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Designing a complete active speaker box

Discussion in 'Audio' started by DiegoTavares, Jul 3, 2018.

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  1. DiegoTavares

    DiegoTavares

    3
    0
    Jul 3, 2018
    Hello guys, rookie electronic technic here :) So, i'm attempting to making a active speaker box to challenge myself.
    My idea is to make a miniature of a automotive speaker box with a good quality and loud sound (and reliable of course) for my PC or whatever.
    It's a little incomplete yet, it's missing the amplifiers (still picking up the best ones) and a final power supply circuit. The crossovers are finished (i hope).

    The specs until now are:
    • 12V/15A Power supply
    • 4 way 2-order active crossover with TL074
    • Mid-High and High amp: 1xTDA2822 (2x [email protected]Ω)
    • Mid-Low amp: 1xTDA7294 (1x [email protected]Ω)
    • Low amps: 1xTDA2030 (1x 10W @8Ω) and 1x TDA7240 (1x 16W @6Ω)
    • Some cellphones speakers to mid-high and high drivers (Moto E398, Samsung Galaxy Y and Moto XT925 for curiosity)
    • 3x C3576 3" [email protected] Sony speakers for mid-low drivers
    • Some 3"/5W and 4"/15W speakers for low drivers
    So, finally, my doubts are:
    According to ohms law, i need 12,5A at 12V to reach 150W. It's pretty easy to get this in a positive power supply, but it's needed a negative too? A 7812 and a 7912 with current boosters will make the work?
    I figure out that a 24V-15A transformer is too expansive :( so i'm thinking in a 12v-15A switching power supply from some online shop...but if i need a negative supply, i can't think in a alternative for now...
    I (hope that) have no problems with the crossovers and the amps, my doubts are about making the power supply. But i accept suggestions :)
     

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    Last edited: Jul 3, 2018
  2. Audioguru

    Audioguru

    2,742
    619
    Sep 24, 2016
    Your power supply is not plus and minus 12V, it will short circuit itself. Why do the filter opamps need 24V anyway? Use the 12V to feed a 7809 and use the +9V to power the filter opamps. Simply bias the (+) input of each opamp at a filtered 4.5V with a voltage divider. Capacitor couple the input and output.

    Why is the low frequency power amp 7.5 times the power of the high frequency amp? The speakers will have almost the same efficiency.
    With a 24V supply, a TDA7294 can produce 15W into 4 ohms, per channel. With a 60V supply it can produce 62W into 4 ohms per channel. With a 80V supply it can produce 150W into 4 ohms with horribly high clipping distortion.
    If you bridge the two channels then the speaker impedance must not be less than 8 ohms and the power will be the same as one channel driving a 4 ohm speaker when the supply voltage is the same.

    The TDA2822, TDA2030 and TDA204 are all obsolete and are not made anymore, but you can buy fake or defective used ones on ebay. The TDA2822 would blow up if powered from 12V when it has a 4 ohm speaker.

    The resistor and capacitor values in your filters are odd, the resistor values are much too low to be driven by an opamp. I always use equal value capacitors (highpass filter) and equal value resistors (lowpass).
    Look at the difference between your 2.5kHz lowpass filter with a 680 ohm input resistance and mine that has a 12k input resistance, the responses are exactly the same.

    Each filter must be driven from the low output impedance of an opamp without your level control in between.
     

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  3. dave9

    dave9

    867
    213
    Mar 5, 2017
    Is it hard for you to source or afford parts? I ask because this seems like a bad ratio of effort to outcome, that this isn't going to be a result worth the effort if someone has the resources to do it a different way.

    Makes little sense to go to the trouble of tri-amping those poor little speakers, nor to design around a 12V PSU unless this "needs" to be powered by a lead acid battery or vehicle electrical system.

    There are 1000 different ways to build an amp, so if you happened to have all these parts lying around, meh I still wouldn't tri-amp nor use 12V since you don't have the PSU yet. Amp chips are cheap.

    I could be wrong. I would understand better if you started with a list of the project requirements, the budget, and what parts you already have.

    To cut to the chase, this is not at all how I would approach this project. It seems very arbitrary and ultimately, a terrible result with the speakers you mentioned.

    You did write "good quality" so I would hate to see you spend a lot of time and build a Frankenstein thing that isn't good quality.

    Have you sought the various audio enthusiast websites? For example if I were doing it cheap and dirty, I would build a "gainclone", powered by a laptop power supply, driving a 2 or 3 way speaker with drivers more carefully chosen, not leftover cell phone parts (???!!) and use passive crossovers. Bi or tri - amping has its merits but the rest has to be right for that to take it to the next level.
     
    BobK likes this.
  4. DiegoTavares

    DiegoTavares

    3
    0
    Jul 3, 2018
    The power supply is my biggest problem, i really don't knew how to get a plus and minus supply. That 24V is because i was making a only +-24V line before i realize that isn't going to work, but i'm going to do with biasing method as you said.

    That difference is because i used my big speaker box as reference to power (1KW subwoofer, 150W horn driver and 100W tweeter driver) almost 10:2 ratio from low to high frequencies amps. Since the low frequencies speakers are bigger and more powerful than the high frequencies speakers, i had to get more powerful amps, right? (I don't get well your phrase)

    I dont really did a hard search on the power amps...do you know a ICs series that would be good? I used TDAs because i found some of they in a electronic store on my city (yeah, the only ones stores in the city are pretty bad on resources)

    About the filters components, i used the texas instrument to design...i don't get a filter class in my technical course, so im trying to learn myself. You mean the level control between the first stage or the last one?

    Well buddy, i started the design with the speakers, i had these speakers on house and i was trying to make advantage of it. Every speaker makes a good and clear sound (even the cellphone ones), so i thought its going to work well, i don't really need a very loud and excellent quality sound, better than bought pc speakers boxes are great

    I'm starting over this project, with more decent thoughts

    Thanks for the advices guys :)
     
  5. DiegoTavares

    DiegoTavares

    3
    0
    Jul 3, 2018
    Taking the ideias of you guys, i'm going to make it right this time:
    I have these speakers, they are pretty loud and has a good sound quality for my idea of usage, so i had a little study and according to they responses frequencies, they make a good 4-way audio system (i thought so)

    High frequencies drivers (4KHz-20KHz): 2x 1W 8Ω Cellphones speakers (Samsung Galaxy Y and Moto XT925)
    Mid-high freq. drivers (400Hz-8KHz): 2x 1W 8Ω Cellphone speaker (Moto E398)
    Mid-low freq. drivers (100Hz-2KHz): 3x 40W 6Ω Sony speakers
    Low freq. drivers (10Hz-200Hz): 2x 5W 4Ω and 1x 15W 6Ω generic speakers

    I'm going to make a wood box for the speakers, and a enclosure for mid-high and high drivers.
    The power will be from any 12v source (if it support the current drain) like a battery or a wall adapter.
    The signal input will be a P2 plug from any headphone output and will be transformed to mono.
    Getting a order of project now: Speakers(already choosen?) > Power amplifiers > Crossover > Power supply.
    I'm not going to do a internal power supply because a 12V/15A regulated switching power supply is way more cheap than a transformer, that is why i wanted to work with a high voltage
    • By now i only have in hands the speakers, so it's needed to choose the amplifiers and a crossover method.
    • The power supply part will be just to regulate any 12V source to any internal circuit requirement.
    • It's worth using a active crossover? Or a passive 2-order passive crossover can make it well?
    • What family of amplifiers are good for this application?
    Low freq. drivers:
    IMG_20180704_182009778.jpg
    Mid-low freq. drivers:
    IMG_20180704_182123621.jpg
    Mid-high freq. drivers:

    IMG_20180704_182312524.jpg
    My idea to final result is more or less like this:
    657814001693824.jpg
     
    Last edited: Jul 4, 2018
  6. BobK

    BobK

    7,674
    1,684
    Jan 5, 2010
    I’m sure.

    Bob
     
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