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Finding the noise-source in a headphone-amp

Discussion in 'Electronic Basics' started by Rikard Bosnjakovic, Feb 8, 2007.

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  1. Greetings

    I found and built this project:
    http://raymondaudio.nl/projects/project7/project7.html

    After I finished and tested it out, I found lots of noise in the
    headphones. Since my skill of analog electronics is way too low to get a
    good guess what might have gone wrong, I took the construction to my
    lecturer at the university. He gave me a couple of advices how to find the
    errors in such piece of circuit.

    Another week passed and after extensive searches, I think I have pinned
    the problem. Here's an explanation of what I did to test and pin:

    The blocks of the circuit are transformer, rectifier, linear regulators,
    sound amplification. Using it plainly, I get lots of noise in the
    headamps. Bypassing the transformer and rectifier-bridge (using a DC-psu
    and feed the DC into the regulators), the noise goes away. Doing the same
    but feeding to the bridge (i.e. "rectifying" my DC), the noise comes back.

    I take this means the rectifier bridge (four 1N4005) is broken in some
    way, but how? Before building, I tested each diode with an ohmmeter, and
    they all showed "infinite" resistance in the wrong direction, and low
    resistance in the correct direction.

    So, what might it be that's happening at the diodes? Magnetic field
    inducing? Short circuits?
     
  2. Eeyore

    Eeyore Guest

    Is it built on a pre-made pcb or have you made it on stripboard ?

    Hiss (white noise) ? Does it sound 'swirly' at all ?

    Doesn't sound much like it.

    I suspect the op-amps are oscillating at VHF due to an absence of local supply
    decoupling. That tends to make them noisy. These 'project designs' on the net
    are often very amateurish.

    Try adding some plastic film or ceramic 22nF capacitors from both V+ and V- on
    the op-amp power pins to ground.

    You should really have something similar on the inputs to the LM317 and 337 too.

    Graham
     
  3. It is built on a veroboard / stripboard.
    I don't think it sounds swirly or noisy, it sounds more like a constant
    "clear tone" ringing all the time.
    [...]

    A lot other people have told me this as well, that the PSU in the
    schematic is really a waste of power. I'm too unskilled to have an own
    opinion, but running into problems like these are probably a good idea to
    learn more things that aren't in the books.

    I will add some caps to the op-amps and the regulators and see what (if
    something) happens.
     
  4. Chris

    Chris Guest

    Hi, Rikard. You've pretty much proven that the issue is with your
    power supply. But I think it's possible the basic problem is with the
    zeners rather than the rectifiers. I'd use a standard resistive
    voltage divider (try replacing the zeners with1.2K and the 200 ohm
    resistor with a 120) instead of the 12V zeners for the 317 and 337.
    Any zener noise is just going to be copied directly onto the output
    line, with poor audio resuts.

    Good luck
    Chris
     
  5. Eeyore

    Eeyore Guest

    Which increases the noise of the 317 / 337 by ~ 10 times, the gain of the reference diode !


    You're talking complete and utter rubbish.

    A. The noise fron zeners isn't enough to be problematic.
    B. Op-amps have very significant supply voltage rejection. They don't care too much about
    in band supply noise.

    Graham
     
  6. Rich Grise

    Rich Grise Guest

    Like a whine, or a whistle? Then it sounds like you've built an
    oscillator. :) Do you have access to a 'scope?

    Good Luck!
    Rich
     
  7. ehsjr

    ehsjr Guest

    Each regulator should have a .1 uF cap from Vin
    to ground. Whether or not that fixes your problem,
    install them.

    Ed
     
  8. Clint Sharp

    Clint Sharp Guest

    More likely you've got one of the diodes in backwards. Check and double
    check.
     
  9. Lionel

    Lionel Guest

    Not a chance. Putting in one or more diodes backwards would either
    make it buzz or stop it working altogether.

    As everyone else has said, it's almost certain to be one or more of
    the chips oscillating from the lack of bypass chips - especially
    seeing as it's built on Veroboard. I'd solder a 100nF monoblock
    ceramic across the power input/output pins of each chip, *as close as
    physically possible*. (And maybe a 4.7uF tantalum to the input side of
    each LM317.)

    Oh, & if I were building that circuit myself, I'd use a 78L12 & a
    79L12 & replace each Zener diode with a pair of 1N4001 diodes to jack
    the power rails up to 13.2V. (Cheaper & smaller)
     
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