audio amp oscillation problems

Discussion in 'The Projects Forum' started by CaliusOptimus, Oct 29, 2009.

  1. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    ive been toying with a few parts ive had lying around trying to build a class A audio amp. heres a schematic

    [​IMG]
    op amps are lm324n's
    transistor is a tip41

    note: RL isnt actually the load, its been around 100ohms for my experiments.....the load would be connected between the collector and gnd

    anytime the circuit is powered on, i get severe oscillations on the output load regardless of the input signal, or lack there of, being applied. the oscillation is a quasi sine wave, right around 53Khz.

    i dont really know what to do to counteract this...any suggestions?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The leftmost opamp is running open-loop, and you've tied it's noninverting input to ground via a 1M resistor. It's getting positive feedback from the output of the TIP41 transistor.

    The 2nd opamp is basically an emitter follower; it's just passing along whatever the 1st opamp feeds it.

    I'm not surprised you're getting terrible noises from it.

    Class "A" amplifiers are very inefficient, by the way. If you had a speaker connected to the collector of the transistor, it had a lot of current flowing through it.
     
  3. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    how is the left opamp running open loop?? look again. the non-inv is AC coupled man....thats why its tied to gnd via a 1Meg..

    the whole circuit is supposed to be a voltage follower using a class A output. the class A output inverts the signal, so i have the rightmost opamp to REvert the signal, otherwise it would cause positive feedback.

    i didnt plan on using this circuit by itself.... im using it as a voltage gain stage to drive a complimentary pair of larger transistors.

    BUT, if i cant get oscillations out of one stage....theres no point in adding another stage.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    There is no direct RC feedback path from it's output, therefore it is running open loop. There is propagation delay through the other opamp and the transistor, which is what is causing the oscillations.

    I saw that.

    This circuit only makes the power company happy.

    So you want to have a REALLY LOUD AND AWFUL NOISE?

    You are absolutely correct on that point.

    It's not easy designing an audio amplifier.

    I have no clue what your goal is, but I suggest that you pay this site a visit:
    http://sound.westhost.com/projects.htm

    There is a great deal of good information posted on that site. Bon appetit!
     
  5. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    SgtWookie, note that he has the output of the collector going all the way back to the first op-amp as feedback, but it doesn't seem to have its own feedback, do you think he should also have feedback with the first op-amp like he does in the second one? I'm incompetent when it comes to op-amps so forgive me, I'm trying to learn.

    CaliusOptimus, any Class-A amp will waste a significant amount of power. I would advise you to go on Texas Instruments website and take a look at their Class-C and Class-D audio amps; you can observe their design techniques.

    Thanks,

    Austin
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The open-loop gain of the 1st opamp coupled with the propagation delays of the remainder of the circuit would result in a horrendous noise; a square wave. Basically, our OP has created a square wave oscillator (but not really, as opamps make for very bad square waves).

    Does that make sense to you?
     
  7. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    well, im trying to build myself an amp, somewhere around 60w PK into 6Ω

    i figured id use the NFB of an op amp to keep the output accurate, and the parts count low. im not hunting for super low S/N ratios or high output power..... im more or less trying to understand the concepts behind a functional amp.

    help me out, do i need faster op amps? a discrete feedback circuit? totally different topology?
     
  8. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    lol, class C amps are narrow band amps for RF freq's. and class D amps are waaay over my head atm.

    what ive posted here is my idea for the first stage of a class AB amp.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why don't you do some reading on the site that I posted?

    It will save my fingers a great deal of typing, and you will get a great deal of good information.

    I'm not on here to type. I'm on here to help.
     
  10. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    lol, ive been reading that site since you posted the link. its a collection of projects, not a discussion of audio amp design. nifty yes, helpful...not really.

    if youve got a link to something that discusses the basics from A to Z....ill be glad to buzz off. until then i need some one to spell it out for me....what can i do?
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    The left op-amp is not actually open loop, since there is a feedback loop around it in the external circuitry. But, I agree the phase shift/increased gain and prop delays through that loop are probably the reason it's oscillating. You always have to be careful putting op amps inside the feedback loop of other op amps. They are not designed to work that way so you have to make sure and cut the gain down.
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Sounds lika an audio amp. You have the wrong circuit, use a push-pull output stage. Lots of circuits around for this.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, sorry that link wasn't quite the one I meant to post.

    Here:
    http://sound.westhost.com/articles.htm
    Scroll about halfway down the page, under "Beginners' Luck - The beginners' Guide to ..."
    and click/read to your hearts' content. :)

    There is plenty of reading material in our E-books.
    Here's where the section on Amplifiers begins:
    http://www.allaboutcircuits.com/vol_3/index.html
     
  14. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    If it were stable it would want to sit a 0 v with no input, AC would, I think clip at 0, trying to go negative. A simple push pull amp with one op amp to drive it would be much better. Use a darlington to get from a couple of ma to a couple of amps. Getting to 60 watts into 8 ohms has voltage problems.
     
  15. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    the ultimate goal is to have a push-pull output..... but i need a class A stage for voltage gain. for the typical push pull topology, you need 20.7v IN, to get 20v out. i can only run an op amp at ±15v, and to get 60w pk into 6Ω i need ±19v to the speaker.

    btw, thanks wookie, that link helped a lot!
     
    Last edited: Oct 30, 2009
Loading...