Problems With Car Amp

Discussion in 'General Electronics Chat' started by Shagon, Jan 6, 2011.

  1. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    Hey guys I have connected a computer power supply to a car amp which I have been using for about a month... It has been functioning great except for this incredible hum which I assumed that it was because of the computer power supply since its a switching power supply and all.

    Now today I have noticed this issue with it... it would work normally on normal volume but when I increase the volume it would play like an old record. It keeps sticking.. I assume that the signal is clipping.

    the power supply is 450w 25A with +12V
    I don't have any idea on the power rating of the amplifier except that above the fuses it says 20ampx2 which I would assume is about 480w? but I don't use the amplifier to its maximum volume capacity.

    Do you guys have any idea of what might be causing this clipping? I am not sure if its clipping but it plays like a faulty record when I increase the volume. and plays normal on lower volume.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    The PC supply is shutting down due to loading during bass reproduction.
    You can improve this by using a battery in parallel with it, but for this there are precautions.
    Another option is to use a super cap that car installation uses, but to use this in parallel with a supply is tricky as you need to slowly charge the cap, as the supply cannot instantly charge high value caps. U will need a surge limiter circuit in between the cap and supply to prevent supply shutting down at power up.
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Car amps are incredibly inefficient in most cases, darn thing might want over 100A of current at peaks.

    It likely only puts out about 100W/ch RMS on a good day and those supercaps are of little help, only good wiring and a battery will solve the problem unless you just break down and buy a decent amp off of eBay or find one being given away on your local craigslist or freecycle websites.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Shooooooot! I forgot.

    You should use thick wires to power the car amp.
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Mine's all lined up if I ever get to it but there will be a big fuse at the source and a small spare battery in the trunk. 168W RMS x 4 will impose an incredible demand should I ever crank it up, not to mention if I add the Class D sub amp I've got which is rated at 500W RMS but I really don't think that I need it considering the speakers I've already put in.

    You can get bits and pieces of this wire from any car audio shop, you want the fine stranded stuff.
     
  6. tom66

    Senior Member

    May 9, 2009
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    What is the rating on the 12V rail? It's likely it's only 80% of the total rating. Running a power supply near its limits is not a good thing. Also, I'd wager that the rating is probably a peak limit. And a car amp works on 11.8V - 14V. Most computer power supplies are nominally 12V, but they can vary anywhere from 11.4V to 12.6V, especially under heavy loads, and that's an average, they'd probably drop lower on sudden load increases.
     
  7. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    My friend have been using this set up for decades with the same speakers.... with the exception that he was using an older PC power supply which wire is a bit thicker than the one I got. And that he preamped the signal before sending it to the car amp. I don't have such Luxury so I sent the signal straight in from the computer to the car amp and use the volume control from computer to adjust volume.

    Would adding another power supply in parallel or maybe adding another 12v wire from the existing power supply solve the issue?

    or what if I preamp it like my friend did?

    Are all the +12v wire within the power supply connected in parallel? or are they coming from a different sources like I mean different voltage regulators?
     
  8. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    Hmmm It actually does drop bellow 11.8v when I increase the volume.... it spikes from 11.8 all the way down to 11.4 but when normal its at 11.8 to 12.01 so I am assuming that the problem... Thanks alot...
     
  9. tom66

    Senior Member

    May 9, 2009
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    You can try power supplies in parallel. Some will work; others will not. Your best bet is to buy a cheap 600W or so power supply on eBay, at least then it should be able to handle the bass.

    Usually all 12V lines are tied in parallel. They might be individually monitored for overload, but that's about it. More expensive supplies may use separate transformers.
     
  10. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    Could you explain to me why it would function properly for about a month and then all of a sudden there is this issue?

    Did it deteriorate because it might have been under to much heavy load all this time?

    I think at one point in time my cat was playing around with the cord and then the PSU went off... I just removed the power cord and placed it back and it worked again... Was that an overload protection?
     
  11. tom66

    Senior Member

    May 9, 2009
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    Ah, I missed that it was working for a month. Clear case of electrolytic capacitor failure, very common in cheap power supplies. The capacitors begin to leak their electrolyte after heating up - caused by heavy load, and not helped by the already high internal case temperature. This causes their capacitance to fall and eventually they lose their filtering capabilities, causing drops in voltage due to heavy load. Which would be another reason to always choose a supply with better ratings. You can try to fix it but it will only be a stopgap solution.
     
  12. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Alternatively, the amplifier, or the speaker(s) may have deteriorated, so that excessive current is drawn. Sometimes loudspeaker voice coils can touch the pole pieces on loud peaks, causing more or less of a short-circuit.

    This is more likely to have happened if you have been playing it very loud, especially if the loudspeakers are not too generously rated for the amplifier output power.
     
  13. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    Hey guys These are my findings

    I am about 90% sure now that its a power supply issue.

    I repaired a power supply that I previously owned nothing big just changed the fuse. This have been the second time that I have "OOPS" sent 240V into it when the switch is on 110V anyways I tried that power supply first that power supply is a lower rating power supply but out of curiosity I wanted to find out what would happen. The same thing happen with this power supply with the exception that I would have to lower the volume even lower for this power supply for the system to normalize.

    This power supply however with load drops down to 10.25volts so that explains everything.

    I tried those two power supply in parallel... Still the same thing.

    One thing I have noticed however that with only one channel it would function fine at a certain volume level but when I plug in the second channel on that same volume level I would get the same problem. I would assume that with the second channel in operation that the amp is drawing out more power from the power supply causing it to clip.
     
  14. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    Ah now this would explain the sudden increase in humming which I noticed this afternoon... whats the best way to test for leakage in a cap?
     
  15. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    /sounds like you have already done the tests.

    Humming sound and large voltage drops under load. Bad filter caps.

    Get a 12 volt battery charger from a neighbor or friend,(one with plenty of amps) The radio will be much happier with the increased voltage and current output. And this will prove whether the amp is shorting at high volume or the supply is sagging under surge current demands.
     
  16. tom66

    Senior Member

    May 9, 2009
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    Turn off the power supply for a few minutes, open it up, and look for physical leakage - the top of the cap may begin to bulge, and electrolyte may form around the base of the cap.
     
    Last edited: Jan 7, 2011
  17. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    Even without these things present could it still be possible that the caps are faulty? cause the caps looks normal to me...
     
  18. tom66

    Senior Member

    May 9, 2009
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    Take a picture, it's usually visually obvious. Best bet is to assume the caps are faulty, especially due to the low output voltage you measured.

    BTW, we're looking at the output caps - the ones away from the mains input. These are the most likely to fail. The one or two big input capacitors are usually okay.

    Even so 12V is actually quite low for a car amp - they expect in the range of 12.5V - 13.8V while the vehicle is running, so you are running it near the limits at 12V anyway. You can buy a dedicated 13.8V supply for powering marine equipment, but it will also work well for your amp.
     
  19. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    What Bugles Me is that... My friend had this set up for years with an even lower rating power supply and it worked fine....

    well the point of the matter is... if I gotta purchase anything its best I just buy a proper amplifier... I am using this because thats what I got at my disposal...
     
  20. tom66

    Senior Member

    May 9, 2009
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    Luck of the draw - maybe his power supply was a low rated power supply but from a name brand, and thus they hadn't cheaped out on the capacitors? Or maybe the rating was more realistic before manufacturers pretty much started making them up? Or maybe he never cranked the volume to the maximum or maybe he didn't use it as much? Or maybe his amplifier was more generously specified on its output power rating?

    It might surprise you but capacitor failure probably accounts for 50-60% of computer and electronics failures I get asked to fix.
     
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