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Converting an existing amplifier into a subwoofer amp

Discussion in 'General Electronics Discussion' started by fulbrightworks, Dec 6, 2018.

  1. fulbrightworks


    Dec 6, 2018
    I'm sure most everyone here is familiar with the Pass A40 (40 watt class A audio amplifier) DIY project from Nelson Pass. (I've tried to upload a screen shot of the schematic but this forum won't let me do that. So here's the link to the article: In the article Mr. Pass says that capacitor C4 frequency compensates the amplifier using a feedforward technique. My question then is, could one change the high-end rolloff to, say, 80 Hz by simply altering the value of C4 (to use this project as a subwoofer power amp)?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Whe had an issue, it's fixed now. Please try again.
  3. fulbrightworks


    Dec 6, 2018
    In the schematic you'll see that C4 provides negative feedback from the first voltage amplification transistor to the differential input stage. He says the high frequency rolloff provided by C4 comes into play around 200 kHz. So I'm wondering if by changing the value of this cap I could bring the rolloff down to about 80 Hz and make a subwoofer amp. Thoughts anyone?

    Pass A40.PNG
    Last edited by a moderator: Dec 7, 2018
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Using C4 for creating a low frequency cutoff is imho not a very precise way due to tolerances of the components involved. This is not important for the desired high frequency roll off in the original application.
    I suggest you put a well defined low pass filter in front of the amplifier to achieve the desired bandwidth limitation. With a single operational amplifier you can easily create a good 2nd order low pass.
  5. CircuitMaster


    Dec 17, 2016
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    From the original article, see link in post #1. Although I'd say this is a class AB model.
  7. Audioguru


    Sep 24, 2016
    A single capacitor makes a 1st-order lowpass filter like the simple tone controls on an amplifier. It is useless for a subwoofer. You need an active 2nd-order or 3rd-order lowpass filter plus matching active highpass filters for the other speaker amplifiers.
  8. BobK


    Jan 5, 2010
    Yes, the article goes on and on about how bad push-pull (class AB) amps are, and his solution is a push-pull circuit!

  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    But he nearly sidesteps that by defining his design a class A. Magic!
  10. dave9


    Mar 5, 2017
    It seems like a bit of a waste to me, I mean you can pick up a 50W+ class D sub amp on ebay for around $8. Granted the PSU for it will cost at least that but if you're only needing 40W then a laptop PSU might suffice. Point being then you have the Pass amp for something else.


    May 20, 2017
    The amp shown in you schematic is a bog standard design for class A/B. It can be turned into a class A by simply cranking up the current in the output stage. This is done by adjusting the values around the Vbe multiplier of Q6 R11 and R12.
    There are improvements that could be made to the circuit. With regard to using for a sub bass, you own solution is not very good for reasons already stated by others. A good low pass filter in front of the amp is the best way. Google Sallen - Key filters. They are easy to build and work very well.
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