Powering a DIY subwoofer amp in a car

Discussion in 'Automotive Electronics' started by programmer6502, Aug 18, 2015.

  1. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Hi,

    I'm looking to add a subwoofer in my car but am not satisfied with a lot of the cheap garbage on the market when it comes to amps. Hence, I've decided to build my own 100-150 watt HIFI amp but have some questions on how to properly power it. Obviously, 12v from the car battery alone won't do the job since I need about 40v +/- to power the amp. So my thinking is (and tell me if I'm out of line) is to use a DC/DC booster such as these for example, but of course rated for what I need in amperage and voltage (and would be fused for safety). The other requirement for the amp to function as well obviously is dual DC power rails, and as far as I know, common DC boosters don't offer that. This is the main source of my roadblocks.

    The amp wasn't original intended to be in a car and this is the suggested power supply:

    [​IMG]

    Here you can see that after the voltage is rectified into DC, the two rail action happens (for lack of better word).

    So what's the best way to get the power I need considering it would be in a car? Wouldn't some dual rail circuitry after the DC booster cut the voltage considerably? Like I said, I could be totally out of line! Any help would be great.

    Thanks
     
  2. Ramussons

    Active Member

    May 3, 2013
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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    That's the high frequency way. Another approach is a low frequency inverter such as a square wave or modified sine wave inverter with a 60 V center tapped output transformer. Much less complex circuit, but the tricky part, as always, is the transformer.

    If you want to buy rather than build, what you need is an isolated boost converter or isolated flyback converter for the negative rail.

    BUT, there is nothing magical about a dual-supply amplifier. With a large enough output coupling capacitor, a single-rail audio amp should meet your quality needs and solve most of the power supply issues. A 20 amp input inverter is not trivial, but at least you can use off-the-shelf magnetics.

    ak
     
    Last edited: Aug 18, 2015
    programmer6502 likes this.
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    So don't buy cheap garbage.. But a well know quality amp.. There are plenty out there..
    No need to cobble your own..
    The time/money you spend attempting your own could pay for a quality amp a few times over..
     
  5. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Possibly, and I'll make sure that doesn't happen by doing the proper homework before ordering stuff. But high quality amps usually come with a price and I'm only looking at $10 to get the components I don't have already have amp wise. There's also nothing like making your own personalized equipment as long as it's safe ;)
     
  6. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Helpful information! I'll do some more researching today. And I would rather buy than build whatever power source I have. The biggest thing I want is for it to be clean and somewhat efficient. That's why I haven't bought a full blown inverter yet even though they're cheap. Using an inverter followed by the original power supply (built for 110v and instead of 230v of course) doesn't sound very efficient.... But in an emergency situation I could power a household device in a pinch ;)
     
  7. AnalogKid

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    Loved the 6502, the first GREAT uP.

    ak
     
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  8. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    I'm starting to think that a full blown 110VAC inverter (probably a modified sine wave type) wouldn't be that overly bad. Reason being, I'm seeing that even small commercial car amps have 15 amp fuses!
     
  9. AnalogKid

    Distinguished Member

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    An inverter won't cause the system to draw less power; it's just a different kind of power conversion stage. Granted it makes the amp design easier, but now you have two power supplies in series followed by the audio amp with its own inefficiencies. Figure at least 200 W out of the car for 100 W out of the speakers - per channel.

    ak
     
  10. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    It'll only be one channel, but I still don't like the idea of that....

    I'm not having the best luck finding a suitable isolated boost or flyback converter though. But I'll keep digging, as I'd much rather take that route.
     
  11. AnalogKid

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    I understand what you want, and why. But the most efficient route is a non-isolated boost and a single-rail amp.

    ak
     
  12. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Alright, I gotcha. So no class AB amps or anything like that? I need to study up again, I'm pretty unfamiliar with single rail amps!

    Hey thanks for taking the time to help me out, I really appreciate it!
     
  13. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Just use two converter units to get the +-40 VDC rails. One does a forward boost to get the normal +40 DC rail and one is set up to do negative boost to generate the negative 40 VDC rail.
     
  14. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    I was wondering about that. The only concern I would have is the two voltages going out of tolerance from one another overtime for various reasons. But as long as they're close to 40v, I wouldn't think it would prove to be a problem.
     
  15. RichardO

    Well-Known Member

    May 4, 2013
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  16. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    I'm just thinking the thing over and have another question: Can a regular DC/DC booster be used for negative voltage if you connect the + and - output leads backwards? Or will it need some modifications? Just don't want to blow anything up
     
  17. AnalogKid

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    Not what I meant. A traditional class AB audio amp can be designed to use only one power rail and ground. Except for a large output coupling capacitor, it does not have to be significantly different from a class AB amp with +/- supplies.

    ak
     
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  18. AnalogKid

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    Yes, as long as you are not combining it with other supplies. A standard non-isolated boost converter circuit has the input ground and output ground connected together. That's part of what the non-isolated tag means. So if you try to combine one with any other non-isolated converter, you have to be very careful about where the grounds are or you'll dead short something.

    ak
     
  19. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    That makes a lot of sense. I think it would be best for me to look into adapting it to a single rail design, just for simplicity.
     
  20. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    I can't believe how much I learn by asking question(s) on this forum! Always a heck of a lot more than I bargained for that's for sure! :confused::D

    So, AnalogKid, I've been looking into that "large output coupling capacitor" that you've been mentioning to allow my amp to operate on a single supply rail. I dusted off some old books on analog circuits and one does indeed verify this, but doesn't give any additional information. Hence, more questions: ;)

    1) Is the output coupling capacitor all that's needed to accomplish this? (My book only states the need for a coupling cap to be placed between the output and the load)

    2) How do I calculate the size of the capacitor needed?

    I'm all in for doing my own homework, but google is filled with nothing but Op-Amps! I would like to know exactly how that coupling cap works in traditional amps.

    Thanks again


    Edit: By re-reading post #17, it looks to me like you may be suggesting that some slight modifications besides the coupling cap are needed.
     
    Last edited: Aug 21, 2015
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