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Alpha Audio Academy – Series 2 – Episode 3 – Filters

In an ideal world, a speaker would consist of one vibrating object that processes the complete signal. The reason this is ideal is easy to explain: minimalism in hi-fi is always good… fewer components means less loss. And in this case, an influential link is eliminated: the crossover. In fact, a so-called crossover poses some challenges. We are going to deal with them in this episode.

Almost every speaker with more than one speaker unit – from dynamic to planar to electrostatic – has a crossover. This distributes the incoming signal among the various units so that they stay neatly within their optimal operating range.

To give an example, an average dome tweeter works fine from about 2500 Hz. If you go below that, most models quickly suffer from a lot of distortion. A woofer works fine up to about 400 Hz, but should not be used much higher for obvious reasons. Midrange drivers work well between 300 and 3000 Hz for example. There are of course exceptions. Special mid-woofers work over a broader range. Say from 50 Hz to 3000 Hz for example. You see those units in two-way models most of the time.

There are also special wideband units. However, you will see that they can either go quite deep or extend nicely into the higher frequencies. This is because it just seems impossible to do both well.

Watch this episode (Dutch)

The challenge

So to paint a complete picture, two units are needed anyway: one for the high and one for the mids and bass. Ideally, a filter works with a super steep slope. Then each unit gets exactly what it needs and there is no overlap of signals, which in turn can cause problems. Think coloration. But we don’t live in an ideal world. A super steep filter is unprecedentedly difficult to make. From4th order onwards, it already becomes very complex to make a good filter. More on that later.

Anyway: possible or not. Creating a nice filter is not a problem, is it? It’s purely a matter of making a choice and go, right? Well… yes. But then again, it’s not. You see, a filter introduces a few problems. The biggest problems are:

  • A phase rotation (90 degrees per ‘order’).
  • Output variations (by overlap of frequencies)
  • Impulse response
  • Loss of definition (due to components in the signal path)

So designing a filter is incredibly difficult. And in many cases, a filter makes or breaks the speaker. If you put a mediocre filter in a high-end speaker, you will immediately notice that the speaker does not sound right; something is off. If you use a very good filter in a relatively simple speaker, you will hear that the speaker plays well beyond expectations.

Think of it a bit like putting a rock-solid speaker on a mediocre amplifier; you immediately suffer from the limitations of the amplifier. And because of that, it doesn’t work as well as an average speaker on an extremely good amplifier… That amplifier lifts the speaker to a higher level.

Active or passive

Back to the filters. There are simply two methods of making a filter: active and passive. A passive filter receives the complete music signal from the power amplifier and separates it into two or more signals of a certain bandwidth. The filter does this with coils, capacitors and resistors. Partly due to the use of capacitors, phase distortion cannot be avoided. Although it can be solved. With a2nd order you get 180 degrees phase rotation, which can be solved, for example, by reversing wires on one unit.

An active filter works differently. A real active filter sits in front of the power amplifier. And an active filter contains active components. Think of op-amps and transistors. These separate the signal before it enters the power amplifier. The advantage is that the power amplifier is also partly less taxed: it only needs to amplify part of the bandwidth and thus the low cannot influence the mids and high frequencies. The result is a cleaner sound; provided everything is done properly, of course.

Digital filters

Another way of active filtering is to use a DSP instead of transistors and op-amps. By letting a DSP separate everything, much steeper filters are possible than with a passive filter. This can in theory approach an ideal filter, if done properly of course. Digital has other challenges. Think of clocking, filters (yes, filters again) and noise from active components.

Order

Who dives into the world of speakers, eventually ends up in the world of crossover filters. And who dives into that, cannot ignore the various orders. First order, second, third and, for example, fourth order.

The higher the order, the steeper the filter: each order is 6dB per octave. So a4th order filter drops 24 dB per octave. And the higher the order, the greater the phase rotation. A first order, rotates by 90 degrees, a second by 180 degrees, third by 270 degrees and fourth by 360 degrees.

The latter is ideal, since we are round again and thus everything is back in phase. The ‘delay’ is so minimal that we cannot perceive it. So why don’t all manufacturers use a fourth order filter? Simple: cost and complexity. And that usually goes together. A second-order system is less complex and a 180-degree phase shift is solved by reversing + and -, so that’s fine. However, the filter is less steep and so there is more overlap. Which again can cause problems.

So what is good?

Ideally, a speaker consists of one unit that reproduces everything without distortion. But well: that speaker does not exist. We must therefore make concessions.

Minimalism is then ‘key’ is our opinion. A beautiful two-way with high quality units and a simple filter with beautiful components can sound magisterially good. Rhythmic, energetic, pure … A similarly priced three-way is often no match for this. However, those who are looking for real fullrange, often can not avoid a three-way. Purely because the midwoofer of a two-way cannot drop to 20 Hz without problems.

Know however that a full-range three-way (or sometimes four-way) costs serious money if you also want the purity and transparency of a two-way system. After all: more components. And more complex filters. And a larger, more expensive enclosure…

Next time

In the next episode: active or passive? Do you find this kind of series interesting? Then consider supporting it by becoming a Patreon member. You will then also have a chance to win great prizes and unique content.

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