Cap It Off

Learn how to use small bypass and safety capacitors to protect your system.

Crossovers are circuits designed to direct audio energy to different loads, depending on the frequency of the incoming signal. There are two basic types of crossover construction: active circuits which use transistors or ICs to distribute the audio at preamp signal levels, and passive circuits which use capacitors, inductors, and, possibly, resistors that operate at a higher power level.

The passive crossovers used in car audio are typically purchased completely assembled or custom-built from raw components. A formula or chart is usually provided to determine optimum values for the capacitors in microfarads (mfd) and for the inductors in millihenrys (mH). But, have you ever wondered about the voltage rating of the capacitors? How many times have you seen an exploded crossover capacitor or perhaps wondered why some brown fuzzy stuff had mysteriously distributed itself throughout the trunk of a car?

Capacitor Voltage Ratings
The voltage rating stamped on crossover capacitors, or any other capacitor, is the maximum voltage that should ever be placed across the cap’s leads. To find out what that maximum applied voltage to a passive crossover capacitor might be, we must first examine the amplifier driving the crossover and speaker. Actually, we want to look at the power supply inside the amplifier. It’s the power supply that provides the “rails” necessary to amplify a signal. (The word “rails” is an audio term for the operating supply voltage of an amplifier.) As an example, let’s say we have an amplifier rated at 50-watts driving a very typical 4-ohm load. The power supply “rails” might be plus and minus 25-VDC (Volts Direct Current). This means that the maximum voltage attainable from this amplifier would be (25-VDC) + (25-VDC) or 50-VDC total. Any crossover capacitor used with this amplifier should, therefore, have a rating at least as high as 50-VDC.

As another example, let’s say that we try using the same crossover capacitor as mentioned above with an amplifier capable of delivering 800 watts into a 4-ohm load. The power supply of such an amplifier might have “rails” of plus and minus 100 VDC for a total maximum output voltage of 200 VDC. If we were to use a 50-VDC rated capacitor in this application, be assured that sooner or later there will be some more fuzzy brown stuff all over the place or at least a quiet speaker somewhere. Be sure of your voltage ratings!

Now, it’s not always easy or necessary to go inside the amp to measure the power supply. However, based on the power ratings, it’s pretty easy to apply a simple formula that will keep us out of trouble. The voltage rating should be about three times the AC rms voltage at the output of the amp. If you don’t know the output voltage, but know the power in watts, a simple formula will get you where you need to be. Multiply the power in watts by the impedance rating and then take the square root of the total. Three times that number will be the safe voltage rating for Caps for that particular amp. For example, for a 300-watt amp at 4 ohms (see Fig. 1).

Improving Passive Performance
When the response characteristics of a component are specified, many attributes are often measured and lumped into a single category called distortion. One particularly annoying type of distortion is common in the design of most simple car audio passive crossovers — hysteretic distortion. Hystersis can be described as a “sluggish” or slow response to AC (alternating current).

All large capacitors, and especially electrolytic capacitors (bi-polar included), suffer from hystersis distortion. The remedy to this problem is to use all expensive mylar- or poly-type caps for crossover building. Unfortunately, if you’re building crossovers for low frequency points and/or low impedance speakers, sometimes obtaining large mylar or poly caps is just not possible or practical. But there’s a solution that works just as good — the practice of “bypassing” the less-than-perfect cap with a small value cap with good high frequency characteristics, such as polystyrene types. Each bypass capacitor should be around .01 mfd and rated for voltage at least as high as the capacitor to which it’s connected across. The value of this bypass capacitor is so small that it will not affect the crossover frequency.

To install the performance-improving bypass caps, just solder the leads in parallel to a crossover capacitor. There’s no polarity to a .01 mfd capacitor, so there’s no need to be picky as to directions. However, polystyrene film caps are much preferred for audio purposes. These capacitors are available from high-end car audio dealers or directly from sources such as Mouser Electronics (800-346-6873) and cost around $.25 each. Another source would be Digi-Key (800-344-4539). Minimum purchase amounts are required at each of these companies.

The improvement in hystersis distortion will be most noticeable in the mid and high frequencies. It’s amazing how just installing the bypass capacitors can sometimes make an audible and measurable difference. For serious SQ contesting, consider adding bypass caps to your passive networks even if you have already used good quality caps!

Protecting Tweeters
Tweeters are expensive, and I’m sure that there’s nothing more frustrating than purchasing a pair of $200 tweeters and having one of them destroyed. Tweeters, and especially horn drivers, are extremely sensitive and fragile. One common failure mode for tweeters in systems with electronic crossovers is failure to provide protection from unintentional low frequency transients. Woofers have been known to move great distances when amplifiers are turned on or turned off or a RCA comes partly unplugged, but what about the tweeters?

If the tweeter is fed from a passive crossover, it will naturally be protected from this phenomenon. But, what if it’s attached directly to a monster amp? Since tweeters were not designed to move more than a few thousandths of an inch, what happens when they get popped by an amplifier? The answer is that they typically suffer from suspension fatigue and become permanently damaged. There’s no remedy for damaged tweeters, and they will produce constant distortion until they are replaced.

Simply plugging and unplugging an RCA lead from an active crossover or an amplifier driving a tweeter will often cause a potentially damaging pop. The pop in a tweeter may not always be audible, but the damage will occur electro-mechanically. (This is one of the reasons that a couple of Japanese amplifier designs included a feature that required a chassis ground connection to the RCA connector before the amplifier would engage.) There is no reason to put tweeters at such risk because the necessary safety precautions are both inexpensive and easy to install.

To protect a tweeter, a bi-polar crossover capacitor should be inserted in series with the + terminal. (It really doesn’t matter, but let’s be consistent.) This “safety” capacitor will block dangerous pops and spikes, yet, it will permit the audio to pass.

The exact value for the “safety” capacitor can be calculated by first dividing the tweeter’s crossover frequency in half. Then a first order crossover filter capacitor value can be calculated by using the equation in Figure 2.

For example, let’s say we’re using a crossover point of 2000 Hz on a tweeter with an impedance of 4 ohms (at 2000 Hz). One-half of 2000 Hz yields an f of 1000 Hz. Solving for C, we arrive at 40 mfd (39.8 mfd if you’re a math buff) for our safety cap. Remember, electrolytic caps can be used as long as they’re bypassed properly. Again, polystyrene film capacitors should be used for bypassing. If this is done correctly, the safety cap will have no affect on sound as it’s too low to have any audible effect, but if any low frequencies slip by they will be removed before they can cause damage. A tweeter safety cap sort of acts like a “subsonic filter” does for a woofer, only it works at the frequencies that are troublesome to tweeters.

That’s it for this issue. I’ve been using these techniques in studio and pro audio applications for decades, and they’ve saved me many times. I hope that you’ll consider including small bypass and safety capacitors in your next system. Someday the tweeter you save may be your own!