Top view of my crossover, click to enlarge. Low pass crossover is the large PCB on the right, smaller PCB to left is the power supply near the toroidal transformer. Note that the high-pass caps between the Input RCAs and High Pass Output RCAs (the 4 rightmost in the photo) are not yet installed as they are amplifier input impedance dependent. Woofer level control is in the center of the rear panel.
Internally, the active low pass crossover is a 3rd order IC based unit, similar to the Dahlquist but with modern components. I actually designed and built the audio PCB several years ago, but recently rebuilt it with better parts. I chose OPA604 opamps, and used the single versions (as opposed to duals) of the ICs and IC sockets for maximum flexibilty to swap them out. The level control is a Noble 100K unit mounted on the rear panel (to help avoid accidentally changing woofer level once set), the caps are mostly WIMA MKP10s, the resistors are mostly Holcos, and the RCAs are Vampire gold/teflon. When I designed the PCB, I used very wide traces both for power supplies and audio signal paths. Instead of the continuously variable frequency controls of the Dahlquist, I chose use plug-in frequency selection modules. These are the small boards labelled 100Hz in the photo above. Gold IC sockets and gold headers provide reliable plug-in connections.
The Low Pass Audio PCB, click to enlarge. Note the empty 8 pin DIP sockets near the front of the board for the plug-in frequency modules. Relays at the rear short the outputs to ground to bypass the crossover (and send no signal to the low frequency amp, full range to the high frequency amp).
Closeup of installed frequency determining module, really just 3 resistors. There is one of these for each channel. My markings on the caps was used to select values that were within about 1% of target values.
Bottom view of the PCB, showing the wide traces. You may note that I designed the board back in 2000!
Schematic diagram of my low pass crossover, click to enlarge. Note that I have since removed the 270K/100pf components from the final IC feedback loop (replacing the 100pf with a short), as they caused popping noise when the relay opened. They were originally an ultrasonic filter, not really needed. The relay is set up to close (short out the output) whenever power is removed from the relay, either by a powerdown of the crossover or hitting the "Bypass" switch on the front panel. The 3 resistors labeled Rx are the crossover frequency determining resistors, which are mounted on the plug-in boards as shown above to change frequencies accurately and reliably.
A great deal has been learned about the importance of power supplies since the Dahlquist crossover was designed. I built a Sultzer regulator with a pre-regulator (see references below), using UF4007 rectifiers and Nichicon Muse caps. I used the Muse caps for decoupling of the power supplies on the audio board as well, near the ICs. The transformer is an oversized toroidal by Avel-Lindberg, suppling 130va via two 22V windings. The rectified DC is just over +/- 30volts, the preregulator brings it down to +/- 20volts, and the regulated output is +/-15 volts.
View of the power supply PCB wired up for testing, click to enlarge. Output reads +/- 14.96V, rock steady at any powerline voltage over about 70volts. The rectifiers are UF4007s, the preregulators are LM317/337s, and the regulator itself is the Sultzer design. There is two 3,300uf caps at the input, and two 2,000uf caps at each regulated output, along with a 0.1uf polypropylene bypass - a WIMA MKP10. On the audio board, there is another 1,000uf decoupler for plus and minus near each channel's three ICs.
Audio IC Op-Amp Applications by Walter G. Jung, 3rd edition, published by Howard W. Sams and Company, 1987. This book contains extensive design information for active crossover networks, see especially pages 166-170.
A L.F. Garbage Filter by Walt Jung, published in Audio Amateur magazine, issue 4/75, pages 14-17. This article describes a low frequency filter, but also more general uses of the filter, including as a crossover.
A High Quality Power Supply Regulator for Operational Amplifier Preamplifiers, by Michael P. Sultzer, published in Audio Amateur magazine 2/80. This groundbreaking article discusses the importance of low impedance power supplies. A follow up article in 1/81 entitled Regulators Revisited adds a preregulator. Additional follow-ups and discussion are in Audio Amateur letters in 2/81 p.58, 3/81 p.48, 2/81 p.62, 3/81 p.54, 1/83 p.13, and 4/85 p.54. Also an enhanced higher current version titled A Wideband Power Supply by Jan Didden was published in Audio Amateur in 1/87.
Passive Crossover Networks - Active Realization of Two Way Designs, by Robert M. Bullock III, published in Speaker Builder magazine, 3/85 pages 14-19.
Schematic of the Dahlquist DQ-LP1 for reference. Click schematic to enlarge.
For more information on the DQ-LP1 and its operation, scroll down to page 13 in this Audio Basics Newsletter.