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LittleBits Mixer

Discussion in 'General Electronics Chat' started by 4pyros, Apr 18, 2014.

  1. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Aww, we still need you. Some how I think there will still be CRTs in this world for sometime to come.
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Except they haven't, and their attempt was laughable :D

    Assuming you actually wanted to make an AC/DC mixer (even though it's a completely bizarre idea), and didn't care about massive distortion dependent upon where you set the controls and the specific inputs) this wouldn't be how you would do it.

    For a start throwaway the bogus AC coupled stage (the only stage that isn't distorted), use a proper virtual earth mixer (separate AC and DC gains can be easily set for it, if required), and throw away the no longer required second 'mixer' attempt, and use that opamp for an inverting buffer (so as to maintain the polarity through out). It's still going to be pretty naff, and dependent upon levels and settings, but at least it's actually a mixer.

    I'm presuming the idea of this is that you can use it EITHER (not both at once) as a DC 'mixer' or an AC one?, in which case why not simply build two separate mixers?, twice as useful, both would work properly, and you'd use less parts than the existing effort.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Not in the UK there isn't! :D
     
  4. dave

    Dave New Member

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  5. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    I think one thing I forgot to share is the fact that most of these modules have input and output buffers. It seems the first opamps in the mixer are buffers?
     
  6. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    I assume the main purpose here is to keep it simple for the end user.
    The bits are made for ages 8 and up and it mite be a bit confusing for a kid with no electronics background to pick the right one to use.
     
  7. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    If I layed out all the requirements for the mixer module we could make a better one?
     
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I think it's more confusing as it is, where it will only work under certain circumstances.
     
  9. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Ya but it works all the time!
    Why would you think it only works under certain circumstances?
     
  10. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Here is the speaker or output module
    It is certainly not high fi but its just a toy.
    As expected its mono, but its interesting how they shift the audio thats above 0 volts back to above and below 0 volts.
     

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  11. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Because there's no biasing for the opamps, and even assuming it's provided from the previous stages adjusting the volume settings will still alter the biasing on the later stages.
     
  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Nothing particularly wrong with the output part, it's all you can expect when you've only got a low supply rail.

    However, it's got yet another non-biased buffer at the front, for no reason whatsoever - that opamp isn't part of any DC system so could have been biased properly and capacitor coupled - for that matter the first buffer stage could have been left out entirely, and use the 100K input impedance of the second stage, or even increased it to 1Meg if there was any need (which I wouldn't have thought there was).

    I'm a bit bemused by your use of 'interesting', for what is a standard and simple technique used in almost every circuit you come across - however it's just DC blocking, it's not referencing it to ground as that's not wanted (the usual place it is wanted doesn't apply here because it's a bridged output stage, a single ended stage though would reference the speaker back to ground).
     
  13. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    I wonder if it is used to keep the proper impedance on the feed thru port. It does have an output port as well.
    I think I could take that as a hit, but I won't, admittedly I am still learning even at my age. At least I can still see things that don't seem quit right and find them interesting.
    I thought it was interesting in this case how they feed the signal into the (IN-) of the output chip.
     
  14. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Only the opamps used on the inputs for buffers have no biasing.
    Why would you want to bias a unity gain buffer?
     
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Sorry, but I fail to see anything 'interesting' about it at all, it simply has the standard DC blocking capacitor as all amplifiers do.
     
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Plus the ones pretending to be a mixer, and the output opamp as well, only the un-needed AC (non-)mixer is biased.

    Because otherwise you get massive distortion from it, it makes no difference at all if it's a buffer or something else.
     
  17. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Can you point me to something to help me understand this?
    As far as I can tell if you bias a unity gain buffer it will not work rail to rail with DC.
     
  18. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Any simple book on opamps?.

    And if you don't bias it then it rectifies an incoming AC signal, giving MASSIVE distortion.

    I've been looking a little more at the littleBits website (confusing as it is), and the previous stages (at least the ones I've looked at) provide biasing for the following buffer stage - but this doesn't prevent the problem of the volume controls changing the bias of the following stages.

    You should also bear in mind that you don't get 'rail to rail' opamps, only 'nearish to rail' ones :D

    Unfortunately the entire littleBits system is crippled by it's lack of supply voltage, and too small a number of connections between modules :(
     
  19. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    I dont think Nigel likes your machine. On the other hand, 12 and likes messing with music, maybe it will spark an interest.
    Its a shame that he isnt into it enough to mess with 555 timers and piezo electric disc's for drums etc.
     
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  20. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    I have looked and could not find anything on biasing a unity gain buffer, as far as I can tell its a voltage follower.
    Thats why I asked if there is anything specific you could point me to.
     
  21. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    For the most part we are talking about square and triangle waves witch should be OK with this right?
     

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