Crossovers and Level Controls
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Crossovers and Level Controls

Replacing speakers will make a big difference in the sound quality, but you'll need to add a couple of components to the mix to help ensure the best overall sound.

Crossovers

We've already mentioned that no single speaker can reproduce the entire audio frequency spectrum: different types of speakers perform better for certain frequency ranges. So how do we split up the audio signal produced by the sound system into these different ranges? An electronic circuit called a crossover (also called a crossover network) does that job.

A crossover takes an audio signal and splits it into different frequency bands; this way each speaker can be presented with only the range of audio frequencies that it can handle. Crossovers can be as simple as a single capacitor or inductor, or they can be built using a complex combination of electronic components. The audio frequency at which the crossover splits the signal is known as the crossover frequency, commonly abbreviated as "Fc".

Since we're replacing the existing "wide range" cabinet speaker with a woofer we'll want to ensure that only low frequency sounds are sent to the cabinet speaker. If you think this means we'll need to add a crossover to the sound system, you're right - with an exception.

Games that use the DCS sound system have a sound board with two separate amplifiers, and an on-board crossover that splits the audio signal into two different frequency bands feeding the individual amplifiers. The low frequencies (below about 600 Hertz) are directed to the cabinet speaker, while the high frequencies (above about 300 Hertz) are directed to the back box speakers.

This means that you don't need to use a crossover in a game with a DCS sound system; the crossover is already built into the sound system. If you have a DCS game you can skip ahead to the section on level controls. If you have a pre-DCS sound system game you should always install a crossover.

The specification sheet for a woofer typically lists the suggested crossover frequency; in most cases it's in the vicinity of 100 to 120 Hertz. You can use an extremely simple crossover composed of just a coil of wire (technically known as an inductor) wired in series with the cabinet speaker. In technical terms this would be a low-pass filter: it only permits low frequencies to pass on to the speaker.

If you're using an 8 ohm cabinet speaker you'll want to use a series inductor of approximately 10 milliHenries (abbreviated mH); if you're using a 4 ohm cabinet speaker you'll want to use a series inductor of approximately 5 mH. These values will produce a crossover point of around 120 Hz. The inductor should be made of 18-gauge wire; an air core unit or a unit on a spool work equally well. Parts Express is one source for inductors: the 5 mH unit is part number 255-284, and the 10 mH unit is part number 255-294. The inductors are priced at $8 and $12 respectively.

Another option is to use a pre-assembled subwoofer crossover. You can buy a Pyramid CR-19 subwoofer crossover for less than $10; I found them listed for as low as $8 via a quickie Web search for "pyramid cr19 crossover". This unit is built on a circuit board with spade lug wire connectors designed to fit 1/4-inch quick-disconnect terminals, making it very easy to mount and connect. Note that most pre-assembled subwoofer crossovers are designed for a specific impedance speaker. The Pyramid model is designed for a crossover point of 120 Hz using a 4 ohm woofer.

Note: As of January 2005 the Pyramid CR-19 subwoofer crossover appears to be in short supply. If you can't find one, you can substitute a Mobile Authority HSX-1 subwoofer crossover instead. You can also use a Parts Express in-line crossover: use part number 266-100 for a 4 ohm woofer, and 266-110 for an 8 ohm woofer.

A third option is to use what is called a "two-way" crossover.  A two-way crossover has two outputs, one connected to the low frequency speaker, and one connected to the high frequency speaker. The sound board output is connected to the input of the crossover. If you use a two-way crossover you'll need one that has a crossover frequency in the vicinity of 400 - 500 Hertz. One model you might consider is the Eminence PXB2-500, available from Parts Express.

NOTE: If a two-way crossover is used you MUST connect speakers to each output of the crossover; if you use only the low frequency speaker connection you'll wind up with distorted sound.

I'll add more information on the use of two-way crossovers in the near future.

Level Controls

The human ear's sensitivity to sound varies by frequency: in general, the ear can perceive high frequencies better than low frequencies. Most woofers are also less efficient at converting electrical signals into sound waves than midrange speakers. Because of this, when you fire up a game with replacement back box and cabinet speakers you may not be very pleased with the resulting sound: the high frequencies produced by the back box speakers will probably be louder than the low frequencies produced by the cabinet speaker. To compensate for this we can add a level control to the back box speakers so you can adjust the volume balance between the back box and cabinet speakers to your preference.

The simplest way to control the level of the back box speakers is to wire a resistor in series with them. The resistor effectively "throws away" some of the power output by the amplifier that would normally drive the speakers.  The trouble with a fixed value resistor is that you'll probably need to tweak the value repeatedly until you find a balance you like; as soon as you change the volume setting of the game you'll probably need to replace the resistor.

A much better solution is to use an adjustable level control instead of a fixed resistor. There are two types of level control you can consider: a rheostat and an L-pad.

Rheostat

A rheostat is simply a variable version of the series resistor mentioned above: the rheostat adjusts from zero resistance to its maximum value by turning the shaft of the rheostat. When set to the zero resistance point all of the power produced by the amplifier will be delivered to the back box speakers. The maximum value of the rheostat will control the amount of power delivered to the back box speakers when the rheostat is set to the maximum resistance point.

Assuming we pick a rheostat that has the same resistance value as the back box speaker combination, adjusting the rheostat to its maximum resistance setting will allow only half of the amplifier's power to be delivered to the back box speakers. That will provide more than enough range of adjustment to compensate for the difference in efficiency of the cabinet and back box speakers. This means we'll want to use a rheostat of between 8 and 10 ohms resistance, and rated to handle at least 25 watts. You could use a rheostat with a greater resistance value, but doing so will mean that much of the rotational range of the rheostat will not have a noticeable effect on the volume level of the back box speakers.

High-power rheostats are pretty expensive, on the order of $20 and up. You can use Ohmite models RHS8R0 (8 ohms, 25 watts) or RHS10R (10 ohms, 25 watts), or a Vishay model MP10A10 (10 ohms, 25 watts). These rheostats are available from Mouser Electronics. Try to find an enclosed model, if you can, as most rheostats are built with exposed windings. If an exposed winding shorts to a ground point you could damage the amplifier on the sound or A/V board.

L-Pad

An L-pad is simply two rheostats connected to a common shaft, wired internally in such a way that the L-pad always presents the same impedance value to the amplifier. This is very important for tube-type amplifiers, which are sensitive to the impedance mismatch that would be produced using a series rheostat. Solid-state amplifiers - such as used in pinball machines - are much more forgiving of impedance mismatches.

The L-pad should be the same resistance value as the impedance of the back box speaker combination. Assuming the back box speakers are 4 ohms impedance and that we will wire them in series, that means we'll need an 8 ohm L-pad; the L-pad must be rated to handle at least 25 watts.

L-pads with these ratings are widely available, and are therefore relatively inexpensive. Parts Express model 260-252 is a good choice; you can add a model 260-270 faceplate that can serve double duty as a mounting point. The total cost for the L-pad and faceplate is around $6.

Which to Choose?

I'd recommend the L-pad. It's not strictly required in an electrical sense, but it is a more generally accepted method of controlling speaker level, and it's also significantly cheaper than a high-power rheostat. However, if you have a box of 10 ohm 25 watt rheostats lying around, feel free to use one instead of an L-pad!

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Copyright 2005 by Joseph A. Dziedzic. All rights reserved.