I don't understand this.

Discussion in 'General Electronics Chat' started by M.F, Aug 20, 2013.

  1. M.F

    Thread Starter New Member

    Jul 13, 2012
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    So i play guitar and i've been into electronics for a while, and this site helped me a lot. I have learnd about op amps and done many simulations with them. Howerver I've never accually put this stuff in reality. So i bought a breadboard last week.
    I had some tl071 from before, so i put one on breadboard and created a simple non-inverting buffer. Connected to 9+ power supply and no biasing. It worked but the volume pot had a terrible stratchy noise and when i turned it all up, there was no signal at the output of the opamp. However, an input cap an a shut resistor after the cap solved the problem. I still don't understand why the signal on the output disappeared when i turned volume all the way when there were no cap and resistor.
    However as i proceeded to improve my buffer, i tried to bias it, but first i put a cap on the output, to block dc i was going to have with biasing. However there is no sound from my amp. As i remove capacitor there is sound again. Then I put a 1k resistor in series instead of cap -- no sound, except when i set input pretty high, apmflifier does horrible noises. I put a 220k shunt resistor after that -- still no sound. I could not understand why there is no signal with a small resistance in series, so i put a trimmer in place of the resistor. When i increase resistance from zero a little more, there is a pop and there is no sound again. I cannot understand why does the resistance in series block the signal?

    Thank you,
    Mile
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Your words are almost impossible to follow or visualize. A schematic drawing would be most helpful in understanding what you are doing. Just trying things that come to mind is a really really poor way to learn about circuits.

    You should start with a design and understand why things work the way they are supposed to work. When you understand a thing you can experiment with variations.
     
  3. M.F

    Thread Starter New Member

    Jul 13, 2012
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    [​IMG]
    Here's the schematic. There is no signal on the output of the second schematic.
     
  4. radiohead

    Active Member

    May 28, 2009
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    The TL071 may be a poor choice for a guitar amplifier. See the attached data sheet. If you are going to use this chip, consult the data sheet and start by constructing some of the circuits in the typical applications section.

    For guitar amplification (for headphones), I would recommend the NE5532 low noise op-amp.
     
  5. cornishlad

    Member

    Jul 31, 2013
    196
    25
    MF. Unless you are trying to build a distorting box that circuit just won't work. Try adding another 220k between the top end of the existing one and the 9v rail and it may spring into life. In a single rail design you almost always have to bias the op amp.

    The output of the chip is relatively low impedance but probably nowhere near low enough to drive headphones. The 1K will make matters much worse.
     
    Last edited: Aug 20, 2013
  6. M.F

    Thread Starter New Member

    Jul 13, 2012
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    Yea I tried biasing, but that makes me add an output cap, and as I stated before, an output cap or resistor mutes the sound. The wierdest thing is that it works without the output cap or resistor (although only one side, since there is no biasing of course).

    And I'm driving a guitar amp, which is made for high output impedance (3-6k).

    Thanks anyway, I appriciate it.
     
  7. cornishlad

    Member

    Jul 31, 2013
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    Sorry, the reference to headphone amp was in the other post. You must add the additional resistor to bias it or it's never going to work. If it was working and the capacitor stops it from working either the cap is too small or it's oscillating. Put 100 ohms close onto the output pin then the cap..which you also must have to block dc form your amp input.. (It's possible of course that the amp has a cap on it's input anyway ) Also put a cap of at least 100uf across the battery.
    If it still doesn't work check the dc voltages. should be 4.5v on the + input and the same on the output. If not you may have a duff opamp.

    ps..you could also try biasing with lower values like 2 x 47k
     
    Last edited: Aug 20, 2013
  8. Papabravo

    Expert

    Feb 24, 2006
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    I'm curious as to why it looks like you have a high pass filter followed by a low pass filter. If the corners are not overlapping, you might have implemented an audio notch filter which probably won't work very well either.

    Nevermind:
    High Pass Corner frequency is 72 Hz. and Low Pass frequency is 2.8 MHz so you obviously weren't trying to do any audio filtering.

    10 nF, your input coupling capacitor, has a huge impedance across most of the audio spectrum. If capacitors are "muting" (attenuating is the correct term) your sound, this is where you should be looking. Across the audio spectrum a coupling capacitor should have an impedance of about an Ohm or less. Certainly 700 Ohms to 7 KOhms is too much.

    The capacitive reactance is:

    1 / (2*pi*f*C)

    where pi = 3.1415926.., f is the frequency in the range [20,...,20 kHz] and C is the capacitance in farads.

    The corner frequency in radians per second of an RC filter is 1 / (R*C)
    To convert the radian frequency to Hertz (Hz.) you divide by (2*pi)

    Single supply audio circuits are tough to deal with. Use a second 9V battery for a bipolar supply and get some 100 uF Electrolytic coupling caps.
     
    Last edited: Aug 20, 2013
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Are you driving a speaker or headphones with this amplifier?

    Papabravo is correct in his last post concerning bipolar supplies and supply bypass capacitors.
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    Slight terminology confusion.

    Coupling capacitors are used between amplifier stages and to block DC from pickup to amplifier. Decoupling capacitors are used from Vcc to GND and Vee to GND.

    Good design requires that there be "bulk" capacitance (100 uF or more would be typical) to supply current demand. Providing a low impedance path from supply to GND requires two three lower value caps in parallel (eg .1 uF || .01uF). It is not the parallel combination of values that is important, but the separate, lower impedance paths to GND, that will improve the rejection of supply noise at the source.

    Also. Anything you do on a breadboard is a guideline to what you need to do in a careful layout. As frequencies get higher, things become more critical.
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    If this is a simulation, double check the connection to the output of the lower amplifier. It seems to not be connected, to me. Otherwise, they look to be identical.
     
  12. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I suggest biasing the input with a couple 1 meg resistors.
    The 220k's may noticeably load down some guitars. That means less signal level and a change in tone quality.
    And 1 meg's will also lower the high pass corner frequency with the 10n cap. As is, the 72 HZ corner will affect the lower frequency's of a guitar.

    The TLO72 is Ok for a guitar preamp because it's got very high input impedance and allows a very high input impedance circuit which is best for guitars with passive pickups. Noise should be minimal when used as a buffer or with low gains.
     
    Last edited: Aug 20, 2013
  13. MrChips

    Moderator

    Oct 2, 2009
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    TL071 opamp is designed to be used with bipolar supply. Use two 9V batteries or a dual power supply.
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You need to generate a virtual ground at 1/2 battery voltage. Op amp on left does that. Right op am does what you had and works fine. See below...
     
  15. M.F

    Thread Starter New Member

    Jul 13, 2012
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    Thanks guys, a bipolar power supply solved everything.
    Have a nice day.
     
  16. ian field

    Distinguished Member

    Oct 27, 2012
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    Sounds like, without the cap - you had a small DC current path that passed through the contact between wiper & track.
     
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