How capacitors work (myths dispelled)
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Racer
Joined: Oct 2002
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From: Newark, DE
How capacitors work (myths dispelled)
First of all, hola from the depths of the internet abyss. I haven't posted in months, but I'm back.
Ok, I'd like to take some time to explain exactly how a capacitor works in an audio system. I've seen more discussions back and forth on this forum about whether caps work or not and when you need them. I'd just like to set the record straight.
Assume you have a car with a 140 amp alternator. Also assume that 100 of those amps are taken by other systems in the car (leaving us with 40 amps for the audio system).
When the output voltage of the amp is zero, the input current is very low (just enough to run the DC-DC converter). As the output voltage moves away from zero, the input current rises. Let's postulate that an output voltage of 40 volts equals and input current of 40 amps.
Now let's take a standard, low frequency audio signal, say, a 30 Hz sine wave. We'll be looking at the output voltage of this signal. Assume that the signal has a 60 V peak voltage.
To begin with, we'll assume that there's no capacitor on the system. Now, when the signal starts, the output voltage is zero, which means the input current is very small. The output voltage rises and the input current rises with it. When the output voltage passes 40 V, all of our available current is used, so the system voltage drops enough so that the total current stays at 140A. The voltage stays down until the sine wave signal drops back below 40V.
Now take the case where there's a capacitor on the system. The input current draws from the alternator and the cap remains fully charged until the sine wave reaches 40V. As the output voltage increases, the extra current is taken from the cap and the charge on the cap decreases. The cap continues outputting current until the output voltage falls back below 40V. When this happens, the amp needs less than 40A, and the extra alternator capacity is used to charge the cap until it's fully charged again.
Most people think a cap is no good if you listen to long, extended bass. In fact, a cap helps then because the only time it is supplying power is at the peaks of the output wave. The rest of the time it is being charged.
If you could draw the output demand on the alternator, the graph would look like the asolute value of a sine wave. When you put a cap on the system, it squashes the graph toward the average value of the wave (which by the way, is the RMS value). The bigger the capacitor, the more it squashes the graph. Something as big as an audio cap would make the graph almost a straight line at the RMS value.
The moral of the story is, if your alternator can handle the RMS power output of your amp, but not the peak power output, a capacitor is all you need to make your system run right. If it can handle the peak power as well, you don't need a cap. If it can't handle the peak or the RMS power, you need a bigger alternator.
As for upgrading the big three, that reduces resistive voltage losses, which are sometimes the cause of dimming lights. However, in doing so, it decreases the overall load on the system, meaning your alternator will put out more current, but your voltage will stay more stable. If you're taxing your alternator with your monster sound system, this is NOT the answer. However, if you're running a 400W amp and your lights are dimming, this should work.
Thank you for your time. Class is dismissed.
Ok, I'd like to take some time to explain exactly how a capacitor works in an audio system. I've seen more discussions back and forth on this forum about whether caps work or not and when you need them. I'd just like to set the record straight.
Assume you have a car with a 140 amp alternator. Also assume that 100 of those amps are taken by other systems in the car (leaving us with 40 amps for the audio system).
When the output voltage of the amp is zero, the input current is very low (just enough to run the DC-DC converter). As the output voltage moves away from zero, the input current rises. Let's postulate that an output voltage of 40 volts equals and input current of 40 amps.
Now let's take a standard, low frequency audio signal, say, a 30 Hz sine wave. We'll be looking at the output voltage of this signal. Assume that the signal has a 60 V peak voltage.
To begin with, we'll assume that there's no capacitor on the system. Now, when the signal starts, the output voltage is zero, which means the input current is very small. The output voltage rises and the input current rises with it. When the output voltage passes 40 V, all of our available current is used, so the system voltage drops enough so that the total current stays at 140A. The voltage stays down until the sine wave signal drops back below 40V.
Now take the case where there's a capacitor on the system. The input current draws from the alternator and the cap remains fully charged until the sine wave reaches 40V. As the output voltage increases, the extra current is taken from the cap and the charge on the cap decreases. The cap continues outputting current until the output voltage falls back below 40V. When this happens, the amp needs less than 40A, and the extra alternator capacity is used to charge the cap until it's fully charged again.
Most people think a cap is no good if you listen to long, extended bass. In fact, a cap helps then because the only time it is supplying power is at the peaks of the output wave. The rest of the time it is being charged.
If you could draw the output demand on the alternator, the graph would look like the asolute value of a sine wave. When you put a cap on the system, it squashes the graph toward the average value of the wave (which by the way, is the RMS value). The bigger the capacitor, the more it squashes the graph. Something as big as an audio cap would make the graph almost a straight line at the RMS value.
The moral of the story is, if your alternator can handle the RMS power output of your amp, but not the peak power output, a capacitor is all you need to make your system run right. If it can handle the peak power as well, you don't need a cap. If it can't handle the peak or the RMS power, you need a bigger alternator.
As for upgrading the big three, that reduces resistive voltage losses, which are sometimes the cause of dimming lights. However, in doing so, it decreases the overall load on the system, meaning your alternator will put out more current, but your voltage will stay more stable. If you're taxing your alternator with your monster sound system, this is NOT the answer. However, if you're running a 400W amp and your lights are dimming, this should work.
Thank you for your time. Class is dismissed.
