Audio Mixer Design Enhancement Suggestions

Discussion in 'General Electronics Chat' started by Shalani, Aug 22, 2010.

  1. Shalani

    Thread Starter New Member

    Aug 20, 2010
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    Dear All,

    I am currently an Electronics Major Student and I've involved myself in an Audio Mixer Design project. Our basic design consists of a crossfader, a 3-band graphic equalizer, a summing amp, voltage indicator and a buffer for the speaker.

    We're using the 741 Op-amp for all of our designs. We've been banned from using IC's since the lecturers feel the need to educate us with raw components.

    Thus, I was wondering if I could get recommendations on enhancements or add-ons I can add to the audio mixer. Our aim is to produce quality output and of course, as many features possible without the use of an IC.

    Here's the schematic to our design if you'd like to have a closer look at the circuit.

    Thanks, Looking forward to your replies!
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. Markd77

    Senior Member

    Sep 7, 2009
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    Can't see the attachment.
    How about an optical crossfader? That should be worth a few bonus marks.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    There is some inconsistency here. A 741 op amp is very old and not that great a device to work with, but it certainly is an IC.

    Perhaps you can tell us what you working definition of "IC" is?
     
  5. Shalani

    Thread Starter New Member

    Aug 20, 2010
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    Whoops, sorry about that!

    Here's the attachment. Thanks a lot! We'll definitely be adding LED's to our crossfader for those bonus marks =) We're actually looking for enhancements of a slightly more complex nature, any more suggestions?

    Thanks so much!
     
  6. Shalani

    Thread Starter New Member

    Aug 20, 2010
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    Well, I'm not exactly sure myself, but I'm guessing that we're not allowed to use IC's that offer additional features in comparison to the 741. However, IC's of the same level but offer better performance should be allowed. Programmable IC's unfortunately, are definitely not allowed.
     
  7. tom66

    Senior Member

    May 9, 2009
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    You can omit the buffer amplifier on the end.

    You will need to add a power output if you want to connect any kind of speaker to it. A simple class AB push-pull driver will get you good marks especially because it shows knowledge of discrete amplifiers. In this configuration, you could put a buffer op-amp before it, and wire the feedback to the output, minimising the crossover distortion. You could even integrate it into the amplifier/volume section, just connect Rf51 to the output side of the transistors.

    I'm not entirely sure of the purpose of Rf11, either. I think you should just connect the crossfader output into a buffer amp. But maybe I'm missing something.
     
  8. Markd77

    Senior Member

    Sep 7, 2009
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    It seems to be missing a few DC blocking capacitors. Common extra features would be individual gain controls and mid and treble cut for each channel.
    By optical crossfader I didn't mean adding a few LEDs, I meant replacing the potentiometer with an optical system.
     
  9. Shalani

    Thread Starter New Member

    Aug 20, 2010
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    Right. Will consider, do what I can with the amplifier and crossfader, and return with more questions!

    Also, we've been experiencing clipping at 20Vpp, which I understand is will happen naturally if the audio signal is larger than the input power supply (12Vdc).

    Is it possible for us to avoid clipping? Do I need to amplify or add power for this to happen? Or is it unavoidable?

    Thanks so much.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Enhancements? If you control the panning by varying the channel gains you could smooth the transition out by adding caps such that the adjustment took place over a time period determined by RC time constants.

    Fancy ICs being "out" eliminates cool bar graph LED vU meters but you could scavenge a set of analog meter movements out of an old cassette deck from a flea market. Peak/overdrive indicator LEDs wouldn't be hard to add and if you aren't limited to just three channels on the EQ (which is a worthless # anyway) you could up that to 5 or more. Bass &/or treble boost/cut switches could easily be incorporated, automatic gain compensation - the list goes on and on but whatever you add should have some real world advantage to it.

    Go look at all the controls on a professional mixer for ideas.

    Clipping at 20V? Most mixer circuits don't deal with signal levels this high as they far exceed the op amp capabilities. Process your signals around the 1V level which is commonly used for line in and out, amplify towards the last only as a headphone driver but again, don't exceed the capabilities fof your circuitry.
     
  11. lalakia

    New Member

    Aug 23, 2010
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    well, i guess you are taking miniproject in multimedia university. I was once suffered for it too..good luck
     
  12. Shalani

    Thread Starter New Member

    Aug 20, 2010
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    Got it, Thanks!

    I've just got word from the lecturer that we're only allowed the usage of discrete components or the 741, nothing else *sigh*. They're looking for different and unique.
    If you do have more ideas on enhancements we could work on with just the 741, your suggestions are more than welcome. If not, thanks anyway guys, will work on what we have.
     
  13. tom66

    Senior Member

    May 9, 2009
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    The 741 is quite an old amp but more than suitable for audio. Its distortion is relatively high but that is difficult to hear. I recommend some kind of clipping indicator. Or you could do a peak/VU meter with 10x 741s (as comparators.)
     
  14. Shalani

    Thread Starter New Member

    Aug 20, 2010
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    Or "you could do a peak/VU meter with 10x 741s (as comparators.)"

    Hey, I'm really sorry to bother you with this again, but do you know where i can find more information on this suggestion of yours? Though it sounds like a really good idea, I haven't been able to find a schematic or any sort of information regarding this suggestion of yours.

    Btw, its not an extension of the voltage indicator right?

    Thanks so much
     
  15. tom66

    Senior Member

    May 9, 2009
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    Basically you use 741's as comparators. They aren't really designed for use as comparators; all op-amps are inferior comparators (they are slow), but they do the job in this situation.

    Have a 10-way resistive divider (1k or so) and use each point on the divider to go to the - input of the 741. The + input goes to the audio input. If you put a buffer and a peak detector + discharge resistor on the input, you can make it into a proper, slow VU meter.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    Most of your resistor values are too low. A 741 opamp and most other opamps have difficulty driving a load as low as 1k ohms.

    Audio goes up to 20kHz. But a 741 opamp design is 42 years old and is lucky to go as high as only 9kHz at full output swing. Use a much better opamp, most go easily up to 100kHz.

    A lousy old 741 opamp adds a lot of hiss to an audio circuit. Many opamps today are low noise.
     
  17. gootee

    Senior Member

    Apr 24, 2007
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    - SEE application notes AN-20 and AN-31 at national.com.
    - Add low-pass filter @ input to limit slew rate to what 741 can do (and to reject RF).
    - Buffer the inputs to get controlled, fixed input impedances. Add DC blocking caps. Add buffered attenuator(s).
    - Limit pots' minimum R values with series R.
    - increase 1K Rs but not to more than 10k.
    - Add feedback R to buffers for minimum offset errors due to input bias current.
    - Adjust "1K R to gnd"s for minimum offset errors due to input bias current.
    - Add R to gnd from + inputs for minimum offset errors due to input bias current.
    - Use 0.1uF and 10uF parallel bypass caps on each power pin.
    - Use "active filter" op amp filter topologies?
    - Use parallel op amps for lower noise?
    - Use differential amps everywhere?
    - Add a DC-eliminating servo to the output? (see http://www.fullnet.com/~tomg/gooteesp.htm )
    - Add RF input filtering?
    - use "star" grounding?
     
    Last edited: Aug 25, 2010
  18. Audioguru

    New Member

    Dec 20, 2007
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    Yes, a lousy old 741 opamp works fine if you filter out high audio frequencies at its input so that its output will be no higher than 3kHz "telephone quality".
     
  19. marshallf3

    Well-Known Member

    Jul 26, 2010
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    His/her instructor limited them to only using the 741 op amps.
    If that isn't "old school" thinking I don't know what it.

    Must be over 100 op amps out with the exact same pinout available nowadays.
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    The instructor doesn't know that people with normal hearing can hear low level noise (hiss) and hear frequencies to and beyond 20kHz.
    Since the lousy old 741 opamp was introduced 42 years ago, then maybe the instructor is 64 years old and/or cannot hear hiss and anything above only 9kHz.
     
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