audio amplifier

Discussion in 'The Projects Forum' started by Deric, Oct 15, 2010.

  1. Deric

    Thread Starter New Member

    Feb 8, 2010
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    I have a project i have started that is a little out of my level of competency. I am a senior mechanical engineering student, so i know a little circuits from the classes, but not a lot.

    The task, i want to take a signal from the head phone jack of my phone, and amplify it enough to power a sub, mid, and tweeters.

    The problem is that when i have tried to teach myself to build the filters i have found so many different variations that i dont know what one to even start with, or which is most beneficial to this application. plus, it would be nice to have some experienced people look at my layout and help me foresee any problems i may encounter.

    important stuff:

    phone output: 200mV 10mA
    ext. power supply: 12v 20A
    (1)subwoofer: 12v rated; 200W rms; 8ohms; 43-250Hz (i know the math doesnt work out, but thats from the manufacture data sheet, rockford fosgate p28s8)

    (2-4)tweeters: 12v rated; 20W rms; 6ohms; 5500-20,000 Hz

    mid range: undecided as of now

    two middle speakers in schematic is just the mid, shown as two speakers on separate filters for now, but its just one. there will be several tweeters though. there will also be a few pots to adjust volume of different channels,

    hopefully someone out there understands all this. any advice on which type of filter is best suited will be appreciated. also, any comments or suggestions to make my circuit better would be great. thank you all

    SORRY ITS NOT LABELED, PHONE JACK IS CONNECTED TO THE BASE OF THE TRANSISTOR
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You've got a number of problems to deal with here. First of all the 741 op-amp is so outdated it isn't funny unless you enjoy no frequency response and a ton of distortion.

    Looks like you're trying to tri-amp or in this case quad-amp something. A few things to remember:

    Tweeters don't need much power at all to drive, 5 W RMS is often enough

    Woofers need almost all the power, it takes a hefty amp for them.

    So, if you insist on separating the frequencies before amplification you're going to have to build different amps for each speaker, each with different capabilities unless you go with large common amps and adjust the input to each to compensate.

    May I suggest you take a look through http://www.partsexpress.com for a bit?
     
  3. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Mechanical Engineering - So Electronics is not your specialty. We can cut you some slack for that.

    The speaker rating is likely to mean they are meant for installation in cars. High powered automotive amplifiers include switching power supplies so they can output high voltage audio from 12V DC power input. Some average car stereo enthiusiasts would be confused if you told them that the speakers were rated for 24 Volts RMS - 40 Volts peak and they didn't know that yes those are speakers for in a car.

    It would be like you trying to explain static and dynamic loading of a bridge to somebody. Telling them that a bridge is rated for a static load of 20000 tons won't help you to convince them that the bridge is only rated for vehicles under 20 tons, but could be used to pass a 2000 ton tractor trailer if the bridge was closed - well inspected - supervised - and the truck kept to a crawl of under 2 mph. Actually that is civil engineering but close enough.

    You really do want check your parts and plans.

    Using Mosfets that way, are you doing a Class D amplifier?
    You are missing the whole switching signal coversion part of your amplifier, which is a big miss.

    Otherwise you look to be making a class C amplifier which is not recomended for audio applications.
     
  4. Deric

    Thread Starter New Member

    Feb 8, 2010
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    I realize the speakers are for cars, that was my original intent when i bought these. I have done many car stereos, but we just buy an amp for that. This project i am doing now is a personal project i want to do, that i also get credit for in school. Well, credit for learning something new to me.

    Basically, i would like to have the ability to get the most out of each speaker, be able to show off what they can do.

    I dont know the difference between 741's or any other op amps. i assumed when i made that schematic that these were basically seperate amps, arent they?

    I also dont know which type of amp, class c or d, i am going for. thats all part of the learning experience to me. as long as i go from the idea, to building a working device, i am happy to learn it all along the way.

    is there a better alternative to the 741? should i be aiming towrds a certain type of filter or amp that i dont know of?

    also, i wont consider buying a pre-made, plug and play system, i am doing this for the learning. thanks for the quick responses so far, it gives me a few things to think about.
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You've got a lot of design work ahead of you.

    I'd start with building one simple amplifier block and if you're going to try and use MOSFETs for the output you should google Class D amplifiers - fairly complex stuff.

    If getting the entire class dancing to the the latest tunes it will really get difficult.

    My guess is for your demo simply building 4 identical mono amplifier circuits would be best then make an active crossover before them all to separate the frequencies, or passive ones afterwards but that gets more difficult and expensive.
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    IF u want the best from ur speakers, then go for active cross over and tri-amping.
    U will never touch another music system ever again. this I guarantee.........
     
  7. Deric

    Thread Starter New Member

    Feb 8, 2010
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    Well i will look into that, i really have no idea what any of that means, or where to go. But thanks for the help. Any good places of reference for me to learn about this stuff
     
  8. R!f@@

    AAC Fanatic!

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  9. SgtWookie

    Expert

    Jul 17, 2007
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    The frequency response of a typical telephone system tends to roll off somewhere around 4kHz, so your tweeters will be pretty much useless. Also, bass response isn't that great, so your bass speakers won't do much either.

    All you really need is decent frequency response in midrange; perhaps 300Hz to 4000Hz, in order to be able to clearly understand the conversation taking place.

    Just to get some ideas, you might look at a TDA2822 IC by ST Microelectronics. In the datasheet (attached), they show a test circuit for a bridged amplifier (figure 2 on the 4th page). The idea of a bridged amplifier is that while one output goes high, the other output goes low, so the 12v supply is effectively doubled to 24v. Unless you use such a bridged amplifier scheme, you will need to build a power supply, which is a whole 'nother can o' worms.
     
  10. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Darn it.

    How come Sgt is the only one that looked at a question that included a phone as the input and bass and tweeter amps and thought to say you don't need the bass and tweeter.

    To be fair the phone could also be an MP3 player.
     
  11. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Also I wonder if anyone knows off hand if there has been any expansion of the frequency response for phones since they have gone hi bandwidth digital?

    Why wouldn't they would be the next question - but I expect that limiting freq range helps keep a focus on the voice audio.

    Who has a voice that drops under 300Hz or reaches over 4k?
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Potato Pudding: There´s phones and phones. What about headphones input?

    http://diyaudio.com is a very good source for amp design and other audio stuff.

    As for the amplifier, do you have any power supply that you want to use or have available? That is the basic thing in designing a high-power amp. As marshallf3 said, tweeters need 5-10W which can be provided by a very simple single-chip amp.
    The problem is going to be the subwoofer. Are you absolutely sure that it can withstand 200W RMS, or is it just some bogus marketing number? This is because you don´t want to smoke it as well as you don´t want to build needlesly powerful amp, especially being a begineer in electronics.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    I "like" the schematic of the amplifier:
    1) The input transistor is not needed and is biased wrong so it is almost saturated all the time anyway.
    2) The opamps are biased wrong and won't work with their (-) pin connected to ground without an additional negative power supply.
    3) The opamps would have a voltage gain of about 200,000 without any negative feedback.
    4) The Mostets are upside-down.
    5) The Mosfets conduct DC through the speakers all the time but speakers must be fed AC, not DC.
    6) If a normal single-ended amplifier is used with a 12V supply and an 8 ohm speaker then the power would be only 2.2W.

    A little TDA2822M bridged amplifier will melt if a supply higher than 6V is used. With a 12V supply its output into 8 ohms will try to be about 4W and its heating will also try to be about 4W. But its max allowed dissipation is only a little over 1W because it is a little guy and is not designed to use a heatsink.

    There are (or were) at least 120 car radio amplifier ICs available. Most are bridged and produce 14W per channel just before clipping into a 4 ohm speaker with a 13.2V supply. Because they are bridged then they don't need a huge output capacitor to feed AC to a speaker.
     
  14. Deric

    Thread Starter New Member

    Feb 8, 2010
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    I am sure my phone will output plenty for tweeters and subs. you have all seen ipods play off ihomes and other systems, just an audio in. my droid incredible plays music way better than any iphone or ipod.
     
  15. Deric

    Thread Starter New Member

    Feb 8, 2010
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    also, i have a power supply that plugs into 120v and outputs 12v @ 20a
     
  16. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    PSU rated at 20A is nothing and will not handle car power amps without shutting down.
    To prevent tht u will need a car battery connected so it can handle good bass, or a super cap with it too will be quite incredible.

    First tell me what kind of speakers u have. The droid can play Audio with a pretty descent bandwidth.

    What u are tryin to built is pretty simple, I wonder why you haven't made any progress.

    Not enuf replies for u ?
     
  17. Deric

    Thread Starter New Member

    Feb 8, 2010
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    i havent made proges cause i dont know where to go. i thought 20a would be enough since i only plan to run 10a through the sub, 2a through each of the three tweeters, and 4 through a mid when i find it. if its plugged into the wall i dont see what a car battery would be needed for. i just want to play music in my house with my phone. basically it will be the same as the ihome, but more incredible.
    seems like everyone has conflicting advice so its hard to tell which direction to go i guess
     
  18. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    OK.. let's do this properly.

    First can u give me the specs of ur sub and tweeters.

    I will get u started. No problem
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    An adult male westerner has a voice that produces frequencies as low as 80Hz. If you cutoff at 300Hz then his voice sounds "tinny".
    The very important silibant sounds in western speech reaches up to 14kHz. Most are well above the 3kHz cutoff frequency of telephones.
     
  20. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Vocals are mostly mid-range. A few can go outside the regular freq
     
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