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Audio Signal Project

  1. Sep 20, 2008 #1
    We're studying BJT amplifiers, and the project that I've chosen to do is to get my Ipod to to play on my music system speakers.

    The idea is to amplify the output signal from the ipod using a biasing circuit and transistor amplifier and use it as input for the pair of speakers.

    Ill most probably be using ac power, so I'll also need to make a bridge rectifier circuit (with a capacitor filter) to get DC to run the amplifier circuit.

    The thing is, I dont know what conventions are used in the audio industry, so I dont know what the ratings mean. I know that the headphones we normally use are rated for 32 ohms impedence, and the speakers I want to connect them to are rated at 8 ohms. Im guessing here that I need the amplitude of the amplified signal to be 4 times greater than the input signal from the Ipod.

    For the biasing, Ill probably be using a Voltage divider biasing ckt, but I cant work out the values unless I know what these ratings mean. Any hints, ideas?
     
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  3. Sep 20, 2008 #2

    MATLABdude

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    You're overcomplicating things ;-)

    8 ohms impedance means just that: the speaker (throughout the range of audio frequencies) has an impedance of roughly 8 ohms. This number is not exact and varies throughout this range (and not just simple w*L inductor impedance either):
    http://www.churchsoundcheck.com/imp1.html

    What your amplifier needs to do is yes, to amplify the signal, but also to be able to supply the required current to drive the 8 ohm load. How much do you need to amplify the (voltage) signal? It depends on the power rating of your speaker. This explains the difference between the 1000W 8 ohm "neighbourhood waker" and the 200 mW 8 ohm speaker in your computer.

    One cautionary tale though: don't hook up a (significantly) lower input impedance load to something designed to handle a higher impedance (say, 8 ohm speakers up to your 32 ohm expecting iPod). However, you can usually hook up a higher impedance. If there's too much current draw, you may end up burning out your audio source as happened to my friend's Discman (the bigger, CD-taking iPod of the 90s and early 2000s) when we were building basic BJT amplifiers for our Active Devices class.
     
  4. Sep 20, 2008 #3
    How would I work around that? Isnt that the whole point of a transistor though? It transfers a current from a low resistor to a high one...
     
  5. Sep 20, 2008 #4

    dlgoff

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    Here's a circuit I would use: A practical amplifier circuit
    My concern is you making a power supply taking power from the AC mains. Eventhough it's not that difficult, it can be extremely dangerous if proper safty considerations are not taken. I would suggest using an AC adapter (wall wort) to get your DC source.

    Regards
     
  6. Sep 20, 2008 #5
    Ive worked with AC mains before, we've even made those circuits in the lab. Also, last year a bunch of us made a very very basic robot that worked on AC mains... that's the easy part :smile:
     
  7. Sep 22, 2008 #6
    It is rather more complicated than you are suggesting.

    Easy way is to use a pair (for stereo) of Integrated Circuit audio amplifiers such as .....LM380.

    http://www.national.com/mpf/LM/LM380.html


    Otherwise you will have to read up and construct a class B push-pull audio amplifier.

    You could build a simple low power class A amplifier but it will use up a lot of battery power.
     
  8. Sep 22, 2008 #7

    dlgoff

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    Isn't that the point?
     
  9. Sep 23, 2008 #8
    Single stage Class A BJT amplifiers are only suitable for low power amplifiers. Anything more than a handful of milliwatts output you want a Class B complementary pair BJT or FET amplifier particularly if you are using battery power. The vast majority of audio output amplifiers used in all sorts of equipment, radios, TVs, Hi-fi amps etc are of the latter type.

    http://www.allaboutcircuits.com/worksheets/bjtamp_b.html
     
  10. Sep 23, 2008 #9

    berkeman

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    Okay, then please answer a few Quiz Questions to make us feel better:

    -1- Are you using a 2-prong plug or a 3-prong plug? Why?

    -2- If using a 2-prong plug, what do you need to do special?

    -3- If using a 3-prong plug, what does UL require that you do with the ground wire? What type of connector?

    -4- Where do the fuse and switch go? What is the fuse rating? What kind of switch?

    -5- What does the term "SELV" mean? How does that apply to an audio amp?
     
  11. Sep 24, 2008 #10
    Wow. We never went over these.

    1: 3 prong plug because you need to ground the input?
    4. Fuse rating is 5A (I think), and fuse and switch will go before the rectifier circuit?

    Where can I look all this up? (I feel like a fool right now)
     
  12. Sep 24, 2008 #11

    berkeman

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    Okay, two things. If you can't yet easily answer those quiz questions, you shouldn't be trying to make an AC Mains input device from scratch.

    Second, there are several places you can learn more. There was a good audio hobby project web page that was posted in a thread here on the PF a while back, and they had a good set of pages about AC Mains safety and construction guidelines. I'm trying to find it with google, but no luck so far. I'll post a link if I find it, and you can try some searching with keywords as well.

    The source for many safety rules and construction guidelines in the US is Underwriter Laboratories ("UL"). They publish rulebooks for different kind of devices, including how the AC Mains connection and circuitry need to be built. The objective of the rules is to help make it very unlikely that a device can deliver a lethal shock to you, or catch on fire.

    SELV is "Safety Extra Low Voltage" (hey, I didn't make up the name, okay?), and is generally defined around 50Vrms or so:

    http://en.wikipedia.org/wiki/SELV

    AC Mains is not SELV, and the device/circuit that you use to go from AC Mains to SELV must meet UL requirements in the US (other agencies in other countries), or you risk pretty hefty liability issues, even for home hobby projects. I'm surprised that your instructors are letting you work with AC Mains voltages without basic instruction in UL regulations and good practices.
     
  13. Sep 24, 2008 #12

    berkeman

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  14. Sep 24, 2008 #13

    dlgoff

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  15. Sep 25, 2008 #14

    berkeman

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    Pretty good quiz. I only got 2/5 right, though. Didn't have time to look up some of the questions in references, though. Thanks.
     
  16. Oct 5, 2008 #15
    Thank you for leading me on to these. I dont know why we never got lectured on all this, even when we work in the lab with ac mains (we use breadboards to make the circuits), we were never taken through these precautions.
     
  17. Oct 5, 2008 #16
    Im sorry for bumping up an old thread.
     
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