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| Build-Your-Own | Main Panel
| Dipole Woofer | Crossover/EQ
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Crossover/Equalizer
Operational amplifier circuitry generates the crossovers
between tweeter and midrange, midrange and woofer, and equalization for the 3-way,
open-baffle loudspeaker. Circuit behavior is described by the following functional block diagram.
The electronic circuit diagram for implementing the above functionality is shown below.
Download the 38xo_eq1.gif file to resize the circuit schematic on your computer for printout. Calculated frequency response curves for buffer, tweeter, midrange and woofer outputs can be seen in xo_eq2.jpg. Subtract a 20 dB graph offset from the vertical scale to obtain the true gain values. You will find measured frequency response data on the | Test | page.
Circuit descriptionThe circuit description follows the signal path from input to the three outputs. 140kLP -6dB - Low-pass filter to block radio frequency interference. 6dB attenuation to compensate for the following 6 dB gain stage. 100-200HP and +6 dB - Shelving high-pass filter with 6 dB gain to transition between the woofer on the floor radiating into half-space and the main panel radiating into full-space. - shlv-hpf.gif 2760NF - Notch filter to account for differences in the ear's free-field and diffuse-field response and differences between sound pressure pickup in recording and playback environments. - inductr1.gif, Design Models - H Buffer - Low impedance drive source for the following three channels. Tweeter channel: 1440HP - First high-pass filter section of 24 dB/oct, 1400 Hz L-R acoustic crossover to midrange. - xo12-24.gif +/-2.5dB - Gain adjustment with 5 dB range and precisely linear dB scale. The preceding two resistor ladder sets the channel gain and provides a nominal 3.5k driving impedance to obtain a 5 dB adjustment range. - gain-adj.gif 1440PH - Two phase shifting all-pass delay networks to compensate for acoustic phase differences between tweeter and midrange outputs in the overlapping crossover frequency range. The phase shift difference results from the tweeter's voice coil being in front of the midrange voice coils and the tweeter's high-pass response. - allpass.gif 1440HP - Second high-pass filter section of 24 dB/oct, 1400 Hz L-R acoustic crossover to midrange. - xo12-24.gif Midrange channel: 1440LP - Two low-pass filter sections for 24 dB/oct, 1400 Hz L-R acoustic crossover to tweeter. - xo12-24.gif 90HP - First high-pass filter section of 12 dB/oct, 100 Hz
L-R acoustic crossover to
woofer. 400NF - Notch filter (series R-C-L resonator) with 2.5 H inductor to equalize response peak from driver's rear chassis and baffle low-pass filter. This affects the 6 dB/oct dipole diffraction slope and makes it steeper. - inductr2.gif 90-500LP - Equalization of 6 dB/oct roll-off of the midrange dipole response and second high-pass filter section of 12 dB/oct, 100 Hz L-R acoustic crossover to woofer. Equalization is carried only to 90 Hz to use the 6 dB/oct roll-off of the panel below this frequency as part of the crossover response. - shlv-lpf.gif Woofer channel: 99LP - Low-pass filter for 12 dB/oct, 100 Hz L-R acoustic crossover to midrange. - xo12-24.gif +/-2.5dB - Gain adjustment with 5 dB range and precisely linear dB scale. The preceding two resistor ladder sets the channel gain and provides a nominal 3.5k driving impedance to obtain a 5 dB adjustment range. - gain-adj.gif 290NF - Notch filter with 2.5 H inductor to equalize 1/4 wavelength peak of 1/2 cabinet length transmission line. - inductr2.gif 10-300LP - Shelving low-pass filter to equalize the 6 dB/oct roll-off of the woofer dipole response. - shlv-lpf.gif 2HP - High-pass filter to block any dc offset voltage from reaching the power amplifier for the woofer. The low cut-off frequency of the high-pass filter reduces low frequency phase distortion for improved realism of bass reproduction. All operational amplifiers are Burr-Brown OPA2134. A
blank printed circuit board, material list, and loading chart
to simplify construction of the PHOENIX crossover/eq electronics is available.
Remote volume and other controlsAs I mentioned in the "PS" to Sound
Reproduction it is very important to have playback volume control at the listening place. Ideally, the
control has digital readout so that one can directly return to the preferred
setting once it has been found for a particular CD or LP. A tilt control can be added to the crossover/eq by adding
circuitry to the buffer stage that drives the three filter channels ( 3db_buff.gif
). One way to obtain remote volume control is to build a separately housed preamplifier which is connected via cable to the crossover/eq. All the desired control functions can be assembled into a small package for convenient adjustment from the listening place. A circuit for this is shown in preamp1.gif, gain-adj.gif. It combines volume, balance, mono-stereo switching, polarity and tilt control functions. The frequency response of the tilt control is given by the lower two curves of 3db_oct1.jpg . The umbilical cord between preamplifier and crossover/eq consists of four coaxial cables for left and right channel inputs and outputs, and two wires with shield for the +/-15 V supply and ground. If you wanted to experiment with a -3 dB/decade tilt control, then you could first build a passive version of it and use it in your existing system. 3db_oct2.gif
Power amplifiersFor many years I have used my own 8-channel power amplifier, which was designed around single National Semiconductor LM12 power opamps of 40 W per channel, for a system similar to the PHOENIX. Today one might select the LM3886 , selling for $9 each. A printed circuit board is not needed for the heat sink mounted power opamps, because the few additional components can be wired point to point using terminal strips and a copper plane for signal ground ( 3886amp.gif ). The voice coil presents a very easy load to the amplifier and there is no risk of high frequency oscillation due to capacitive loading. Use one opamp per driver. I have not found it necessary to provide speaker protection and drive even the tweeter from a full bandwidth power amplifier. If you want greater voltage or current capability, then use opamps in bridged or paralleled configuration. Bridging two devices doubles the voltage swing, but does not increase the maximum output current capability. It is useful for higher impedance loads. Paralleling two devices doubles the maximum output current, but keeps the same voltage swing. It is useful for driving low impedance loads. Several opamps can be paralleled to increase current capability n-times. A 200 W amplifier can be built with two paralleled LM3886 in each leg of a bridge configuration. Check out the new LM4780 dual power amplifier. It has two 60 W amplifiers in one package which gives close thermal tracking, important when bridging or paralleling amplifiers. The power supply for the multiple operational amplifiers needs to be able to
supply large amounts of instantaneous current which requires a low impedance
transformer and large filter capacitors. ( 3886pwr.gif
) More power than 40 W per channel is desirable, because it tends to impart a
greater sense of ease to
the sound when playing back at high volume levels. Amplifiers that I have used with my speaker designs,
and that performed extremely well, were made by For other options check out the following web sites
which have interesting power and preamplifier designs for the DIY audio enthusiast: While the woofer is best driven from a solid-state amplifier, the main panel is well suited for low distortion, low power tube amplifiers. The 8" drivers generate a SPL of 99 dB above 250 Hz at 1 W / 1 m, decreasing to 92 dB at 100 Hz. The lower tweeter power sensitivity of 87.5 dB is still adequate, because of the generally low power demand of most program material. (Design Models-I, FAQ16, FAQ20) In all cases, most important are low output impedance (<0.4 ohm) and low distortion (<0.1%) of the amplifier over a 5 Hz to 50 kHz frequency range.
| Build-Your-Own | Main Panel
| Dipole Woofer | Crossover/EQ
| Supplies | |