I think I have seen a thread on this subject but will ask the question again. I believe the previous answer was "if your headlights dim when bass notes hit, you need one".

I am looking at adding the following and am wondering if I will need to add a capacitor:

Rockford Fosgate Punch 600.4
Specs: RMS Power: 75W x 4 @ 13.8V
Bridged RMS Power: 300W x 2 @ 13.8V
THD at Rated RMS Power: 0.05%
Frequency Response: 20-20,000 Hz
Channel Separation: 65 dB
Signal to Noise Ratio: 100 dB
Fuse Rating: 50 amps

This will be the only amp in my setup...any experience with this or similar amp? If needed, what do I buy?

 

Because of the way our HID headlights works, they will never dim. It's conceivable you could load your system to a point they turn off, but you'd probably blow your alternator first.

I'm guessing you're going to use one pair of channels bridged for the sub, and the other two for the fronts. Subs are what cause the big draws that dim your lights - if your sub can handle all 300W those two channels can dish out, then it's possible you might notice some very minor dimming.

But (in my VERY humble opinion) you won't need a capacitor. "Conventional" wisdom states that you use 1 farad of capacitance for every 1000 RMS watts your system puts out. At most a .5F cap will be what conventional wisdom says you "need". But again, I think it's a waste of money.

I'm using an 800.4 (400x2 bridged) just for the sub. Very big bass notes cause the interior and brake lights dim a bit, but only at volumes you would only listen to for a very short while, not all the time. Definitely not a problem worth spending a couple hundred bucks on to fix.

If you get everything hooked up and decide you do need a cap, you can always add it later...

 

thanks for the input...

correct on your assumptions...running front components off 2 channels and bridging the other two for a sub....just trying to get a handle on total out of pocket costs to add the amp/sub/enclosure and everything else (wiring, capacitor, my time etc.)

 

KaCee,

This is probably one of those questions where there is no right or wrong answer and you have to try it to see if you notice a difference.

What the cap. is supposed to do is store energy for the amp when it need a big power surge. What the cap does is provide extra power to the amp quicker than the battery can because it is closer to the amp. Some people say that it sounds better with the cap in because of the quicker power delivery...again try it for yourself and decide if there is a difference. If you do not want to spend alot of money on a cap for testing go to an electronics surplus store and find one...it will be alot cheaper than one from an audio store.

If your headlights dim...it will probably only happen at idle as the alternator is probably (not 100% sure) not putting out it max. capacity. This would probably not happen at any higher rpm. Adding a cap could actually make this worse (I think) because you need to constantly charge the cap. and run the car/electronics.

Modifry did a lot of testing on this, you may want to search for his results.

Just my 2 cents.

 

matrix,

I'm not picking on you, but I want to clear up a couple of points...

1) You got it all right about what the cap does except why it does it better than the battery. You can actually put the cap further from the amp than the battery and still see better results. They work because the cap is designed to provide a "faster" delivery of the same amount of juice than a battery can (better slew rate, for all you technogeeks).

2) As jzr pointed out, since our headlights work off of a ballast, they won't dim with decreased power, but they WILL turn off if power goes too low. Not quite a strobe effect, since the ballast isn't fast enough, but it might look pretty interesting at a car show

3) Looking at all of the items that WILL dim, the cap won't make things worse, it can only help. Remember, you're not charging the cap up to full potential with every bass hit. The cap is charged to full potential when it is first connected to the battery, then it has a portion of its energy drained with each bass hit, which is then recovered through the alternator/battery combination.

IMO, 300W isn't worth a large expenditure, but as jzr said, you can always add one in later.

 

By now you should be leaning toward not needing a cap. I agree (just one more opinion). I don't think your system is big enough to get much benefit, and like they say, you can always add one later.

One piece of advice - use the fattest power wires you can reasonably get by with. In the limited testing I've done, series resistance seems to be the biggest factor in limiting amp performance. You can strangle your amp with a small gauge power wire.

I've heard a lot of people make assumptions as to why and how capacitors help high-powered amp systems (and they do, but not by much), but I've not seen any substantiated information. I mean, how hard would it be for a cap manufacturer to do some tests and prove exactly what their product does for a big system? Not hard at all, if the cap makes a measureable difference, but I've never seen any real data. Any serious engine performance companies always do testing to prove their products work as intended, and then publish results so everyone can see. I've never seen this done in the audio world.

I did some testing on a small system (50 watts per channel) but did it with a seriously over-taxed power supply to simulate a bigger system's drain on the alternator/battery, and the best gain I could measure was 1 db more peak power with a cap.

MacGyver - I did some more testing, and I can't find any high slew rate current surges in the amp power line. I agree the cap can probably provide higher slew rates than a battery (and I'm not 100% convinced), but I can't verify the need actually exists. I can't even find the expected high-frequency switching currents in the supply line - I bet the internal amp capacitors are taking care of this.

 

modifry,

I'm wondering if you would see more of a pull with a really large system, say 1000W. The micro-sized tests you've done in the past were sound (no pun intended ), but I tried looking at it from a different perspective. With a big bass hit on your micro system, the energy pull difference is (for argument's sake) a rapid jump from 0W to 50W over X amount of time. Now, over the same X amount of time (the bass frequency hasn't changed, nor has its attack time), pull from 0W at rest to 1000W...it HAS to have a much faster spike (higher slew rate) if the amp is going to keep up with the music.

I'm sure the internal caps are smoothing things out quite a bit, but then I have to look back at the micro/macro argument. Let's say the total internal capacitance of an amp is 0.1 F. This works great at smoothing out supply surges for a 50W system. Now let's go crazy and jump to a 5000W system...still think that 0.1 F of capacitance can do as good a job? Probably not...

This has always been one of those arguments that I cannot convince myself one way or the other until I spend a good deal of time running the numbers through on paper and looking at test results, such as yours. At the moment, I could probably convince myself of either argument, if I only argued each side for a few minutes.

 

MacGyver, I do not view your points as picking on me . Actually when I reread my post, I can see where most of your comments are coming from.

I do and don't agree with point #1. Everything about slew rate, definitely. If you are trying to increase slew rate, keep the cap close. Not sure I agree with the cap doing it better than the battery though, as the battery is basically a big cap. that is why I did not mention slew rate and only mentioned that it was closer.

[QUOTE]2) As jzr pointed out, since our headlights work off of a ballast, they won't dim with decreased power, but they WILL turn off if power goes too low. Not quite a strobe effect, since the ballast isn't fast enough, but it might look pretty interesting at a car show

 

I'm hoping someday someone here in Atlanta will get a disgustingly large system in their S and have nothing better to do on a Saturday afternoon but stand around and watch scope traces.

[QUOTE]Originally posted by MacGyver
[B]I'm wondering if you would see more of a pull with a really large system, say 1000W.

 

Modifry, I agree with you 100%, in a previous car I had about 1000W and instead of choosing to replace the battery every so often and beat on the alternator, I installed a second alternator. Never had power problems and never used a cap. Never had lights fading etc, etc.

As I stated a battery is basically a big cap. that is already in the car. Only benefit I can see is that the cap can be installed closer to the amp.

Just my 2 cents.