COMPONENTS

The Passive Frequency Crossover

With multi-way loudspeakers, frequency ranges are subdivided into several ranges which are reproduced by drivers that are specifically designed for a particular frequency range (see Figure 6: "Loudspeaker Frequency Ranges"). A typical three-way loudspeaker may cover the following frequency ranges: bass drivers cover 30 to 800 Hz, the mid-range cover 800 to 6,000 Hz and the high-range is covered above 6 kHz. Each frequency range is covered by a specialized driver.

The only bstacles are the overlapping frequencies between the various drivers, since they don't operate only in a strictly defined range but have a slow roll-off above and below that area. In other words, specific frequencies are emitted simultaneously by two drivers. This leads to alterations of single frequencies, phase shifts, etc. That is the reason why multi-way loudspeakers are equipped with passive crossover networks.

The Right Type of Filter

It's extremely difficult to "calculate" a passive frequency crossover, since its application always leads to considerable deviations from the theoretical ideal. That's why, after having developed our systems and having established the theoretical ideal values, every single system undergoes meticulous fine tuning and accurate, empirical investigation. This is a very time consuming and complicated procedure, however the results make it worthwhile.

What kind of frequency crossover is used depends on the drivers chosen. The filter rate, the bandwidth of the driver, and its phase characteristics have to be considered at the same time. Furthermore, the resistance of the driver, depending on the frequency, has a detrimental effect on the operation of the frequency crossover.

The mechanical and electrical performance of the loudspeaker requires a specific filter design. Basically, there are various types of filters available with differing roll-off rates. The roll-off rates are measured in dB/octave and indicate the reduction of voltage at the loudspeaker with a specific type of filter.

Commonly Used Filters

The Chebyshev filter was developed to show the highest filter effect, however, its impulse behavior is poor. The Bessel filter shows the best impulse and phase behavior. But the filter effect in the cut-off frequency area is very low. The Butterworth filter is a compromise. It achieves a good filter effect and acceptable impulse behavior.

Depending on the roll-off rate (order of filter) and the type of filter, distinctive overlapping occurs which means that loudspeakers have to operate linearly even outside their application range. This leads to considerable problems around the frequency crossover.

Butterworth filters or Chebyshev filters of higher order bring about considerable advantages for the systems because they offer an increased roll-off rate. Thus the drivers are not "misused" having to work in a non-designated frequency range. However, this has a detrimental effect on the impulse behavior. That's why those kinds of crossovers are always a compromise.

The attempt of optimizing the impulse behavior therefore automatically leads to the use of filters of the first order. Usually, these filters lead to large overlapping. In this case, the high-range loudspeaker has to cover additional two octaves below the cut-off frequency. In order to apply a first order crossover, the high-range loudspeaker has to bear considerable amounts of mechanical wear and the mid-range loudspeaker has to show excellent impulse behavior.

 
Figure 6 -- Loudspeaker Frequency Ranges. Via filters, a cross-over assigns a particular frequency range to each driver. A low pass filter allows passing of low frequencies, a high pass filter allows the high frequencies to pass. A band-pass filter is used in the mid-range frequencies filtering frequencies below and above a specified range.

The CDC-System

In general, there are only few loudspeaker systems on the market that can be operated with the above mentioned frequency crossovers. By virtue of our CDC-system we've been able to eliminate this problem. CDC stands for "Controlled Dispersion Characteristic" and induces the driver to work only in a precisely pre-defined frequency range. The way CDC works is very simple:

The lower cut-off frequency of a horn loudspeaker is mainly determined by the size of the horn's throat area, which means the larger the horn throat area, the lower the frequencies the loudspeaker can emit. Below the cut-off frequency of the horn, the response falls off steeply at 18 dB/octave. Horn loudspeakers thus operate only to their cut-off frequency limit. Below their natural cut-off, they decrease their volume automatically without any filter.

The upper frequency response is mainly determined by the driver, however, it can also be influenced acoustically by the horn. For this purpose, a small chamber is placed between the driver's membrane and the horn throat (see Figure 7: "CDC Acoustical Low-Range Pass Filter").

Figure 7 -- Acoustical Low-Range Pass Filter. The volume between the membrane and the horn throat filters high frequencies and thus induces a monitered attenuation of the frequency response above a defined area. CDC is a kind of acoustic frequency crossover which operates without any electronic components in the signal path.

The driver doesn't emit directly but via a small air chamber into the horn throat opening. This air volume operates as a kind of band-pass filter and automatically filters frequencies above the resonance volume of the chamber (at 6 dB/octave). By choosing an adequate driver with a natural roll-off at 6 dB in this frequency range, we obtain an acoustic attenuation of the frequency response of 12 dB without any passive frequency crossover.

Controlled Dispersion Characteristic thus causes our loudspeakers to only operate within their operational band and steeply fall off at the transition points. CDC allows us to equip our systems with the quickest filters and to obtain the best solution for an optimised response characteristics. The drivers are tuned to harmonize with each other in terms of their dynamics and frequency response behavior.

The steep slope attenuation at the low cut-off frequency of the spherical horns, combined with a high power handling capability of the driver systems, enables us to use phase neutral high-pass filters. These filters are first order types and in the mid-low range we can do without any passive filters in the signal path. Consequently, all our satellite systems in the lower frequency response range are not limited by passive crossovers, which means our satellites operate in full range mode.

If you take a look at our frequency crossovers, do not be surprised to find only few electronic components (see Figure 7. "CDC Acoustical Low-Range Pass Filter"). This doesn't imply exaggerated economy measures, on the contrary, it shows the balanced design of the system as a whole. At this point, the saying: "Less is more!" is appropriate. The frequency crossover in our system makes up one part of the entity of our conception, and doesn't serve to amend errors in other areas. The more disharmony there is within one of the loudspeakers, the more complicated the construction of the passive crossover will become. Hence, the simplicity of the frequency crossover bears witness to the high quality of our systems.

Quality of Components

The major purpose for selecting quality components isn't to perfectly adhere to the values, but rather in obtaining exactly equaling frequency crossovers for the right and left channel. The major effect can be observed in the transparency and imaging of the program. That's why all our frequency crossover components and of course our drivers are selected in pairs.

The quality of a loudspeaker is predominantly determined by the quality of the material used. Just imagine a concert piano made of plywood. The very same applies to the components of a passive frequency crossover.

The steep slope attenuation at the low cut-off frequency of the spherical horns, combined with a high power handling capability of the driver systems, enables us to use phase neutral high-pass filters. These filters are first order types and in the mid-low range we can do without any passive filters in the signal path. Consequently, all our satellite systems in the lower frequency response range are not limited by passive crossovers, which means our satellites operate in full range mode.

Less is more! Our frequency crossover allows absolute freedom to the drivers. We simply use  two resistora, a coil and a capacitor.

If you take a look at our frequency crossovers, do not be surprised to find only few electronic components (see Figure 7. "CDC Acoustical Low-Range Pass Filter"). This doesn't imply exaggerated economy measures, on the contrary, it shows the balanced design of the system as a whole. At this point, the saying: "Less is more!" is appropriate. The frequency crossover in our system makes up one part of the entity of our conception, and doesn't serve to amend errors in other areas. The more disharmony there is within one of the loudspeakers, the more complicated the construction of the passive crossover will become. Hence, the simplicity of the frequency crossover bears witness to the high quality of our systems.

Quality of Components

The major purpose for selecting quality components isn't to perfectly adhere to the values, but rather in obtaining exactly equaling frequency crossovers for the right and left channel. The major effect can be observed in the transparency and imaging of the program. That's why all our frequency crossover components and of course our drivers are selected in pairs.

The quality of a loudspeaker is predominantly determined by the quality of the material used. Just imagine a concert piano made of plywood. The very same applies to the components of a passive frequency crossover.

Capacitors

The capacitors of our systems are among the best available on the market. The brilliant sound qualities are achieved by virtue of a specialized induction-free coil technology. Two capacitor windings are combined in such a way that they mutually interact, neutralizing the induction of one another. Both windings are connected in series. Thus this design incorporates four times the material of a normal capacitor.

Polypropylene foil insures the capacitor will have very low electrical losses, thus making this capacitor extremely fast. The rigid material used for the housing of the capacitor eliminates feedback caused by microphonic effects. Although the price is about 20-times higher than a good quality standard capacitor, the resulting transparency, speed and inner detail make it all worthwhile.

Coils

What applies to capacitors, also applies to the coils. All components in the signal path must be of highest quality. We've paid special attention to the DC resistance that above all, considerably influences the impulse behavior and performance of the loudspeaker.

The resistance of a coil can either be reduced by using thick wires or iron cores. We are using air coils, since iron cores run into saturation at higher currents. This happens with overdriving and produces distortion. Similar phenomena can also be observed with tape recorders.

With regard to impulse linearity and low distortion behavior, air coils are unbeatable. Moreover, our coils are manufactured with a special type of wire. An additional lacquer coating on the wire is heated to its melting point after the wire has been wound. After cooling-off, the lacquer has perfectly glued together all the windings of the wire.

Normal coils tend to vibrate when electrical current flows. This "howl-back" effect, which means the transfer of mechanical into electrical oscillations, is added to the original signal as an additional piece of information. Details of the original signals are thus being overlapped or made unrecognizable. Hence, the music loses its body and transparency. Joining together the exterior of the coil wiring preserves all musical subtleties, endowing the music with its natural liveliness and brilliant body.

Resistors

Resistors with metal oxide layers don't show residual inductance as wire resistors do. As optimal impulse speed is of major importance for us, we use resistors with metal oxide layers exclusively.

Wiring

Avantgarde Acoustic hornspeakers utilize highest quality maximum-purity copper cables. This holds true not only for external but also for the internal wiring. The wiring is characterized by extremely low inductance and minimal capacitance. This results in hardly measurable losses due to damping.

No colorations are introduced by "exotic" cables that could interact with your amplifier. You'll hear what we heard when we designed them.