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| Build-Your-Own | Main Panel | Dipole Woofer | Crossover/EQ | Supplies |
| System Test | Design Models | Prototypes | Active Filters | Surround | FAQ |

 

Crossover/Equalizer

Operational amplifier circuitry generates the crossovers between tweeter and midrange, midrange and woofer, and equalization for the 3-way, open-baffle loudspeaker. 
    Crossover frequencies are set to 100 Hz (12 dB/oct L-R) and 1400 Hz (24 dB/oct L-R). The 6 dB/oct open-baffle diffraction slopes of woofer and midrange are equalized and also the half-space to full-space transition between floor mounted woofer and free standing midrange. Psycho-acoustic equalization is provided for the 3 kHz region.

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 description

The 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. 
The opamp drives an attenuator with specified output impedance of 5.2k controlling the notch depth of the 400NF filter together with the 4.22k resistor. - attnrout.gif

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
Circuit components may be purchased from suppliers such as Digi-Key and Mouser .

A blank printed circuit board, material list, and loading chart to simplify construction of the PHOENIX crossover/eq electronics is available.
General topology printed circuit boards WM1 and MT1 can be used to build selected sections of the circuit. 

 

Remote volume and other controls

As 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. 
    To correct for occasional channel imbalances at the listening position, it is nice to have a balance control handy. 
    Channel polarity switches are sometimes desirable. Recording techniques can be investigated by reversing the polarity of one channel. The audibility of absolute polarity is tested by reversing both channels.
    Finally, for a speaker like the PHOENIX that is flat on-axis and similar in its power response, some recordings may sound bright because the microphone pickup was too close to the instruments. To correct for those situations I have found a frequency response downward tilt of 3 dB/decade to be a subtle, yet effective remedy. The tilt is also beneficial when listening very close to the speaker in a small space, or when listening at lower volume levels. 

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 ).



The response of the tilt circuit is given by the top two curves of the 3db_oct1.jpg graph with the toggle switch set to on and off.

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.gifgain-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 amplifiers

For 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 )
Alternatively, you might consider a battery supply, where the batteries are trickle charged. The power supply constitutes the major cost for this type of DIY amplifier solution. 

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 
Hafler (models DH-220 and DH-200),
ATI (Amplifier Technologies Inc., models AT1504, AT1506, AT1802, AT6012),  
BEL (Brown Electronic Lab, +1 408 259 8648 voice, model 1001),  
Muse Electronics (model 160),  
Jeff Rowland Design Group
(models MC-6, Concentra, Monoblock 6). 

For other options check out the following web sites which have interesting power and preamplifier designs for the DIY audio enthusiast:
The Self Site
- Doug Self
Borbely Audio - Erno Borbely
The Audio Pages - Rod Elliott, e.g. single chip 50W/8ohm power amplifier

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 |
| System Test | Design Models | Prototypes | Active Filters | Surround | FAQ |

 

 

What you hear is not the air pressure variation in itself 
but what has drawn your attention
in the streams of superimposed air pressure variations 
at your eardrums

An acoustic event has dimensions of Time, Tone, Loudness and Space
Have they been recorded and rendered sensibly?

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Last revised: 06/12/2017   -  1999-2017 LINKWITZ LAB, All Rights Reserved