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Thread: How to change the MIDI input circuit for 6N137 instead of 6N138?

  1. #1
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    How to change the MIDI input circuit for 6N137 instead of 6N138?

    I'm trying to figure out what modifications to make to the MIDI input section of this circuit to use 6N137 IC instead of the 6N138 shown.

    Click image for larger version. 

Name:	td_libs_MIDI_sch_t3.png 
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    In my searches I'm finding incomplete and conflicting info. Any solid answer? (I'm using Teensy 4.0 and 4.1 if that makes a difference)

  2. #2
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    6n137 vs 6n138

    use the teensy 3.3v as pull-up on 6n137. See https://electronics.stackexchange.co...d-opto-coupler

    Quote Originally Posted by dannytaurus View Post
    I'm trying to figure out what modifications to make to the MIDI input section of this circuit to use 6N137 IC instead of the 6N138 shown.

    Click image for larger version. 

Name:	td_libs_MIDI_sch_t3.png 
Views:	29 
Size:	4.9 KB 
ID:	25568

    In my searches I'm finding incomplete and conflicting info. Any solid answer? (I'm using Teensy 4.0 and 4.1 if that makes a difference)

  3. #3
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    Thanks for the link jimr but I decided to go the easy route and source some 6N138 instead. I see a lot of conflicting information about the 6N137 and I'm too new to this to make sense of it.

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    Quote Originally Posted by dannytaurus View Post
    Thanks for the link jimr but I decided to go the easy route and source some 6N138 instead. I see a lot of conflicting information about the 6N137 and I'm too new to this to make sense of it.
    @dannytaurus:

    Have a look at <this thread>, which appears to describe a successful implementation with the 6N137.

    Mark J Culross
    KD5RXT

  5. #5
    Senior Member oddson's Avatar
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    While not qualified to say, what I read is the 137 can tolerate a much higher impedance pullup and so the using the internal one makes sense to me.

    I believe lower value pullups would still work and that higher values are recommended to mitigate high-frequency noise from the fast switching speed.

    The Darlington pair in the 138 switches much more slowly and needs a more aggressive (lower impedance) pull-up to work, even an MIDI speeds.

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