Dipole Bass

I finally got around to replacing the sealed box Bass with a Phoenix type dipole bass. Wow! What a difference. I have been very happy with the "Ugly" subwoofer but I was using it up to about 50 Hz and extending the two Dynaudio 24W75 9 inchers down to 50 was great as long as I didn't try to play very loud. When really stressed, they protested loudly, sometimes bottoming with a great clacking sound. Now that I have a dipole bass that will easily work up to 100 Hz I changed the crossover frequency up to 100 Hz were it belongs and am much happier. My Dynaudios are much happier as well.

This is a good place to give credit were credit is due. The concept of this woofer system is entirely due to Siegfried Linkwitz. The W-baffle Box design is his, the crossover circuit design is his, and I give him complete credit for the great sound coming from these speakers. Thank you Siegfried. All of the information about how to design dipole woofers and the specifics of implementing them are on Siegfried's web site at Linkwitz Lab

I started out with four of the Lambda Acoustics SB-12 sealed box woofers. These worked really well but there were times that even these couldn't do all I wanted of them. So eventually I upgraded to the Adire Audio DPL-12. These have the low distosion and high x-max that I need! More on them farther down this page.

As most of you know, I don't like to build square boxes so I got some 2 inch radius quarter round MDF from AIT and made myself a box with rounded corners to match my main speakers. I built these a little wider than really necessary so I could have some flexibility with drivers. The inside dimensions of the cabinet are 13 inches high by 19 inches wide. Then I made a removable U shaped piece that actually holds the drivers. Removing a few dozen screws allows me to remove the piece holding the drivers, change out the drivers with ease and then slide the piece back in. This works better than I imagined and will make driver changes or upgrades a breeze.

This is the front of the speaker with the driver carrier installed.

This is a rear view of the speaker. The inside of the cabinet is just bare plywood. I applied a little paint for appearances but nothing fancy. I would normally build something like this from MDF. It is easy to work with, easy to finish and is fairly inexpensive. Plywood is much more problematic. It splinters, has huge voids between the plys and shows all the grain when painted. I only used it here because I had it. Normal particle board or MDF are much better choices.

This is the front of the speaker with the final paint job applied. You can see some light streaks on the inside of the cabinet were the paint has been scraped off by the driver carrier being test fit. The outside of these cabinets was treated to a layer of 4 ounce fiberglass cloth and several coats of polyester resin. This is the same finish as my main speakers and eliminates the problem with shrinking and expanding that plague cabinets made up of different materials. The final paint was misted on from a spray can and produces a nice "pebbled" finish. The complete story about this finish is back in the section about the main panels.

Here is the carrier with the drivers installed. I attached a brace at the open end of the U to give it a little more strength when installing the heavy woofers. This could be a piece of all-thread and then it could just be left in place.

Here is the carrier ready to install. The brace has been removed. These drivers are the older model 4-spoke Lambda sealed box woofers. They work pretty well but are limited to about 3/4 inch peak-to-peak excursion before they get real noisy.

The carrier is exactly 13 inches tall and the inside of the box is also 13 inches. It is quite easy to slide the carrier in and out. I actually built the carrier first and then used it as a jig to build the cabinet. That way it has to fit!

I wired the speakers up to a Radio Shack dual banana plug that is screwed to the floor of the cabinet. I drilled and countersunk a hole right through the plug and then attached it with a short sheet rock screw. Beats the hell out of $50 binding posts! I had a few cans of "Wrinkle Paint" hanging around so I finished the inside of the cabinets with it. It is more trouble than it is worth, but looks cool in the right application.

Here is the whole system in my living room. I want to put the dipole woofers next to the floor-wall intersection but I don't really have one. Someday I will have a room dedicated to Audio and it will be symmetrical! I am really pleased with the sound of my system now.

Change to Adire DPL12 Woofers and Integration of Stryke350 Amp into Cabinet

After listening to the Phoenix-Type Dipole woofers with the Lambda sealed box drivers a few things became apparent. First, I really liked the sound of these woofers. It was the most natural sounding Bass I had ever heard. Second, the Lambda woofers made too much aerodynamic noise when pushed to extremes. These are some great woofers and are designed for a sealed box. When used in this application, they are fantastic until they get up to about 3/4 inch P-P excursion. Then the pole vent starts huffing and chuffing like "The Little Engine That Could". When these are in a sealed box as they are intended, this noise is never heard.

I started looking for a new set of drivers that would have low distortion and low aerodynamic noise at the same time with great excursion. Along came the Adire Audio DPL12 ( http://www.adireaudio.com/diy_audio/drivers/adire/dpl12.htm ). This is a purpose built woofer just for this kind of application. Very large pole vent and a vented spider along with an Xmax of 14.3 mm and a Fs of 16 Hz. Just what I needed. I also looked at the Lambda dipole woofers. These have great potential also, but are way more expensive, have less Xmax and as of 2003 will no longer be manufactured or supported. Nevertheless, I bought 6 of them and will build something that optimizes there capabilities in the future. I just don't think they are the right driver to bury deep inside a Phoenix-type box. They are beautiful and deserve to live out in full view!

I replaced the Lambda woofers with the Adire DPL12 woofers and immediately heard a great improvement. I was using a Definitive Technologies plate amp with about 250 watts output and variable boost at 30 Hz and volume control. Using LAud to look at the in-room frequency response and adjusting the controls I got a pretty good sound from this combination. It was not perfect, but sounded great. I used the boost to provide the 6 db dipole correction slope. Neither the high pass or low pass frequencies are adjustable.

Next I ordered two 350 Watt plate amps from Stryke Audio. These have variable crossover frequency, phase and volume. I hooked these up to the DPL12 woofers and using LAud equalized as best I could. This was a little better that before but was still not what I wanted. I have worked on these Speakers for nearly six years now and I wasn't going to compromise at this point. I decided to bypass all of the filter circuits in the Stryke amps and build a circuit based on Siegfried's crossover. I already had an active crossover for the Dynaudio Mids and the power supply for that would easily handle a few more op-amps.

Using Siegfried's circuit design and measured data from LAud I entered the data into CALSOD and came up with a circuit that worked for these woofers. It was mounted in the same case as the active crossover for the mids. The two stacked boards on the right are the left and right sub crossovers.

The measured frequency response using a microphone right at the front plane of the speaker cabinet and the actual filter response shows that the filter is doing just what it is supposed to do. The 6 db dipole correction is in the filter response but does not show up in the driver response since it is close mic'ed

These are plate amps and measure about 12 inches by 12 inches. There was no place to mount them on a Phoenix type W-frame subwoofer. What to do? Cut the amps into two smaller pieces and put them inside the woofer enclosure. Hot Damn! This might actually work!

I built an umbilical using some Molex connectors and spare wire so I would be able to disassemble the whole contraption later. I cut off the lower portion of the control panel and remounted the IEC Power connector to make the parts a little smaller. I am not using any of the speaker terminals still on the board, but it was easier to leave them on.

I bypassed most of the filter circuits built into the amp with one simple wire. The only thing left is the volume control. The jumper takes off at the R25/C13/U3A junction, which is just after the volume control, but before the phase and boost circuits. The resulting frequency response is flat out to about 10K. Plenty good enough for subwoofer duty.

The driver carrier, described earlier, was the scene for the mounting of the main part of the amplifier. It is really crowded in there but with a little trial and error the transformer just cleared the spokes on the woofer basket. It is not visible here but it is right behind the woofer magnet.

Side view of the driver carrier with the amp installed. Here you can see some of the details of the Adire Audio DPL12 woofers. The vents around the spider have screens over them to keep out mice and other small animals. Twelve narrow spokes and 1.25 inch pole vent.