC parts and two opamps.

The datasheet of the LM386 shows that its maximum allowed heating is about 1W. With your 12V supply and 4 ohm speaker its heating is 1.1W when its maximum output power is only 0.25W.
With a 9V supply and an 8 ohm speaker its heating is 0.6W when its maximum output power is 0.5W.
With a 12V supply and an 8 ohm speaker its heating is 0.8W when its maximum output power is also 0.5W.
This makes a lot of sense and I am designing the sallen-key filters right now. My question now is how I should connect these two op-amps to get a single output that I can feed into the LM386. I appreciate your patience and your feedback.
 

Thread Starter

fujigoata

Joined Mar 22, 2022
5
Thank you for the response! I tested the sallen-key frequency response in multisim and got what I desired. I'm gonna try to find an 8 ohm speaker instead. I will also lower the power supply voltage to the LM386. Your input was very helpful and I now have a much clearer idea of what my error was. The original circuit attached was filtering exactly how you described it in your first response.
 

Thread Starter

fujigoata

Joined Mar 22, 2022
5
I just have one more question now that I have attached my LM386 it is slightly adjusting my cutoff frequencies, is there any way to prevent that from happening?
 

Audioguru again

Joined Oct 21, 2019
4,693
Which filter circuit do you have now? Your original double highpass, the passive bandpass that I posted or the Sallen-Key bandpass that I posted?

The speaker and its enclosure affects the frequency response of the very low frequencies.
The LM386 has a 50k ohms input resistance that would affect the frequency of an input passive filter.
The Sallen-Key filter must be fed from a very low resistance like the output of an opamp.
 

Thread Starter

fujigoata

Joined Mar 22, 2022
5
I did do the sallen-key filter and it does work to the design specifications I have. I was trying to pass this output into the LM386 and then output it from there. I guess the input resistance is having an affect then.
 

Audioguru again

Joined Oct 21, 2019
4,693
I did do the sallen-key filter and it does work to the design specifications I have. I was trying to pass this output into the LM386 and then output it from there. I guess the input resistance is having an affect then.
The Sallen-Key bandpass filter I posted has the + input of the 1st opamp biased at 0VDC then the opamps must be powered with a plus and minus supply so that its output is the 0VDC that is needed for the input of the LM386 power amplifier.
 

DickCappels

Joined Aug 21, 2008
8,710
I did do the sallen-key filter and it does work to the design specifications I have. I was trying to pass this output into the LM386 and then output it from there. I guess the input resistance is having an affect then.
Highly unlikely because Audioguru again's filer has a very low output impedance.
 

Audioguru again

Joined Oct 21, 2019
4,693
The 1000uF output capacitor on the LM386 power amplifier produces a 40Hz cutoff frequency into a 4 ohm speaker.
Then 40Hz is -3dB, 20Hz is almost -9dB (about 1/3rd the higher frequencies level),10Hz is almost -15dB and 5Hz is -21dB (about 1/10th the higher frequencies level).
 
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