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Posted: Sat May 15, 2004 4:58 pm
I've been heavily into car audio the past few years. <p>One question that is often posed is whether or not capacitors are useful in providing a stable input voltage to the amp. Many believe that a capacitor will eliminate or greatly reduce the voltage drops that occur when the audio amplifier draws more current than the alternator can put out. Which, if true, would result in a higher output power in amps with an unregulated power supply.<p>I've been determined to find a set answer the past few days. I've never noticed a difference with or without the capacitor inline with the amp, but me voicing my opinion with no proof does not convince anybody.<p>I found this test where two tests were performed. One test was without a capacitor and the other test was with a 15 farad capacitor installed inline. Keep in mind that the supposed rule of thumb is 1 farad per 1000 watts. This was a 2-3k watt system using over 5 times the recommended capacitance. <p>The link...<p>Richard Clark's Test
<p>As you can see, the test with the capacitor had a lower overall voltage and was slower to return to the idle voltage.<p>My question is...<p>1) What are your opinions on this?<p>2) Why do the much smaller filter capacitors in an amplifier seem to do their job? Are the dips in voltage much more minimal? <p>I've heard the following from a someone, but he wasn't sure if it was totally correct...<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>A cap's value is determined by it's capacity to store x number of coulombs per
volt. For a 1 farad
capacitor, you have exactly one coulomb per volt. If you have one coulomb of
through a point in a circuit in one second, you have one amp of current. If you
capacitor's voltage down from 12 volts to 11 volts in one second, you had a
current flow of 1 amp.
If you draw it down in 1/10 of a second, you had 10 amps of current flow. If
it's done in 1/100 of
a second, you had 100 amps of current flow. Many people try to suggest that a
capacitor stores a
certain amount of energy and that the total stored energy can be used by the
system. This isn't
the case because to use the total stored energy, you'd have to draw the voltage
down to zero
volts. <hr></blockquote><p>Is this the way it works? I just want to make sure.<p>Thanks for reading. Didn't realize I made it so long until I finished. :p
Posted: Sat May 15, 2004 7:06 pm
well, many home amplifiers use large power supply caps, for preventing AC ripple and also since to be able to deliver enough power to drive an amp at full power without caps to store power so it has enough power for each boom. to deliver full power without the caps would take a huge transformer.<p>cars have many amps of power, so it isnt as big of an issue. plus,i am sure that they have them built in.
Posted: Sat May 15, 2004 8:14 pm
The main problem with the car amps is that they're usually located in the trunk with the battery in the engine compartment. Since the high-power car amps draw current in the hundreds of amps when putting out their rated power, you can get a voltage drop on those power leads back to the amp unless they're made of copper welding cable. For normal operation, it isn't problem. But if there's a high-power transient sound, e.g., a bass drum punch, then the amp may clip and distort since the voltage will drop with the higher transient current demand.<p>One fix is to install a large capacitor near the amp and it will store a short term bucket of energy that'll supply the amp for that transient. The cap is right there at the amp so there's virtually no voltage drop for that quick surge. The cap recharges during the longer duration between the bass transients.<p>Another fix that should work just as well is to treat your car like an RV and install a second 12v battery right there by the amp, PROVIDING THAT THE AMP IS IN A SPACE VENTILATED TO THE OUTSIDE SO THAT YOU WON'T GET ANY HYDROGEN BUILD-UP THAT COULD EXPLODE. The battery, like the cap, is in parallel with the 12v feeder line and will charge right along with the car battery, but will be able to supply high current pulses without the voltage drop of the long wire. Overall, the cap is probably a better deal.<p>Note that if your favorite form of music is classical pipe organ, that cap won't do you a bit of good. In fact, most car amps will stumble and crash on the long, sustained notes of a good 32-foot pipe organ recording. If an amp is going to clip, it's going to clip on that. But if you're interested in reproducing the transient bumps and thumps of those disgusting bass tapes/CDs or that nasty rap stuff, then the cap will work best. Safety-wise, the cap is a better way to go over the second battery. There's just too much of a temptation to install that second battery inside a closed-in compartment of the car, creating a very dangerous situation.<p>Dean
Posted: Sun May 16, 2004 12:39 pm
The theory in your quote looks ok to me.
Posted: Sun May 16, 2004 8:49 pm
Was that a "15 farad capacitor", or 15 uF (micro Farad capacitor?<p>I had a power supply with 150000uF (0.15 Farad) capacitor in it for the 5V supply. That cap was 3 inches in diameter and 8 inches tall. A 15 Farad cap would be 100 times the volume! It would fill the trunk of a car.<p>There are super-caps. 1 Farad at 2.5V max is 10mm diameter by 22mm tall. They can be ganged in series to get the voltage up, and parallel to increase capacitance. They are meant to provide power to small circuits while batteries are replaced. Their internal impedence is too high to be used for power supply filtering.<p>At 2-3KW, if the cap was 15uF, that is much too small to make any difference. Hey, wait a minute! ...<p>To drive 3KW into 1 Ohm (four 4 Ohm speaker in parallel) requires, at least, a positive 55V and negative 55V supply. (Or a humungous audio output transformer, unlikely.) Putting a capacitor between the battery and the amplifier power supply (that increases the voltage) would have no effect
! This is because the power supply must be a switcher. It has to chop up the 12V anyway, then filter the higher output voltages. Noise on the 12V line is irrelevent.<p>As Dean says, dips in the 12V caused by transients and a long power line to the trunk may be a concern. In a good design, this would have been thought of, and compensated for. An el-cheapo copy-cat amp may have this problem, and a capacitor would help. Unless you've got Dean's ears and listen to organ music; a drum thump is just a thump, you won't hear the difference.<p>-----------------------------------------------
summing up:<p>I find a 15 Farad capacitor hard to believe, I think the decimal point is in the wrong place. If it really exists, where can it be purchased, I might need one someday?<p>Do not play a 3KW amp wide open when you drive down my street. I'll call the cops
<p>C U L -
Posted: Sun May 16, 2004 11:56 pm
Dean : <p>I have dual runs of 0 gauge to my amps. They draw around 300 amps at full tilt. <p>Recombinant Gas batteries can be used inside a vehicle, I believe.<p>Not sure about the caps ability to stay charged even for music with quick beats. Based on the test, it drops in sync with the system with no cap. It seems like if it was supplying power for even a few 1/10's of a second, there would be a slight delay.<p>
Dale :<p>Nah...there's quite a few that have even more capacitance.<p>Alumapro is just one that has caps up to 50 farads...<p>Alumapro 15 farad Cap
<p>Just 9"x9"x1.6" for the 15 farad cap.<p>Don't worry, I have 3kw's, but I'm not into going down the street waking up people.
<p>Those are the people that give a bad name to everyone interested in car audio.
Posted: Mon May 17, 2004 4:55 am
do you mean this??<p>http://www.alumapro.com/cap.html
<p>the site you sent us to is about their subs.
Posted: Mon May 17, 2004 4:56 am
do you mean this??<p>http://www.alumapro.com/cap.html
<p>the site you sent us to is about their subs.
Posted: Mon May 17, 2004 6:23 am
That's a C.A.P. not a capacitor. An electronic device. (C.A.P. propably stands for Capacitance Amplified Power) And, it appears Richard Clark didn't connect it properly. He said "no relays or fuses", which probably means the black wire in Figure 1 at www.alumapro.com/cap15dia.html
was not connected
. So the test was performed with the C.A.P. turned off
! No wonder the yellow and red lines on the graph look so similar. Also "added
6 inches from the amp with 4 gauge wire" sounds like he simply connected two C.A.P. output leads in parallel with battery leads going to the amplifier. The C.A.P. is supposed to be in series
with both the +12V and ground connections.<p>That was not
a fair test. <p>I was kid'n about the street, I'm sure you wouldn't "blast" me
Posted: Mon May 17, 2004 10:39 am
Posted: Mon May 17, 2004 3:42 pm
That ebay 100F cap has an ESR of 3.6 ohms? With an ESR like that, what's the point? It could never, ever source the current that you're wanting for these transients. 3.6 milliohms would be more reasonable, providing an internal drop of about a volt at 300 amps.<p>Vincent, the ability of the cap to charge and discharge will depend upon the duty cycle of the "beat". You can bet that the cap's nearly worthless when running one of those "bass tapes" or "bass CDs" that the car audio kids like since they use synthesized percussion with "transients" that extend out far more than a legitimate transient.<p>I'm not a car audio guy at all and don't condone even the amps, let alone the extra foo-foo. I'm usually quite contented with the factory stereo. As a matter of fact, when a kid pulls up within a few cars of me and his thumping drowns out even loud conversation in the car with the windows rolled up, my wishes for the life expectancy of the kid's electronics are not very nice.<p>Dean<p>
Posted: Mon May 17, 2004 11:21 pm
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Dale Y:
That's a C.A.P. not a capacitor. An electronic device. (C.A.P. propably stands for Capacitance Amplified Power) And, it appears Richard Clark didn't connect it properly. He said "no relays or fuses", which probably means the black wire in Figure 1 at www.alumapro.com/cap15dia.html was not connected. So the test was performed with the C.A.P. turned off! No wonder the yellow and red lines on the graph look so similar. Also "added
6 inches from the amp with 4 gauge wire" sounds like he simply connected two C.A.P. output leads in parallel with battery leads going to the amplifier. The C.A.P. is supposed to be in series with both the +12V and ground connections.<p>That was not a fair test. <p>I was kid'n about the street, I'm sure you wouldn't "blast" me
I don't understand Dale.<p>The C.A.P. is just a unique name for that capacitor as far as I know. There's even 100 farad ones called capacitors. <p>Capacitor Link
<p>Is there a difference between them? The only thing I don't understand is that most caps are wired in series, but these are wired in parallel. <p>
The small relay is only there because the amp and the capacitor will draw more current from the remote turn-on lead of the radio than it can supply. In the test, he probably just ran a jumper from one of the nearby power wires. The only reason you would connect it to the radio with a relay is for everyday use so it turns off when the radio is turned off.<p>The larger relay looks as if the only purpose is to discharge the capacitor after use which is why the 5 ohm resistor is there.<p>With the experience he has, I'm sure Richard Clark knows how to hook up a capacitor. Truthfully, I don't even know if this is the 15 farad cap he used. <p>On the Ebay auction, its obviously wrong. I wouldn't trust the specs put by ebay sellers at all. Here's the actual ESR...<p>ESR = .0018 ohms
<p>And I didn't mean to get all the old people riled up over the misuse of amplified music.
Like I said, a few idiots give everyone a bad name. It makes me just as angry as you when a ignorant kid repeatedly drives through a parking lot trying to set off alarms, etc.
Posted: Tue May 18, 2004 3:12 pm
I have limited experience with amplifiers, but I'm very interested in the subject, and am currently working on a modest 250W model whose schematic was posted here some time ago. <p>Anyway, in the test link you posted the only reason I can think of for the similarity between the tests is the rating of the amp. My 250W unit will have 8x10,000uF caps, 4 each in parallel for the positive side and negative side with respect to ground. That's the power supply - BEFORE the amplifier. These caps exist both to minimize the AC ripple and to act as reservoirs. I must believe that a professionally designed amplifier of the size you mentioned would have enough capacitance included to take care of the bumps and thumps they tested with. <p>Just noticed this: it is interesting to note that Mr. Clark doesn't seem to have the concept of a legitimate test cemented into his mind just yet. Check this out (quote from that site):<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>For our first test we played the system with the engine off and no cap. The result
was the purple trace at the bottom. We played the system as loud as we could get it
that seemed to produce no audible distortion. <hr></blockquote><p>Notice that? NO AUDIBLE DISTORTION. By definition this means that the amp itself is able to handle whatever you asked it to do. For the next test with the cap installed and engine on they specify that the volume control is left precicely where it was before. Of course you aren't going to notice much difference in sound - you didn't push it hard enough in your baseline test! Wanna try this at home? Push the amp until it clips (watch out you don't blow your speakers) and get a good feel for what it sounds like. then connect the cap and try it again at THAT volume. See if you don't notice a change in sound.<p>Hope this helps!
Posted: Tue May 18, 2004 6:19 pm
i should get into the car audio business:<p>"today, we will be seeing how loud your amps can go without blowing a speaker"<p>thats about as good of a job as the person whose job is to test electronics for safety by blowing them up!!
Posted: Tue May 18, 2004 8:42 pm
Glenn,<p>THD wasn't really what they were testing for.<p>The cap was put inline while everything else was left the same. The voltage dipped to the same level with the capacitor while the input signal was left the same.<p>So, if the capacitor does nothing to prevent a voltage drop, the amp is going to clip at the same time as it would without the capacitor.<p>Your 250 watt amp is runs off of AC voltage doesn't it? Therefore the capacitor would not have to provide near as much current to prevent ripples. On the car amps due to the limited supply voltage of 12 volts, the 2800rms system in the test draws 300+ amperes after amp efficiency is figured in.<p>Feel free to correct me, though. I'm not sure how home amps work exactly.