I found an excellent resource concerning capacitors used for auto audio:

http://www.sounddomain.com/ubbthreads.php/ubb/showflat/Number/342776/page/1#Post342776

For those of you using 1, 2, or even 5 farad capacitors, this may be an eye opener. Check out the last couple of paragraphs at the end:

What happens to the terminal voltage of our cap when 300amps is flowing? Well for starters, the voltage tries to drop nearly 5 volts inside the cap before it can even get out. Not in a short time but instantly. There is no time constant in the formulas for ohms law. They are instantaneous calculations! But wait. The voltage doesn?t really drop to 9 volts because we have our battery sitting in reserve waiting at 12.8 volts.

Our cap lets our poor alternator down as the voltage plummets and when things hit 12.8 volts our battery jumps in and starts to take over. The battery with its enormous storehouse begins to provide vast amounts of current until things lighten up for our poor cap and alternator. Of course we could add another cap to halve our ESR loss to only 2.5 volts but that would still cause the cap terminal voltage to drop to 11.5 volts.

Let?s see how many caps of this spec we would have to add to keep the voltage at 13.5 for even a few milliseconds. We would need a cap bank with a total ESL of about .001 ohm. Gee it looks like it would take over thirty caps paralleled to maintain 13.5 volts at 300 amps for even a brief instant. And let?s hope we don?t need to do this for long, as the total power contained in thirty units is only about what is in a dozen 9v alkaline batteries!

 

Its been ages since I was in EE classes...

I read the article linked.
Im not involved in competition car audio soundoffs.

I always thought that caps were added to an car audio system to "smooth" out the voltage spikes (caused by the audio amplifier pulling more current than the alternator could supply)

I = C * (dv/dt)

so - if dv/dt is high (i.e. the the audio amp makes an instaneous pull of power for a bass note) the cap will smooth out the voltage drop.

Smooth out.
Not prevent.
The cap cant hold the voltage at 13v
But it can force the dv/dt to be slower.

And thats where its important - to slow down the dv/dt so that the "note" passes and the demand for current ceases so that the "distortion" caused by SHARP fluctuations in the amp's poweor supply dont propagate into the audio chain.

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In other words - the OP's "methadology" is very good - but thats not really the pointof a cap in the system - i.e. the MARKETERS have it wrong as well.

Of course and ad copy that states "chage the slope of the dv/dt curve!" isnt going to sell many amps either

IMHO the use of CAPS in a power supply chain is just a mask for a poorly design amp power supply. (or its there for the bling factor).

Take the infamous Crown MacroTech amps. They were BIG and HEAVY.
That HEAVY was a huge transformer - which stored current capacity in a giant magenetic field. Any huge current draw (bass note hit) would have to collapse the field - which by laws of physics was NOT an easy task. Thus the field could supply "temporary" power.
At the expense of
--weight
--heat
--mag field
--phase shift (? - Im guessing here)
--inefficient power consumption (keeping the field up consumed current)

Class D amps are far lighter and efficient but use a fast switching supply to make up for the power... I dunno if those would benefit from a cap more than a class A or A/B amp would...

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Way back int he early 80s when car sound offs were just beginning - there was a top level competito that used a Crown D75 amps (*only 35 watts per channel*) dunno what he did for CD power supply - maybe a high end TrippLite DC to AC converter (i.e. true sine wave)

 

ewong, phenomenal explanation. I understand the cap to provide current when needed, ie- under surges of current above the available current in the existing electrical system and not something that will continuously provide current in-lieu of the existing electrical system providing that current.

Some car audio systems may need it, but most 'non-competition' car audio systems do not need it. I have 720 watts RMS from 2 separate amps and have no need for one. It is very clean and does not dim lights or pull the power down on my voltage meter on my dash when played loud. I did have a problem with that before, but corrected that by simply updating the wiring from the alternator to the battery and grounding.

 

Like I said - its been oh 20 years since I was sleeping in the back of EE-101.
I didnt get very good grades either
I had to go to summer school (do over) so take this explanation with a large grain of salt.

The physical property is that

I = C * (dv/dt)

What does this mean in common english?
Of interest to us at this time is this statement
If the current draw is BIG (big bass note) and the capictance is LARGE (big cap)
- a capicator will not allow a rapid fluctuation of the VOLTAGE (dv/dt)
[i.e. dv/dt must not be large]

Note I said fluctuation, not HOLD voltage.

The consequence of slowing down a voltage drop is of course that the voltage "appears" to say high. But of course it will fall off eventually.
If the current source (Alt, battery) can continue to supply current - then the voltage will stay steady.
If the current source can NOT supply the current (alt cant pump out - battery cant convert chemicals fast enough) then the current (I) drops and of course the dv/dt (or voltage change curve) can get steeper - i.e. the voltage will fall faster.

Why is this importatn to us?

Well most amplifiers do just that - thye "amplify". They take a voltage curve (the music signal) and they "step it up" - so they take a 5v music signal and maybe boost it by 10 to get a 50v speaker signal.

The problem is that the multiplier relies on the power source voltage being "steady".

The analogy (Im unsure of ampfier physics so IM GUESISNG here) is this - to get a 5v signal to 50v the amplifier stacks ten 5v power supply voltages on top of each other...violia - 50v.

But if those 5v power supply voltages were to "dip" due to high current demand on the supply side to say 4.5v - now we have a peak of 45v and not 50v.

The problem isnt that we cant do 50v, the problem is that we are TRYING to get to 50v and can ONLY get to 45v.

So we have "lopped off" the top of the signal voltage curve....becuase instead of hittign a 50v peak - we have a 45v plateau.
**ALL of the signal voltages above 45v have been squashed back to 45v.

What does this signal look like - looks like a CLIP to me.

And why is a CLIP bad?
(again I was asleep in class for this part)

A FFT (Fast fourier transform) of a CLIP shows that there are both low frequency compnenets and HIGH frequency components at the same time.
The low comonents carry the bulk of the signal.
The high components "fill in the sharp corner" at the edges of the clip.

My interpetation is that the bass speaker attempts to move both slowly (low freq) and quickly (high freqency) at the SAME TIME.
A speaker cone that tries to be in two places at the same time usually RIIIPS.

But I have a Frequency Filter on the signal!

Sorry charlie - the hig frequency components are generated at the AMP - after the filter.

But I have frequency Filters on the speaker wires AFTER the amp!

Wonderful. You have added an RLC circuti to your high powered speaker wires
a) these circuits bleed power - kissl SPL goodbye
b) these circuits add phase shift effects - kiss your soundstage goodbye
But hey - you protected your speakers

Again - Im not familiar enough with the differnt amp topologues (class A, class A/B, class D) to know the pluses and minuses for the power supply effects....

 

Real competitors do NOT use caps, they do NOT help... if you need, add a second battery and/or alternator... caps are a waste...

 

bigger alternator is the answer. ^_^ biggers better.

 

Thanks for the info

BTW - are competitors allowed to hook up to land power or does it all have to be self contained?

Are they allowed to "top off" before the SPL test from a land power line?

 

I believe youre allowed to hook up to a battery charger, but every competition is different like where you "top up"... ex if it's indoors, outdoors, etc, bumpin_yota' would know much more than me....

 

There are no rules against it, but it depends upon the physical location most of the time.

Most of the Super Street and Extreme competitors in dB drag need a battery charger running and to that end bring a generator. I was in a street division and I too have a BEEFY battery charger, and prefer to use it over my alternator to recharge the batterys. But at the smaller events its pretty common to not have a place to plug in....

Also having a beefy alternator is handy in eliminations as you get 2 min to recharge the system before you go up against the next competitor...

ps - u will never see a cap in my competition truck.....they hurt spl.

 

bump to the top for those that just recommended a cap for someone with dimming lights.

read this tread first and then redo your wires. If you need to after that, add a second battery.