LM386 - Input filters

Discussion in 'The Projects Forum' started by vol_, May 8, 2016.

  1. vol_

    Thread Starter Member

    Dec 2, 2015
    93
    3
    Hello, hello..

    I uploaded a schematic from LTspice for an LM386 electric guitar amp and its frequency response.

    Can someone help me about whats going on with the input filters and output filters?

    NOTE: The LM386 has an internal 50K resistor to ground to set the impendance.

    So, i added, after the MPF102, a 33K resistor (R4) and a 0.69nF cap (C2) that form a low pass filter with frequency response corner at about 7kHz. After, the 100K resistor (R1) is in parallel with the LM386s' internal resistance giving a total resistance of about 33K. This resistance combination with the 0.1uF cap form a high pass filter, with corner frequency at about 48Hz.
    FInally, the output coupling cap (C3) form a high pass filter with the speakers impendance giving a frequency corner at about 90Hz.
    Is these right? Or am I missing something?
    Because LTspice is giving me other values than these. The overall frequency response is calculated by LTspice at 370Hz and 15kHz.

    Cheers everyone!
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,796
    Congratulations. You discovered that you can't just string filters together and assume they retain their design specifications. This is because the filters load each other and change things. Now there are ways to design things to avoid this, but that is a lesson for when you've acquired more math skills. If the new values are acceptable, then it looks like you have a win. If not just play with the values until you get something you like better.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,067
    3,836
    The LM386 is a noisy, hissy little jelly bean. No matter how much filtering you add, you won't be able to prevent this chip from sounding like an AM radio playing a distant station. The hiss it plays through your speaker when your strings aren't even moving will drive you crazy.
     
  4. Bordodynov

    Active Member

    May 20, 2015
    643
    188
    low pass filter freq=1/((33k||100k||50k)*C2/(2*Pi)
    There are still elements that determine the frequency response: C6 and R5
     
  5. vol_

    Thread Starter Member

    Dec 2, 2015
    93
    3


    lm386-top.JPG

    and about the gain capacitor, neglecting 47R and LM386s' internal 150R resistance, i have only a 15K resistor in parallel with the C6 capacitor (the previous image is for gains' C6 capacitor frequency setting). Is this the mechanism that C6 sets the frequency?

    papabravo i ll try to take care of loading then..
     
    Last edited: May 9, 2016
  6. Bordodynov

    Active Member

    May 20, 2015
    643
    188
    freq=1/((150+47)*2.2u)/(2*Pi)= 367.224Hz and + R_C_Load ==> 370Hz
     
  7. vol_

    Thread Starter Member

    Dec 2, 2015
    93
    3
    I think i managed to make the LM386 sound like an FM radio playing a distant station, with some speakers that match the circuit. I'll send the schematic at the future.
     
    GopherT likes this.
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