3 Band Equalizer

Discussion in 'The Projects Forum' started by xz4chx, Nov 6, 2013.

  1. xz4chx

    Thread Starter Member

    Sep 17, 2012
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    For our electronics project, we are building a 3 band second order op amp.

    We were told that we should use 741 op amps for this project as it shouldn't make enough problems for the equalizer.

    Analysis:
    U1 is the low pass filter with an db breakdown of -6 db at 500hz (Sallen-Key op amp configuration)
    U2 is the high pass filter with a breakdown at 2000hz (Sallen-Key op amp configuration)
    U3 and U4 make the band pass (Sallen-Key configuration)
    U3 is a high pass filter with breakdown at 500hz and U4 is low pass with breakdown at 2000hz. U5 is an inverting configuration s because U3 and U4 together create an extra 180 degree shift, so the inverting is there to shift another 180 degrees to match it with the other two passes.

    After all of the bands, the outputs are sent to a weighted summers configuation (U6). The 500k pots are there to make the gain from -20db to 20 db (-R15/(R19+R16(Pot))).

    After the weighted summers a NPN and PNP transistor combination with the op amp is used in order to increase the current in order to drive the speaker we are playing music out of.

    By theory, everything seems to check out okay. When we built the circuit in lab, our output seemed had static which we came to find out was from saturation. Music is played at around 300mV so I don't see out we would be getting saturation at such low levels. The max gain is 20 db (10x the input) with is only 3V. And our op amps should only saturate at +/- 15V.

    Everything separate seems to work out perfectly but when combined we start having problems.

    I don't understand what would be causing this saturation at such low voltages.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The output stage is not wired correctly. The lower PNP transistor is upside down.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I have no idea what that means, but a 741 is the lowest garbage grade device there is.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Even when the output stage is correctly wired it will have significant crossover distortion since the stage is not biased into the active region. Thus there will be a flat-spot in the output as the output stage drive from U7 has to go from +0.7V when the NPN transistor turns on to -0.7V when the PNP transistor turns on (which U7 can't do instantly).
     
  5. alexfreed

    Member

    Oct 8, 2012
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    May not be a major issue: the feedback loop correctly includes the output stage. Yet still for any sound quality it is much better to bias the output transistors say with 2 diodes and use better op amps. Say NE5532.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    It likely will give noticeable distortion for any but low frequencies, since the frequency response and slew rate of a 741 is rather slow, limiting the minimum time it takes to traverse the 1.4V difference. The two added diodes and a better op amp as you suggested are really needed.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The absence of input offset correction perhaps?
     
  8. xz4chx

    Thread Starter Member

    Sep 17, 2012
    71
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    So if we change the op amp at the transistor, will that fix our problem of having those "saturations". I did notice that when we lowered all the Vcc and Vee of the circuit from 15V to around 4-5V that there was less cutoff at the peaks of the voltages. That is directly related to the slew rate of the 741 with the transistors?
     
  9. xz4chx

    Thread Starter Member

    Sep 17, 2012
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    Also, would the 741 cause problems with the bands and my weighted summers? If I use a different op amp then the 741 I need a legitimate reason to be switching besides just the little noise the 741 creates compared to others.

    Thank you so much for all the help. Thank you thank you.
     
  10. Experimentonomen

    Member

    Feb 16, 2011
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    And i dont see why ppl call this a three band equalizer when all it is is a bass/mid/treble tone stack.

    In order to truly be called an equalizer you need a bare minimum of 6-7 "bands"
     
  11. xz4chx

    Thread Starter Member

    Sep 17, 2012
    71
    1
    I'm sorry?...
     
  12. xz4chx

    Thread Starter Member

    Sep 17, 2012
    71
    1
    If I check the output after U6 with the oscilloscope, everything works perfectly fine, but then if i plug in the speaker, the signal goes to saturation. I am not 100% what the op amp voltage follower (U7) and transistor combination does. Our professor said it would help power the speaker but we just started to learn transistors.

    So i wanted to know if the speaker does not have enough current to it, does the speaker cause the signal to go into saturation at smaller voltages? If we add more current to the speaker would that cause the signal to not saturate so quickly?
     
  13. xz4chx

    Thread Starter Member

    Sep 17, 2012
    71
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    Also I changed the transistor part of the circuit. I know one is pointed the wrong way.
     
  14. tubeguy

    Well-Known Member

    Nov 3, 2012
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    The 100k resistors R22 and R23 prevent the associated transistors Q1 and Q2 from supplying any useful current to the speaker load. Those resistors could be eliminated. Try adding a 100-220 ohm series resistor instead between the output of the op-amp and the bases of the transistors.

    What is the impedance (ohms) of the speaker ? Even without R22 and R23 those transistors can only provide very low power to an 8 ohm speaker load.
     
    Last edited: Nov 8, 2013
  15. xz4chx

    Thread Starter Member

    Sep 17, 2012
    71
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    Usually they are around 8 Ohms
     
  16. xz4chx

    Thread Starter Member

    Sep 17, 2012
    71
    1
    The one we are using is 8 Ohms
     
  17. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Have you studied voltage dividers ?
     
  18. xz4chx

    Thread Starter Member

    Sep 17, 2012
    71
    1
    Yes, so the resistor and speaker would work as a voltage divider, which would lower the voltage going to the speaker.

    I'm making a educated guess here.
    Since the speaker was turning on but going into saturation at the extreme voltages at the end of (U6), you add in the (U7) to take into account for the voltages between -.7 [V] and .7 [V] and transistors Q1 and Q2 in order to have a gain in current since the base current (current from output of op amp) is much smaller than the emitter current. The voltage divider lowers the voltage going to the speaker but the transistors increase the current to the speaker so that the speaker doesn't go into saturation but still has loud enough sound come out of it?
     
  19. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Ok. The transistors are intended to follow the voltage output of the op-amp, and boost the current.
    To the OP:
    How much current would it take to produce 2.8 volts across the 8 ohm load? If that current flows through a 100,000 ohm resistor, what is the voltage drop?
     
    Last edited: Nov 9, 2013
  20. xz4chx

    Thread Starter Member

    Sep 17, 2012
    71
    1
    It would take .35 mA to produce 2.8 voltage across the 8 ohm load. The voltage drop would have to be 35 kV. Okay that makes sense lol.

    If I am hearing sound through the weighted summers, do i even need this circuit configuration (my teacher said i would need it for a speaker)?
    It seems that this is needed if the speaker is not getting enough from the voltage and the output current from the op amp.

    Also I am having saturation issues and we figured out what the problem is. Whenever we add a bigger resistor at the load (500 ohms) the voltage (output) does not have any saturation not matter how much we increase the voltage (to a value lower than Vcc and Vee), but when we change it to a 4-10 ohm resistor or the speaker there is saturation at the output at unity from the input.

    Why is this? This is our main concern.

    Besides this all of our circuit components work perfectly and we had a little DC offset so we just added a capacitor at the input and output to get rid of that.

    Also thanks for all the help. I appreciate it a bunch.
     
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