GB2095073A

GB2095073A - Loudspeaker crossover networks

Info

Publication number: GB2095073A
Application number: GB8200127A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-01-06
Filing date: 1982-01-05
Publication date: 1982-09-22
Also published as: GB2095073B

Abstract

A crossover network in a loudspeaker system shares voltage equally between separate parts (Ra, Rb) of the system and the sum of the voltages supplied to separate parts (Ra, Rb) of the system remains substantially constant in the region of the crossover frequency. Preferably a bandpass filter (C3, R1, L3) renders the input impedance substantially linear in the region of the crossover frequency. <IMAGE>

Description

SPECIFICATION The design of voltage crossovers for loudspeakers Introduction It has been suggested that constant input impedance crossover circuits can be constructed by combining R-C impedance correction networks with loudspeakers to produce constant resistive loads for the crossover circuit. This equally divides the electrical power available between the two loudspeakers at the crossover frequency and, if correctly designed, provides an approximately constant resistive load to the electrical supply. (Ref. Ferguson Radio Corp.

British patent application No. 20198/52).

These proposals neglect the fact that two loudspeakers operating in the same frequency range have a greater acoustic output than either loudspeaker operated singly and fed with the same total power. The result is usually a peak in the acoustic output occurring at the crossover frequency. It is common practice to mask this effect to on-axis measurements by reversing the phase of one of the loudspeakers.

Research has indicated that the acoustic output obtained from two loudspeakers of similar efficiency operating in the same-frequency range, will be approximately equal to the acoustic output of either one operating singiy, if the sum of the voltages supplied to the loudspeakers operating together is equal to the voltage supplied to either loudspeaker operating alone. As electrical power is proportional to V2, the crossover must act as a band rejection filter in the region of the crossover frequency. It must therefore have a rise in input impedance at that frequency.

British Patent Application 8100161 described a voltage crossover and showed how the rise in input impedance can be corrected by placing a bandpass filter in parallel electrical connection with the crossover. It is the purpose of this patent application to further describe voltage sharing crossover networks and the bandpass filters which may be used to linearise their input impedance.

Description Provided the loads, Ra and Rb, presented to a voltage-sharing crossover are purely resistive and the crossover network itself is lossless, the total power supplied by the crossover at the crossover frequency will be given by:

where v is the input voltage to the system.

If Ra=Rb then V2 power= 2Ra and the impedance of the crossover, RX, will be given by: R,=Ra+Rb at that frequency.

The rise in input impedance may be corrected by connecting a band-pass filter across the input to the crossover. This must have impedance characteristics such that parallel electrical connection with the crossover results in as nearly constant impedance as possible. Provided the loads to the crossover are purely resistive and equal, the band-pass filter will have a resonance frequency equal to the crossover frequency and contain resistance equal to the sum of the crossover loads.

The circuits that follow consist of crossovers containing either single or twin element filters.

However, within the limitations outlined above, an LRC band pass filter may be constructed on a similar basis to linearise the input impedance of any voltage-sharing crossover.

Loudspeakers do not, as a rule, present a constant purely resistive input impedance. It is therefore intended that CR or LR impedance correction networks be applied where necessary or resistive attenuation networks may perform a similar function. Their use is illustrated in the embodiment of this application.

Crossover employing single element filters L,=low pass filter C1=high pass filter Crossover network C3,R1+L3=Bandpass filter Ra and Rb represent the input resistances of the low and high frequency loudspeakers respectively.

At the crossover frequency, fo, the reactance of components L, and C, are to be equal in magnitude to X Ra and 3 aRb respectively.

In order to linearise the input impedance, the reactance of L3, at fo, should be equal to 3/2 Rb and the reactance of C3 at fo, equal to 3/ Ra.

Best results are obtained in this respect if Ra and Rb are approximately equal, as is usually the case.

Ri=Ra+Rb.

Crossover employing twin element filters L,+C2=low pass filter } Crossover L2+C1=High pass filter Network C3, R, +L3=Bandpass filter Ra and Rb represent the input resistances of the low and high frequency loudspeakers respectively.

At the crossover frequency, fo, the reactance of components L, and C2 are to be equal in magnitude to 2Ra. Similarly at fo the reactances of L2 and C, shall be equal to 2Rb. In order to linearise the input impedence, the reactance of L3 at fo, should equal Rb and the reactance of C, at fo, equal Ra. Obviously, the best results will be obtained in this respect if Rb and Ra are approximately equal. Again R,=Ra+Rb.

Envisaged mode Bass Unit=ELAC 8DC 384 Impedance correction network, C4=12.7 ,uF, R2=8.2 Q H.F. Unit=ELAC 25 DT--03C Attenuation network, R3=1 .2Q, R4=33Q or R3=1.0Q.

Crossover impedance approximately 8Q Crossover frequency approximately 3 kHz C2=C2=3.3 ,aF, L=L2=0.8 mH R1-=15P, C3=6.6 ,aF (or 6.8 MF) and L3=0.4 mH Component vaiues=ltl 0%

Claims

1. Electrical filters are commonly included in loudspeaker systems in order to select regions of the frequency range to be reproduced by different loudspeaker drive-units which compose the system. These filters form a crossover network, where at the crossoveF frequency, fo, they send equal electrical signals to be handled by separate parts of the system. At a frequency higher than fo, the output to one part predominates, at frequencies lower than fo the output to the other part is the larger.

The applicant claims the monopoly on all loudspeaker systems characterised by the inclusion of electrical filters which share voltage equally between parts of the system at the crossover frequency and where the sum of the voltages supplied to separate parts of the system remains substantially constant in the region of the crossover frequency.

2. Passive electrical filter networks which share voltage, rather than power, suffer a rise in input impedance in the region of the crossover frequency.

The applicant claims the monopoly on all loudspeaker systems characterised by the inclusion of a bandpass filter acrossthe electrical input to the crossover network which compensates for this rise and renders the input impedance substantially linear in the region of the crossover frequency.