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Need audio preamp/buffer constant current (power) for bussing

Discussion in 'Audio' started by jeff kloth, Jan 20, 2017.

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  1. jeff kloth

    jeff kloth

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    Dec 16, 2015
    I have a project that is driving me insane! The basic idea is multiple Mp3/wav stereo players play their sounds of an organ note/voice. One sound per audio channel. So 5 devices plays 10 tones. Then the idea is to switch (like organ keys) the tones on and off into a "buss" that goes to a mono preamp.

    The problem I am having is whatever schem I use to buss the tones, there is always a changing load presented to the outputs and thus they change their strength or volume in relation to relative nominal levels. So I assume I will need some sort of buffer at each player output that can regulate level or power as the loads change.

    Due to the the fact that there will be many of the mp3/wav players, I want to keep parts count, pin count and cost down for this project.

    I hope you folks with way more electronical experience than I can suggest a solution! Thanks in advance, Jeff K.
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Show us a circuit diagram please.
    cheers
    Adam
     
  3. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Yes, please post some of the schemes, at least so we don't suggest something you've already tried.

    There are several ways to create a summing buss, some (like constant current drivers) more complex than others. With the most simple, as each note is added the output volume increases algebraically. So the perceived loudness of a single note will be considerably softer than the loudness of a 6-note chord. How do you want the output signal to behave between these two conditions?

    ak
     
  4. jeff kloth

    jeff kloth

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    Dec 16, 2015
    I do not have any useful diagram but will elaborate here: The mp3/wav player is a circuit board that takes 5 volts as power and outputs two channels referenced to ground. There is no circuit schematic available for it but I will assume it is a pic type processor or dedicated mp3 decoder chip and minimal support components onboard. I tried "bussing" the outputs together by using a 5kohm resistor on each hot output and combining them "multing" at the preamp input. This is basically the classic passive mixer.

    It does work as a mixer but remember, I want to keep turning the outputs on and off to by the keyswitch. The above still results in the volume of the tones changing when keyed in and out. If I used very high resistors as above, it will work but is prone to rf as there is alot of wiring and I need to feed a low impedance transformer anyway. This is how it was done in the original Hammond organ which fed the transformer from coil-based pickups (kinda like a guitar pickup," picking up a magnetic field changing from a metal (magnetic) disc.

    So again, the goal here is to have the output tones unaffected by the loads changing as more or less keys are pressed. By the way, each tone is its own fundamental frequency so there is no interaction of the audio to factor in.
     
  5. duke37

    duke37

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    Jan 9, 2011
    A virtual earth amplifier can have several inputs, each not affecting any other input. The amplifier has the feedback to the - input of the op-amp, (edit) [with the + input to earth].Input resistors should be sized to give the amplifier gain required. Output will be inverted.
     
    Last edited: Jan 20, 2017
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    without knowing more specifics, it is extremely difficult to give useful suggestions

    how about, at least, photos of these boards
    where did they come from ?
    why no schematics ?
     
  7. jeff kloth

    jeff kloth

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    Dec 16, 2015
    These are simple, cheap Asian-made ebay boards that can be built into a custom case, etc. Hence, no ducumentation. What may help is we are talking about a l/r output capable enough to drive a set of earbuds to reasonable but not particularly loud volumes.

    To your point about a preamp-- Lets take a guitar amp front-end for example. If only one channel design with multiple 1/4" inputs, yes i believe 2 guitars with passive or active pickups would be mixed satisfactorilly but remember, the may be interacting slightly level-wise and we dont detect it due to the tones are not steady-state as in an organ. Also, they are not being switched on and off against each other constantly... To your point, I have tried opamps and even a little lm386 amp chip on each output and then trying to mult those. They acted the same.. Unless the resistors on each line were very high, the outputs added to each other will have thier own resostive load to become interactive. I hope this is making more sense.
     
  8. AnalogKid

    AnalogKid

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    Jun 10, 2015
    The basic concept of a virtual ground summing amplifier has been around since the 30's. Here are a few variations on the theme:

    https://images.search.yahoo.com/sea...p=summing+amplifier+schematic&fr=&fr2=piv-web

    This one has the equation. You can vary R1, R2, R3 etc. for different gains for each channel, vary Rf to increase or *decrease* the summed gain, etc.

    http://www.tradeofic.com/Circuit/4624-SUMMING_AMPLIFIER.html

    Note: all of these circuits assume that the output impedance of whatever is driving the input resistors is zero ohms, or at least *way* less than the input resistor values.

    ak
     
  9. jeff kloth

    jeff kloth

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    Dec 16, 2015
    I agree one hundred percent about the virtual ground summing amplifier as described. It cannot work for my implementation though because in the existing Hammond organ keying system to which I am feeding these tones into, the busses must connect to the tone generation points directly. This means the connection is before the resistor or opamp in the example above.

    As I am sure this is getting more and more confusing, I apologize. To clarify, in the original Hammond organ tonewheel system, each pickup when keyed may combine with many other pickups being keyed.

    Here is a link to the Hammond schematic. My goal is eliminating the original tonewheel/pickup scheme and inserting my tones generate (recorded from an organ) and played from my solid-state cards.

    http://theatreorgans.com/hammond/faq/files/schematics/b3sch-a.pdf
     
  10. jeff kloth

    jeff kloth

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    Dec 16, 2015
    This thread problem has been solved. By adding one resistor, I was able to use the factory matching transformer without loading... thanks jk
     
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