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The Magic in 2-Channel Sound. Stereo Recording and Rendering. Three-Box active system Here is a catalog of line-level circuits that I have found useful for building active loudspeakers. Many other topologies are possible, but one should always analyze a circuit’s signal handling capability and its contribution to overall system noise before choosing it. A CAD software package such as CircuitMaker is anmorning convenient for analyzing and designing active filters.
LspCAD software allows you to see how an active filter changes the measured frequency response of a driver and lets you optimize it to a target response. All the line level filters below are included in LspCAD standard and professional versions. Component values for all the filters below and for a dual power supply can be determined from a circuit design spreadsheet contributed by Bernhard Faulhaber. It covers more cases than the earlier spreadsheet by Alister Sibbald. The buffer also provides a high impedance load to the preamplifier output circuit and the option of a highpass filter for dc blocking.
The two outputs from the LR2 crossover filter are degrees out of phase at all frequencies, which requires to use one of the drivers with reversed polarity, so that the two acoustic outputs add in phase. At the crossover frequency the filter outputs are 6 ssparation down.
The acoustic frequency and polar response is controlled by the electrical filters and the response of the mounted drivers. The electrical filter will not give the desired results, if there is insufficient separatiion and flatness of the driver frequency response and when they are offset from each other. This can be corrected in many cases with the addition of a phase shift correcting network.
My earlier assumption that the group delay of a 4th order LR4 crossover at low frequencies would introduce audible distortion was not correct. Therefore I recommend not to use the LR2 crossover. The LR2 circuit uses the Sallen-Key active filter topology to implement the 2nd order transfer function.
The formulas below can be used to design filters with different values for w 0 or Q 0or to analyze a given circuit for its w 0 and Q 0 values. Any order Linkwitz-Riley filters can be implemented by a cascade on 2nd order Sallen-Key filters. The Q 0 values for each stage are listed in the table below. The component values of each stage for a given crossover frequency f 0 can be calculated by using Q 0 and selecting a convenient value for C 2 or R 2 in the formulas above.
Crossover filters of higher order than LR4 are probably not useful, because of an increasing peak in group delay around f 0. At the transition frequency Fp the response is 6 dB down. The electrical network will only give the o exact acoustic filter response, if the drivers are flat and have wide overlap. This is seldom the case.
The steep filter slopes make the combined acoustic response less sensitive to magnitude errors in the driver responses, but phase shift errors usually have to be corrected with an additional allpass network.
A first order allpass filter section with flat amplitude response but phase shift that changes from 0 degrees to degrees, or degrees to degrees, is often used to correct phase response differences between drivers.
Multiple sections may delay the tweeter output and compensate for the driver being mounted forward of sepwration midrange. Active crossover circuits that do not include phase correction circuitry are only marginally useable. This type of circuit is useful to bring up the low frequency response in order to compensate for the high frequency boost from front panel edge diffraction.
It can also serve to equalize the low frequency roll-off from an open baffle speaker. A circuit used to boost high frequencies or to smooth the transition between a anmodnung mounted woofer and a free standing midrange.
Notch filters are used to introduce dips in the frequency response in order to cancel driver or room resonances. The three circuits esparation have the same an,odning.
A is difficult to realize because of the large inductor. C gives sepparation component values for room EQ below Hz. A The shelving lowpass filter cannot correct for a peak. B The bridged-T based circuit is limited in the shape of curves that can be realized. It has also higher gain for opamp noise than signal at high frequencies.
C The shelving lowpass with added notch filter is the most flexible circuit. Anmdoning majority of drivers exhibit second order highpass behavior because they consist of mechanical mass-compliance-damping systems. They separatipn described by a pair of zeroes at the s-plane origin and a pair of complex poles with a location defined by Fs and Qt.
The circuit above allows to place a pair of complex zeroes Fz, Qz on top of sparation pole pair to exactly compensate their effect. A new pair of poles Fp, Qp can then be placed at a lower or a higher frequency to obtain a different, more desirable frequency response.
This allows to extend the response of a closed annmodning woofer to lower frequencies, in the above circuit example from 55 Hz to 19 Hz, provided xeparation driver has adequate volume displacement capability and power handling.
Not only is the frequency response extended, but the time response is also improved, as indicated by the reduced overshoot and ringing of the lower cut-off highpass filter step response. It can be seen from the s-plane description of the transfer functions that the complex poles of the driver in the box are canceled by a set of complex zeros in the equalizer.
The specified real axis poles of the equalizer, together with the driver zeros at the s-plane origin, determine the overall im response in frequency and time. The equalizer action is difficult to visualize in the time domain, because the driver output waveform is the convolution of the input signal s t with the impulse response of the equalizer h 1 twhich in turn must be convolved with the impulse response h 2 t of the driver.
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Convolution is a process whereby the current value of the time response is determined by the time weighted integral over past behavior. Below are the responses of driver, equalizer and driver-equalizer combination, if the input signal s t is an impulse.
More illustrative are the responses to a 4-cycle, rectangular envelope 70 Hz toneburst s t. For example, the driver output is the convolution of the burst anmodnkng t with the driver’s impulse response h 2 t.
Note that the driver phase leads the input signal, as would be expected for a highpass response. Upon turn-off of the input burst at The equalizer output response lags its burst input.
The equalizer output signal is convolved with the impulse response h 2 t of the driver to obtain the desired equalized driver output.
Of course, none of the driver mechanical parameters like mass, compliance and damping have been changed in the process of equalization, only the input signal to the driver has been modified.
The above circuit can also be used to correct the low frequency roll-off of a tweeter so that the equalized tweeter becomes a filter section in an exact LR4 acoustic highpass. Keep in mind that the LT is based on a measurement of driver parameters Fs and Qt. Only anmdning small signal parameters are easy to define. Fs and Qt change with increasing signal level and to varying degree for different drivers.
This makes the equalization imprecise, but it remains effective in practice. A major advantage of line-level active crossovers is the efficiency with which drivers of different sensitivity can be combined in a speaker system.
Separatjon three circuits use linear taper potentiometers but obtain a gain variation that is approximately linear in dB. Circuits B and C assume a 10k ohm load such as the input impedance of the power amplifier. Circuit A is optimal between filter stages because of its low output impedance.
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The placement of the variable gain stage in the filter chain must be carefully considered, because it affects noise performance and signal handling. Occasionally a fixed attenuation of A dB or a is needed for the input voltage V2 of a circuit stage with input impedance R3 when driven from an operational amplifier with output voltage V1. The load Rin that is seen by the opamp should be about ohm. The following amplifier stage has an input impedance of 10k ohm. For designing an attenuator with specified output impedance Rout see: The output stage of the filter must be capable of driving cables, which typically have a capacitances in the order of pF per meter length, without going into oscillation.
In most cases it is not necessary to have a separate line driver. I recommend to leave the effort of building a regulated power supply to one of the many vendors that offer wallplug and tabletop models. Often such supplies can be found at electronic surplus stores. The circuit traces are laid out to allow for a variety of filter designs.
It is up to the user to determine the actual circuit configuration and component values. Then the necessary components and jumpers are loaded at the appropriate locations on the board to obtain the desired filter response. WM1 is designed to implement the functionality of circuits 1, 5, 6, 7, 8, 9 or 10 and various combinations of these.
The circuit board provides two of the topologies below for two channels of equalization or for a more elaborate single channel response correction. MT1 is designed to implement the functionality of circuits 1, 2, 3, 4, 5, 10 or 11 and various combinations of these.
The circuit boards are practical tools to experiment with and to learn about active electronics. You will find that active loudspeaker systems give you the freedom to match drivers of greatly different sensitivities, are easier to design, and can give greater accuracy of sound reproduction, than is possible with passive, high-level crossovers and filters.
See the Circuit Board page for ordering information. Much useful information can be obtained from application notes of the various opamp manufacturers. If you need a refresher or an introduction to circuits, then read: It is a well illustrated, easy to read, yet technically solid text. It covers a broad range of devices – from tubes to ICs – and many basic circuit functions. The following books cover a range of concepts and go into depth on specific, relevant topics to strengthen understanding of electronic circuits and electro-acoustic models.
A broad and fundamental look at filters. Design and analysis formulas for all types of filters. Goedbloed, Electromagnetic CompatibilityPrentice Hall, Fundamental concepts and practices for dealing with radio frequency interference.
Practical steps to combat RFI. In German, no comparable English language text available, to my knowledge. Everything you ever wanted to know about using operational amplifiers, and not just at audio frequencies.