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LM368 Audio Amp — Parallax Forums

LM368 Audio Amp

RaymanRayman Posts: 14,801
edited 2008-01-29 15:25 in Propeller 1
This chip isn't a very good match for the Prop, I'm only using one because it's the only audio amp I could find at Radio Shack.·

I read some earlier posts where people had trouble getting it to work.· I just got mine working so I'm posting the circuit:

Note:· The 1.5uF cap shown before is actually 0.15uF (now corrected)...



Post Edited (Rayman) : 1/29/2008 1:57:39 AM GMT
LM386N.png
424 x 181 - 3K

Comments

  • RaymanRayman Posts: 14,801
    edited 2008-01-27 22:28
    PS: I find Parallax font is useful for typing out quick circuits. Just wish it was one of the fonts allowed by the text editor on this forum... (I'd guess they could add it, if they wanted too...)
  • deSilvadeSilva Posts: 2,967
    edited 2008-01-27 23:02
    (a) You have a low-pass with cut frequency 100 Hz followed by a high-pass with 1Hz?
    (b) In general the parallax font works, when copying from the Propeller Tool (see my postings with the I2C drivers)...
  • grasshoppergrasshopper Posts: 438
    edited 2008-01-27 23:07
    Rayman said...
    This chip isn't a very good match for the Prop, I'm only using one because it's the only audio amp I could find at Radio Shack.

    Man Radio shack sucks. A good replacement is Digikey www.digikey.com

    Yes you have to wait for shipping but the cost savings and selection is far better than Radio shack
  • RinksCustomsRinksCustoms Posts: 531
    edited 2008-01-28 02:24
    Actually, i found that www.mouser.com has a better selection and more availability than digikey, just look up temperature sensors, lol

    And the 386 is good for about 1W in a 8ohm speaker when setup properly, are you feeding it about 9-12V? (might want a heat sink at 12Vjumpin.gif·) Need more? A typical T03 transistor (PNP or NPN when heatsunk and biased) can produce 5-25W+ by itself. I know·that suggesting someone·bias a transistor is like telling an ITT·grad to use a stone to fix a TV, but it's cost effective and quick!

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    E3 = Thought

    http://folding.stanford.edu/·- Donating some CPU/GPU downtime just might lead to a cure for cancer! My team stats.

    Post Edited (RinksCustoms) : 1/28/2008 4:30:12 AM GMT
  • DgswanerDgswaner Posts: 795
    edited 2008-01-28 06:28
    Rayman, I have to disagree, I've used the LM364 in about 4 different types of amps. you can use just about any resistor capacitor combination and it will work. and I use one with my Propeller. it can take a beating and still work. the only down side is that it's mono. I use it to power a 1Watt computer speaker for my bot. it's nice to have audio feed back for trouble shooting.

    look at www.runoffgroove.com for all sorts of different amp configurations.... mostly for Guitars.

    I can send you a schematic and PCB layout of the circuit I'm using.. it cost about $5.00 to build.

    good luck

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    "A complex design is the sign of an inferior designer." - Jamie Hyneman, Myth Buster

    DGSwaner
  • RaymanRayman Posts: 14,801
    edited 2008-01-28 13:39
    The good things about the LM368 are availibility, DIP package, and fairly low voltage requirement...

    My main problem with it is the built in voltage gain of 20X.· This requires using a pot to reduce the Prop output down a factor of ~20, which I'm not too happy about...

    There are plenty of ICs, like the one on the demo board that are better suited, but it's hard to find something in DIP.

    I think a better idea is a high-ouput current rail-rail opamp set to unity gain.· I think the TLV4112:·
    http://focus.ti.com/lit/ds/symlink/tlv4110.pdf

    might be good.· But, audio isn't mentioned as a target app, although "coil driver" is, which I guess is a similar thing...
  • RaymanRayman Posts: 14,801
    edited 2008-01-29 01:54
    Till now, I was just testing with oscope and sine waves... But, I just hooked up an SD card to this board and the sound is pretty good, considering the 1" speaker it's hooked up to...

    Note, that I did correct a typo in one capacitance value in the circuit... (maybe this makes deSilva happier?).
  • RinksCustomsRinksCustoms Posts: 531
    edited 2008-01-29 02:26
    idea.gif·MAO!! I found a better amp for you, should be effective enough!!

    http://sound.westhost.com/project117.htm·skull.gif
    Scroll down to figure 1, enjoy...lol.gif

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    E3 = Thought

    http://folding.stanford.edu/·- Donating some CPU/GPU downtime just might lead to a cure for cancer! My team stats.

    Post Edited (RinksCustoms) : 1/29/2008 3:13:56 AM GMT
  • deSilvadeSilva Posts: 2,967
    edited 2008-01-29 02:54
    You have still faded off all high frequencies smile.gif
    Your filter reduces the AC voltage swing so that the gain of 20 will not be unused. On the other hand all bass signals will be over-amplified which will give interesting effects. But "sound processors" are good for tiny speakers smile.gif
  • RinksCustomsRinksCustoms Posts: 531
    edited 2008-01-29 03:48
    deSilva said...
    You have still faded off all high frequencies smile.gif
    Your filter reduces the AC voltage swing so that the gain of 20 will not be unused. On the other hand all bass signals will be over-amplified which will give interesting effects. But "sound processors" are good for tiny speakers smile.gif
    & as you know desilva, one must move a certain ammount of air before it can even be percieved at arms length, so is there a point to a small speaker (such as a 1") having the bass boosted?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    E3 = Thought

    http://folding.stanford.edu/·- Donating some CPU/GPU downtime just might lead to a cure for cancer! My team stats.
  • DgswanerDgswaner Posts: 795
    edited 2008-01-29 03:58
    I'm no EE but I know the circuit works and sounds well with a propeller. I used the circuit provided in the data sheet, I hardwired the gain tho. my brother in law cranks up the gain on them and uses them as an effect pedal for is guitar. we love them, I've built several in Altoids tins. they work great as Ipod external speakers. but yes Bass? ya there is no bass on a 1" speaker

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "A complex design is the sign of an inferior designer." - Jamie Hyneman, Myth Buster

    DGSwaner
  • tekochiptekochip Posts: 56
    edited 2008-01-29 15:11
    This is the circuit I use at 5V.· The LM386 is a great general purpose amplifier, but you will need to pad down the signal a little.··The `386·has a fixed gain of 20, and that will cause some clipping as deSilva mentioned, so R25 and R31 will attenuate the signal and give you good performance at 5V.· If you have a 9V supply you can reduce R25 to 4.7K and get a little more volume.
    bmp
    bmp
    Crop.bmp
    293K
    Crop.bmp 292.8K
  • LeonLeon Posts: 7,620
    edited 2008-01-29 15:25
    It's a very old design and rather noisy. I prefer the TDA7059, it's very quiet and needs less components.

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