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Audio Isolation Transformer — Parallax Forums

Audio Isolation Transformer

Lord SteveLord Steve Posts: 206
edited 2010-02-28 23:50 in General Discussion
So, I've been working on a project which uses the Propeller to generate audio.· I use the audio portion of the Propeller Demo Board schematic, including MAX4410.· I have found that plugging in the headphones into the jack shorts some things and causes the project to shut off.· To remedy that, could I interpose a small, surface-mount·isolation transformer·on each channel (left and right) between the MAX4410 and the headphone jack?

Comments

  • LeonLeon Posts: 7,620
    edited 2010-02-28 21:09
    If the headphones work OK on something else the jack is faulty. Try replacing it.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Leon Heller
    Amateur radio callsign: G1HSM
  • Lord SteveLord Steve Posts: 206
    edited 2010-02-28 21:52
    I think all jacks short the neighboring sections of TRS (Tip-Ring-Sleeve) connectors.· I tried more than one jack and after·checking the continuity between pins during insertion,·I have verified that it shorts both the Left channel to the Right channel and·both channels to ground during insertion.· My circuit just needs·some robustness against that.

    EDIT:· I know that it shorts at least one channel to ground, perhaps not both.

    Post Edited (Lord Steve) : 2/28/2010 10:18:24 PM GMT
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-02-28 22:26
    Lord Steve,

    I've never noticed the behavior that you note happening on the Demo Board itself. Are you sure you've got adequate filter and bypass caps on your board? If not, the short may be causing a momentary current surge that draws Vdd below the Prop's reset level. Also, Vdd for the audio amp should route directly to the regulator, without including the power feeds to the Prop in its path.

    -Phil
  • Lord SteveLord Steve Posts: 206
    edited 2010-02-28 23:08
    I think it does not happen on the demo board because the wall wart power supply most people would use with·the demo board·supplies enough current to keep VDD high through an short during plug insertion.· Your tip about the power supply to the amplifier is noted.· Although I didn't plan it this way, it turns out that the MAX4410 amplifier is powered off a branch different from the Propeller supply path.· Also, I put a 47uf cap on the Prop in addition to the four 1uF caps around the it, and also a 47uF cap on the MAX4410.· No success.· The 3.3V step up regulator just can't handle the short (the supply voltage dips below 2ish volts during the short).· And it shouldn't have to.· The MAX4410 datasheet says it has short circuit protection but it didn't see any more info about that.· Am I completely wrong in believing that most jacks would short the inputs when inserting a plug?·

    So how about those isolation transformers?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-02-28 23:50
    I'm not so sure that most jacks will short the inputs during insertion. The plug's tip and ring should be beyond the connector shell before they touch the signal contacts. Where you get into trouble is plugging a monaural headphone into a stereo jack. Also, the "protection" alluded to in the MAX doc just means that a short won't destroy the chip, not that it won't create a current bump.

    You could try a transformer, but it might not help. If the amplifier is pumping out audio, a short on the secondary will look like an AC short on the primary. Anyway, make sure the impedances on both primary and secondary match that of your headphones.

    -Phil

    Post Edited (Phil Pilgrim (PhiPi)) : 2/28/2010 11:57:38 PM GMT
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