Rrrrolla

iTrader: (3)

Caps are filters, that's what they do. It's just semantics, no sense argueing over it. In our application, they do "filter" the current drain on the alternator. Or "stiffen" the power suply, whatever you wanna call it.

GPGT1

Ok in a DC power supply, we are refering to DC here we're talking about amps. Your AC 60 hertz goes through whats called a bridge rectifier, 4 diodes set up to convert your AC to DC but its not a constant DC source its ripply. so you put caps and inductors in a circuit to hold the voltage. this is your rail voltage (you dont need this part in car audio). In amps they add even more capacitance because of the varied draw created by converting the DC back to AC. thats why your 500watt home amp only has a 300watt power supply and that what we are trying to accomplish when adding caps in a car environment, it has nothing to do with filtering.

Dukk

I whittled your response down to this one question.

Me personally - I use a cap to help combat headlight dimming. After applying the cap, the headlights dimmed less. done.

Not everything requires a formula..

theboy

Are they not dimming and recieving half a volt less??????????

Tom.F.1

iTrader: (1)

Zoomer,
You're right, on a quick search, i can't find any real tests about how caps help in cap audio. But i did find out that stiffening caps were used in power supplies long before anyone put one in a car. They were used to stablize voltage, escepially supplies used for multiple and varied loads. There are lots of sites explaining capacitor therory.
So, i went looking for 'expert' opinions. Here is an excerpt from an article in caraudiomag about beefing up your power supply.
http://www.caraudiomag.com/specialfeatures/0207cae_audio_system_power/index.html

And here is the excerpt on the subject of caps:
"Feed the Need
A capacitor is a good solution for the quick power response needed for bass-heavy sound systems. A capacitor, sometimes called a stiffening capacitor, is a device that stores electrical voltage and releases it as needed. This is extremely helpful when your electrical system is near its capacity and the music you are playing suddenly hits a thunderous, long bass note. A capacitor would release the needed power to meet the demand without stressing the system and then would recharge itself immediately, ready for the next big note. There also are batteries designed to function like capacitors. The Xstatic Battcap is an example. They contain a larger charge (measured in farads) than do most capacitors.

If your charging system is significantly short on current and you need more than a capacitor will easily handle, then the best option is to add a bigger alternator. Alternators are the fastest power source, meaning they can respond to power needs more quickly than the battery or even a capacitor. Of course, adding an alternator is usually a more expensive solution to your power needs; but it also may be the best solution. Again, though, if you decide to install a bigger alternator, then the question is, how big?'

hershey

The charging system needs to be part of the overall plan of the audio system. If you look to spend $10,000- then part of that should include charging( alternator, batteries, etc....). Some audio stores will sell you anything and everything without any thought to the charging system.
cheers

scroudt111

thats a great write up, explains alot with little writing

zoomer

iTrader: (3)

Um, please read up on your basic AC circuit theory. A cap and a resistance or inductance is a filter. A cap is an open circuit at DC. So if your car power system would be only DC then the cap would not do anything. But a car power system has noise on it generated by the alternator and all the other stuff, plus the amplifyer while it is playing music is also an AC draw on the battery/Alternator system.
We can look at this battery, alternator, wiring, cap, power amp as a complete system and analyse it using normal AC and DC circuit analysis techniques or simulations. I have done that and will show some results in the next days. I am in a hotel in Ottawa right now and gotta go to work. I will try do so something this weekend.

Basically what I have found is that the capacity of a capacitor to supply current during bass tones is too small to be relevant. I does even out the pulses a very very tiny bit. To be really usefull it would have to be used with a cable that has a reasonable amount of resistance, so that it can operate as a proper low pass filter, to filter out the voltage variations caused by the amp while it plays bass tones. But using thicker wire then adds extra loss. Bottom line, despite all the good hopes and dreams, well, you heard me say it before...

GPGT1

I know that a cap filters in AC but we're dealing with DC

What? This is 12 volts DC like i said there is no filtering going on you should read up on your electrical theory, and thicker wire would have much less loss at our power levels smaller wire could never carry that load.

well heres some crude math, now not taking into effect what the altenator is putting out or anything. lets say you were running a 60hz tone at and the amp is drawing 1000watts. for half a wave it takes 0.0083 seconds. at that power it will take a 1 farad charged to 14.4volts cap 0.2seconds to discharge on its own with no power supply(alt or batt.). After your half wave was done playing you would be at 13.81volts, not too bad. Now if the batt and alt were connected, that cap would have a chance to recharge between peak cycles. Now with caps when they are empty they take longer to discharge then to charge and when they are full more time to charge then to discharge. giving that 1 farad cap the same time to charge as we did to discharge it would be at 14.37 volts from 13.81. so this is where having an good charging system comes in. To give that cap slightly more time to charge, the alt would need to be able to handle a sufficiant load.

I hope you dont work with electricity

Haunz

iTrader: (3)

I agree with dukk.....

voltage drop in a vehicle is primarilly caused when we exeed the current draw that the alternator can provide which is related to its RPMs and the IR losses in the windings... from there we should expect that our batt can provide gobs of current, however batteries also suffer from IR loss as well as voltage drop due to concentration polarization (and a few other factors)....

and of course we have more IR losses in the wire....

A cap should help stiffen the voltage at the amps, however they too have an internal resistance and suffer losses, as well as inductance... the size, ESL, and ESR of a cap dictates the amount of time it takes for the cap to charge/discharge and the amount of current it can provide. (ESL is usually somewhat related to the ESR so that spec is generally good enough to look at, and is usually the only one provided by the manufacturer)

I've seen graphs of amplifier output vs time with and without certain caps and it was clear that a cap with an esr of under 1-1.5 milliohms can generally help the situation... once you get over that figure the cap may hinder as it can't discharge fast enough to help on the very short transients and once it has started to discharge it will later draw current along with all of it's losses...

Either way even with a bank of 'good' caps I doubt there will be an audible difference and further more for the cost of them you are better off buying a more powerfull amplifier with a greater amount of headroom.... further, highend spiral cell batteries exist with ESRs down to 3milliohms which can provide much much much more energy then a small 1-3F cap at the same or less cost of a 1-3F cap.... and a pair in parrallel should rival the perforamnce of any cap on the market today...

If you believe in RF, they can show you a test report showing thier 100F carbon caps can significantly improve amplifier performance.... of course we all know that the test was probably flawed and that RF's marketing department loves to spew fluff...