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PDB Audio Gain — Parallax Forums

PDB Audio Gain

Mike CookMike Cook Posts: 829
edited 2005-12-16 01:13 in General Discussion

Is there any way to increase the audio gain on the PDB (LM386)? I'm using it with the EMIC module and can't seem to get the audio loud enough.

Thanks,

Mike

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Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-12-09 13:51
    No, the components are fixed for the onboard speaker. If you connect an external (e.g., "bookshelf") speaker that is properly enclosed you'll notice a lot more volume.

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    Jon Williams
    Applications Engineer, Parallax
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2005-12-09 13:55
    You betcha!

    It's the gain programming capacitor between pins 1 & 6.· See Page 6 of 10 of the attached PDF (it's page 5 of the datasheet itself.)

    There are a few versions of the LM386, the DIP-8s have different output powers.· So, there may be a power limitation and matters of distortion with increased gain.
    pdf
    pdf
    LM386.pdf
    298K
  • Mike CookMike Cook Posts: 829
    edited 2005-12-09 14:00
    Thanks Jon & PJ,

    I've been using some amplified computer speakers to increase the EMIC's audio but I'm getting a lot of buzzing from these. I'll try your book shelf speaker idea.

    PJ - I'll check out adding the cap to 1 and 6 on the LM386

    Mike

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    Product web site: http://www.allsurplus.net/Axiom/
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2005-12-09 14:09
    Honk, Honk!

    Those would be pin 1 & 8, Mike.· Sorry, I had a presbyopic episode.
  • Mike CookMike Cook Posts: 829
    edited 2005-12-09 14:42
    Thanks PJ,

    I'll check out the data sheet on the LM386 tonight.

    Mike

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    "OEM NMEA GPS Module" Now available on ebay for only $19.99, FREE shipping until 2006!

    Product web site: http://www.allsurplus.net/Axiom/
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-12-09 15:10
    Here's the schematic for the amplifier that we use on the AP-8 audio player.· When the power switch is moved to position 2, a 10 uF cap is connected between pins 1 and 8 on the '386.

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    Jon Williams
    Applications Engineer, Parallax
    LM386_Amp.jpg
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  • PJAllenPJAllen Banned Posts: 5,065
    edited 2005-12-11 01:49
    OK, I'm not busting anybody up, but...

    the·schematic in the post immediately preceding shows the gain capacitor's Positive electrode going (via the switch) to the LM386's Pin 8 ("Boost B"), whereas the device datasheet indicates that this capacitor's Negative electrode should go to Pin·8 and the Positive electrode should go to Pin 1.
    386gain Cap.JPG
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  • John BJohn B Posts: 82
    edited 2005-12-16 01:13
    Hi there,

    · Sorry for not addressing this sooner - I've been terribly busy.

    · The function of the cap is but to bypass a resistor and thereby help establish the gain.· The LM386 cares not about the polarity of the cap.· In fact we could (perhaps should) have used a non-polarized 10uF cap and things would have worked just fine.· The orientation of a polarized·cap is selected empirically by· measuring the difference between the AC voltage present on pin 1 and pin 8 at any given time.· The pin that is more positive gets the positive leg of the cap.

    · During development of the AP-8·our testing showed that pin 1 was more positive than pin 8 half the time.· So in this case it appears that the cap is happy in either/neither orientation.

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    John Barrowman
    Engineering
    Parallax, Inc.
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