View Full Version : What's the point of a Capacitor?

02-11-2000, 07:44 AM
When I was into car audio 10 years ago, I don't recall these caps being around.

What's the point in having them?
How do you know what size to get?
And can you tell a difference with it hooked up?


02-11-2000, 09:16 AM
Capacitors give quick bursts of energy to an amp when the bass hits. It takes a little strain off the alternator, and yes, I could definitly see the difference when I added mine. The most obvious is the lights dim a LOT less.

02-11-2000, 01:59 PM
I could also tell a huge difference, when I recently installed mine, it gives the amp enough extra juice to cleanly reproduce the deep bass notes, instead of distorting them due to not enough power. I would reccomend to anyone that doesn't have one, to get one.

02-11-2000, 02:21 PM
I have read 1 farad per 1,000 watts.

02-11-2000, 02:26 PM
Thats the idea, but I use 1 farad per 500 watts of RMS power. It seems to work better that way.

02-11-2000, 09:29 PM
basically they store power in them for when the amp really needs it. For when the bass hit's hard and your lights and things dim. they just store power.

02-11-2000, 11:24 PM
The cap will help meet the amps current demand which is max when reproducing low freqs. This helps take some of the load off the cars electrical system. 1 farad per 1000w is a good rule of thumb. Trust me you will tell the difference once the cap has been installed.

02-12-2000, 12:16 AM
do I need a cap or would I benefit? I haven't installed it yet but I am pushing my mts 300watts x 2 bridged mono to give my 2 mtx 7000 12" 8 0hm subs over 1000W total. ALso I have a 75 W x 2 runnning my 6.5" component set up front with stock rear speakers. My deck is a premier deh-p900r. I called my local volkswagon dealer and he said my alternator in my 98' jetta is 90 amp. Do I need a cap? would I beneift? by how much?


02-12-2000, 12:19 AM
do I need a cap or would I benefit? I haven't installed it yet but I am pushing my mts 300watts x 2 bridged mono to give my 2 mtx 7000 12" 8 0hm subs over 1000W total. ALso I have a 75 W x 2 runnning my 6.5" component set up front with stock rear speakers. My deck is a premier deh-p900r. I called my local volkswagon dealer and he said my alternator in my 98' jetta is 90 amp. Do I need a cap? would I beneift? by how much?


02-12-2000, 01:00 AM
Are the power ratings that you have given rms or max? What is the effiency of each one of your amps? last but not least about half of the stock alternators amperage will be used by the car while using all of the accessories(sp)? I think that it is pretty safe to say that would benefit from a cap.

02-12-2000, 01:25 AM
my dealer said my am will be bridged mono to give me 1000 watts, so I'm not sure if its rms or max or if what u r asking. each 12" (I have two) is rated at 400 watts rms and 1000 peak, so I hope this helps. I'm not sure what the effiecency is of my amps. How about a pre-amp? would this help? cost? how about a pre-amp and a cap? too much or just cost too much?

02-12-2000, 02:40 AM
O.k. we can use the numbers you have provided. If you have the specs on the amp it should list the rms rating. to produce a 1000watts of power on a battery voltage of 13.8volts your amp should draw about 100amps worth of current. If the 1000watts is max rating you will just expirence lights dimming etc. If it is an rms rating then you might want to look at a bigger alternator.

02-12-2000, 02:50 AM
Thanks for the info, guys. I called Matrix Car Audio today, talked to Zach, and ordered 3ea. JL Audio 10W3-D6's and a PG cap. I'm sure I'll be pleased with the choices I made.


02-14-2000, 07:28 AM
not only does it help on the sub amp to but also on n e other amp as well when run with a sub amp

02-14-2000, 03:33 PM
Just don't make the common assumption that it will bail out a weak electrical system.

02-14-2000, 11:23 PM
Are y'all putting these caps in parallel in series or parallel with the speakers? It seems to me that I've seen them all connected in parallel with the speakers, but when I do the calculations on this, it doesn't make sense. For frequencies around 20 Hz to 40 Hz, the capacative reactance (resistance at a certain frequency) is from .00796 ohms to .00398 ohms for a one farad capacitor, respectively. So if one were to put this cap in parallel with the speaker, it would be acting about like a short in comparison to the speaker. So the overall load on the amp would be slightly less than the lesser of the two parallel resistances, meaning, the resistance would be less than one one-hundredth of an ohm!! A cap that big is not really going to store any kind of energy when you're talking about 20 Hz and up. It's just going to let the signal pass right through... kinda like putting a wire across the two leads of your speaker, or just tying the output leads of your amp together. I really don't see how connecting it in series would help either. Can anyone WHO ACTUALLY KNOWS the electronics theory behind this explain, please?


02-14-2000, 11:33 PM
THC420, these aren't the caps for filtering the signal to the speakers. This discussion is about the caps that are used for power (wired with the power wire right before the amp). They provide a quick burst of energy when the amp needs it (like low, loud bass hits).

02-14-2000, 11:36 PM
OH! Well great. I thought the laws of physics had broken down for a minute, there. Cause I know thousands of people aren't doing that and saying it's great because they don't know what's going on, and yet, I couldn't justify the actual theory behind what would be going on if you put a 1 Farad capacitor in parallel with the speaker. http://www.caraudioforum.com/smile.gif Thanks for clearing me up on that.

02-15-2000, 12:25 AM
Thanks SMX for explaining the cap before the amp theory to THC420, So I don't have to. Yes, Believe it of not there THC there are a number of people on this forum that are wise in the ways of the electrons.

02-15-2000, 05:54 AM
I went overboard. I have two 1 Farad
caps and i only have a 70x2 amp and
a 360x1 amp. I have them both on the 360x1
amp power line.

02-15-2000, 06:17 AM
you can never have too much capacitance

02-15-2000, 07:26 AM
I second that motion. Never too much.

02-15-2000, 07:28 AM
I second that motion. Never too much.

Don't mess with me

02-16-2000, 03:05 PM
Just to clear things up, momad, it's not that I think there is no one on this system who knows electronics theory - I'm sure there are many. I just didn't feel like having a bunch of people with no clue what they're talking about interjecting their "thoughts". http://www.caraudioforum.com/smile.gif But yeah, thanks for the info, too.

02-16-2000, 08:40 PM
HOw much does a cap cost? canadian

02-16-2000, 09:29 PM
I have a cap that was draining my battery when the car was off. I don't know why but
I think the installer messed up, but they said the cap was bad. I doubt that a phoenix gold 1 farad cap would go bad in a day.
Does anybody have an idea why it is drawing
@3volts instead of 12v? This thing is killing my battery. When I hook it up, besides charging it, is there anything else that I should do? My ground is smaller than my power, could this do it?

02-16-2000, 10:20 PM
btal40: Like all electrical components, your cap could have been defective from the factory. The dielectric material could be screwed. As for the ground, your ground should go directly to the body and be no more than 18 inches long. I've also heard that it should be the same guage if not bigger than you power wire.
THC: People do wire in capacitors with their speakers. They are very low capacitance. These act as passive filters or crossovers. That's the theory behind those bass blockers Crotchfield sells.
russell: Wow. Just wow. Why would you do that? Does it really do that much?
General: If you have a high power amp, does that necessarily mean it will be drawing so much current from your electrical system that it will dim your lights (or worse)? Won't a good high current alternator fix the problem?

[This message has been edited by BigE171 (edited 02-16-2000).]

02-17-2000, 12:04 AM
Hey big e like skydogg said A cap is not a fix for a weak electrical system. If your amps are drawing so much current that your alternator can't keep up then your are 100% correct a high output alt is what you need. You have to remember that just because you have a 140 ampere alt that does not mean that when the car is at idle the alt will put out 140 amp. Normally you will need to get to a set rpm in order for the alt to put out 140 amperes. If your lights are dimming while your are at idle and you add a cap that will normally remedy that problem because the amp will have another source from which it can draw the current it needs for that brief period of time. If your lights are dimming at highway speeds then you have a much bigger problem on your hands. btal40 your ground wire should be as short as you can make it and should be AT LEAST as big as your power wire. As big e stated your cap could have been bad from the factory, but I doubt it. Charge that bad boy up, pull it out of the circuit and see if It will hold a charge. If it does get that handy dandy multimeter out and find the short. If it doesn't your cap maybe expirencing Dieletric leakage in which case it would be bad.

Team Drew
02-17-2000, 12:20 AM
I have 2 mtx 225ho running 8 bluethunder 15's. I added a 1 farad cap and then I got a 15 farad cap from alumapro and I gained 3 1/2 dbs imediatley.

02-17-2000, 06:38 AM
That is my bad there thc. I know what you mean. I am sorry that I took your reply the wrong way.

10-21-2000, 10:32 PM
bwahahahaha!!!! gained 3 1/2 dB!!! That's a freakin miracle!!! I used to actually believe this stuff... it's amazing what the truth does to these falacies.

Digital caps simply have the "volt reading" on top of them. This increases the ESR and give you even less current than you would have anyway. I think RC explained a bit about this, but I don't remember.

[This message has been edited by shootme5150 (edited 10-21-2000).]

10-22-2000, 05:55 AM
what is the diffenence between a digital cap and a plain cap? someone told me to stay away from the ditial caps he didn't say why though.so whats up?