Crossovers For Dummies

[myorgan]

I've seen several posts recently about knowing crossover points, and in some (many?) cases, a person may not understand the answers (s)he is being given. Is it possible to provide a basic explanation of what crossovers do–so basic, anyone could understand it?

For example:

• What crossover capacitor values are and what they do,
• What effect do different voltages have,
• What about coils
• What are cores and what is the difference between ferrite vs. laminated steel,
• What do inductors do,
• What do resistors do,
• Passive vs. active,
• What are filters and what do low pass vs. high pass do,
• What are orders,
• LPads,
• Series notch filters,
• etc.?

I'm sure it's impossible to address all this in one post, but as I read posts regarding crossovers, it makes me wonder if perhaps there is a handy-dandy tutorial somewhere or a means for a beginner to understand what crossovers are and/or what the different components do.

Michael

[Admin]

Here's a quick and dirty response to some of the questions. Crossover networks are simply a collection of filters. There are three types of filters used in speaker crossovers:

• low-pass: passes the low frequencies and blocks the highs
• high-pass:, passes the high frequencies the and blocks the lows
• band-pass: passes a band of frequencies and blocks frequencies above and below the band.

The filters can be used either in series with the speaker, e.g. a low-pass filter in series to block higher frequencies from being fed to a woofer, or in parallel, e.g. a low pass shunting low frequencies in parallel with a tweeter to block low frequencies.

The "order" of the filter determines how steep the filter cutoff is. A single order filter rolls off at 6 dB/octave from the cutoff frequency. A second order filter rolls off at 12dB/octave. Higher order filters with steeper roll-offs are not typically used in crossovers. The cutoff frequency of a filter is the frequency at which a 3 dB roll-off occurs.

Passive filters have no active components, e.g. transistors or op-amps, and use resistors, inductors and capacitors. Active filters utilize active components. Active filters are typically used in bi-amp configurations, where the woofers and tweeters are driven by separate amplifiers.

Capacitors and inductors (coils) are the AC equivalent of resistors. Their reactance (think AC resistance) varies with frequency. The reactance of a capacitor varies indirectly to the frequency fed to it. The reactance of an inductor varies directly with the frequency fed to it. A single capacitor or inductor forms a first-order filter, that it is, it rolls off frequencies 6dB/octave from the cut-off point.

Resistors attenuate all frequencies passed to them equally so they can be used to adjust the balance between the low and high frequency drivers. An LPad is a resistive attenuator circuit that presents a constant load impedance to its source over its adjustment range.

[myorgan]

Excellent synopsis of the subject, and I really appreciated the definitions of various components and what they do. I especially found the link useful.

So with that information, could a person make a wise decision about crossovers included in various speakers?

Michael

[michaelhoddy]

One comment from the pro audio world (my industry): higher-order filters are actually pretty common in active crossovers used in multi-amplification (crossover upstream of the amplifiers), with 3rd or even 4th order filters being very common. The upside is tighter control around the crossover point, the trade off is typically lesser phase coherence through the crossover.

[Larason2]

That's a useful summary Admin, thanks. This is one of those topics that's pretty complicated, and I'm not sure I understand it thoroughly myself. There are articles and caculators online I have seen to help with crossover design, and I think the first step is to point them to what Admin has written, and to those. Beyond that, I think they have to be invested in the project, keep learning and continue to ask us questions. At some point past the basic two and three driver passive designs, I think they need a pretty good grasp of electrical engineering to design a crossover.

[myorgan]

So, on the simpler level: Does a low-pass filter allow only the low to pass through the filter, or everything except the low?

Michael

[Admin]

The lows pass through that's why it's called a low pass. It attenuates the highs, but keep in mind that we're talking about what the filter does, not its function in a crossover. Conceivably, it could be used in parallel with a driver to shunt the lows around it insuring that only highs are fed to the driver.

[myorgan]

Thank you, Admin. That's what I was thinking, but I did see a similar filter in a 3-way (I forget the order) system. Ideally, that would be used in a sub-woofer crossover to send only certain frequencies from the bass to the woofer.

I'm entertaining the idea that if I can't find another Allen sub-woofer crossover, I may try to get one from somewhere else, but need to know what to ask for. I now have the speakers for the infinite baffle I hope to set up, so the next step would be to feed it with a signal.

Michael

[michaelhoddy]

Something around the semantics of this which might be helpful to clarify, as it was alluded to, but not specifically said: a "crossover" is typically comprised of two or more different filter circuits. For instance, a 2-way crossover will have a low pass filter for the woofer and a separate high pass filter for the tweeter. A 3-way crossover will have a low pass filter for the woofer, a band pass filter for the midrange, and a high-pass filter for the tweeter. And so on.

Also, something like the Allen approach to subwoofers has a passive 3-way crossover in the HC14 (or similar) main speaker to feed its 3 component speaker drivers, and a second 2-way crossover before the amplifiers with a simple high-pass to the mains, and a low-pass to the subwoofer. This is effectively a "biamplified" system, and is a very common approach in pro audio sound systems as well.