gain increases with frequency issue

Discussion in 'The Projects Forum' started by logans-electronics, Dec 5, 2009.

  1. logans-electronics

    Thread Starter Member

    Sep 1, 2009
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    Hello, I finished building this amp at home. Once I powered it up and played music, it sounded like a high pass filter was active. I went back to my multisim circuit and noticed that as I increase the frequency of the input the output gain increased....basically at 20Hz has severe crossover distortion, at 200 HZ it trys to clean up and at 2000HZ and above it looks fine.......any ideas on why no gain or crossover distortion at the lower frequencies??:(


    Oh by the way, I am running polarized capictors in the circuit...multisim shows polarized..I did know if has anything to do with anything..Either way both the amp and the multisim will not play low end frequencies.....Thanks!

    I can email my multisim to you if you need it. :)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Just for starters, I extracted your image from the Word document, and converted it to .PNG format so it would be easier for everyone to view. No extra software is needed for people to view a .PNG format image file, and it loads very quickly. .PNG is the best format to use here for posting schematics.
     
  3. logans-electronics

    Thread Starter Member

    Sep 1, 2009
    36
    0
    I fixed it ! problem was C2.

    I installed a 1uf cap instead of a 100uf as shown on the drawing. I guess it was acting like a high pass filter was active. Let me run a few calculations on this. It sounds fine now...:)
     
  4. logans-electronics

    Thread Starter Member

    Sep 1, 2009
    36
    0
    Thanks for helping me out...now another question...

    I am using your design from a few months ago... I actually finished building this yesterday and am very impressed with the sound quality and output.:)

    Just wondering if you could possibly explain the purpose and calculations for R1, R2 and R4...I know I used different values than you did. I just played around in multisim and got just a little bit more power out of it (like 1 or 2 watts)...nothing major.:)
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Do you mean the attached schematic?

    If so, that's a design that AudioGuru posted some time ago. I just put it in a simulation program to view its' characteristics. I did add a volume control pot to the input.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    Some very odd things going on here. I received an email reply notification for an unrelated topic on AAC but when I checked there had been no new replies to that topic. While reading the board I decided to check out this thread because I had asked a question about R5 within the last few days. That reply had vanished along with other replies from Sgt.Wookie and possibly others. I checked again a bit later and by George they were there again, including mine. So if anyone is curious why I'm asking this again, it's because they're gone again, or were when I posted this.

    Anyway, my question was why is R5 wired as shown? More common practice, and for good reason, would connect the sig gen signal to the high side of R5 and the wiper would connect to C1. Since no one has mentioned it am I to assume there's a reason why it's wired like it is?
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Looks like a newbie's mistake to me.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    The value of R5=100K also seems a tad high for use with BJT's. I would think 10K would be more like it on the high end and 1K on the low end. A middle of the road moderate would put it at 5K. :)
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    C2 is now 100uf. When it feeds a 6.4 ohm speaker then the -3dB cutoff frequency is 250Hz. It is still a highpass filter because all the bass frequencies are gone.
     
  10. logans-electronics

    Thread Starter Member

    Sep 1, 2009
    36
    0
    Thanks for identifying that!

    I will go back to mutlisim....I am running (2) 4 inch speakers so reproducing the lower frequencies or hearing them would be hard for me.:)
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    My pc speakers are only 3" but produce bass frequencies very well.
    For years my 4" speaker was my sub-woofer at the beach.
    Maybe your 4" speakers are very cheap.
     
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