The Proteus - To D' Appolito or not to D' Appolito, that is the question

By Tony Gee
27 Feb 2001

The cabinet.

If you have been designing and building loudspeakers for more than seventeen years, then by now you would probably have rather a lot of pieces of MDF left over. Seeing as I didn't want to build a very large cabinet I thought it was time to use some of these leftovers. All external walls are made of 22mm MDF except the top and bottom panel, which are made of 30mm MDF. Fig. 2. Image 2 A higher resolution drawing is available on request.

The central construction is formed by the reflex-port/tweeter cabinet combination. Fig. 3. Image 3 It can best be described as the capital letter "H" with two horizontal bars. The area between the two bars is closed off on one side. This forms a separate sub-enclosure for the tweeter to stop unwanted vibrations in the box reaching the rear of this unit, a piece of foam-rubber is pressed between the back of the tweeter and the rear of the sub-enclosure to stop any vibrations in the tweeters rear cover. The cabinet is internally strengthened with this "H"-part and by planks all made of 18mm MDF. There is no voodoo behind the different widths of the planks used in the photo; this is what you get when you use leftovers. Fig. 4. Image 4

Placing the reflex ports symmetrically to the left and right of the tweeter is a very wise thing to do. Seeing as it is a two-way system a lot of midrange energy will be coming out of the port. If you place the ports in the rear panel then this energy will arrive delayed compared to the midrange energy coming from the speakers. This will result in some blurring of the stereo image. As this is a D' Appolito design there are two ports placed centrally around the tweeter as the woofers are.

All internal walls except near the ports are covered with heavy-duty carpet tiles and wedge moulded foam to minimise vibrations and standing waves. The carpet tiles have a nice heavy backing that adds mass to the enclosure and they come in all kinds of lovely colours and patterns. Glue them to the MDF using lots of carpet glue. Adding a few screws helps hold them in place and stop them curling up while the glue dries. Fibre damping material is rolled up and placed directly behind each woofer. [fig.05_damping.jimage5pg] Depending on the positioning of the speakers in the room and personal taste this fibre can be rolled up densely or loosely to give a tighter and dryer or a fuller and warmer sounding bass. I used Monacor MDM-3 damping pads that consist of 2/3 sheep's wool and 1/3 polyester fibre and found that one bag per speaker was okay. If you have a small listening room or your speakers have to stand close to a wall; maybe some extra fibre placed in the middle of the cabinet will be necessary.

The baffle has two extra pieces of 30mm thick MDF mounted on it. Fig. 6. Image 6 The edges are routed with 4mm's at 45 degrees for looks and to remove the sharp edges that would cause baffle diffraction problems. The ends closest to the tweeter are covered with thick soft felt (the blue things) to stop reflections so close to the tweeter. The add-on parts have two main reasons. First of all they time-align the two woofers with the tweeter, the acoustic centres of the drivers are all in line with each other. Later on you will see that this only works for first-order networks. Higher-order networks give steeper phase shifts and therefore the thickness should ideally be changed. Secondly they reduce the amount of vibrations caused by the woofers reaching the tweeter. Between these add-on parts and the box there is a 2mm layer of felt that forms a sort of barrier for vibrations. Fig. 7. Image 7 These together with the internal bracing and the carpet tiles form a very rigid cabinet. For a single walled box the vibrations that can be felt on the outside, when playing them loudly, are minimal.

The total weight of each cabinet is 27 kg (61 pounds). To stand this firmly and level on the ground you need spikes. The cabinet rests on three spikes so that there is always optimum contact. I used three gold-plated SPS-10/GO spikes and protectors by Monacor. To bring the speaker to ear height it stands on a T-shaped stand, which is also spiked to the floor. This stand is made of three layers of Finnish birch plywood of which the middle layer has a large cut-out. This hollow space is filled with dry silver-sand to add mass and form a sturdy base for the speaker to stand on; the total weight of each stand is 14 kg (31,5 pounds). So that the speakers are in line at the listening spot the stand is angled 3 degrees upwards. You can of course build a straight stand but then it should be taller so that the tweeter is exactly at ear height.

The internal volume is about 32 litres and the bass-reflex ports are tuned to 45Hz. Fig. 8. Image 8 The images show the separate output for the parallel woofers and the reflex port as also the summed output. Fig. 9. Image 9

The crossover network.
Here it all boils down to the question: To D' Appolito or not to D' Appolito. Both have their advantages and disadvantages so I designed two series-filters, one for each. With a series-filter the electrical signal is evenly divided across the drivers because the drivers and filter are wired together in series as one total system. The pass-function of the components is an exact mirror image of the stop-function. The total voltage measured across the drivers is identical to the voltage measured across the amplifier outputs (minus the losses in wiring etc of course). For example if you take a basic first-order two-way series-filter consisting of a capacitor, an inductor, a woofer and a tweeter Fig. 10. Image 10 and you shift the cross-over point of the tweeter upwards by choosing a smaller value capacitor the cross-over point of the woofer will go up with it as the capacitor runs parallel to the woofer.

More information about series filters can be found at:
http://users.hunterlink.net.au/~mbbarg/Clayton%20Oxendine/clayton-oxendine1.htm
http://www.lcaudio.com/dele.htm

The D' Appolito version.
This uses a second-order series-filter, this isn't what Joseph D' Appolito originally specified but it seems to work. This must be a world premier, if anybody can find information about D' Appolito systems using series filters please let me know I would be very grateful. The whole filter is nice and simple using only four active components for three drivers but still giving a crossover with slopes of 15dB's per octave. The only extra components are the resistors to tame the tweeter slightly. Depending on your equipment and taste the series resistor can be anything from 1,5 ohms to no resistor at all. The inductors are air-core type using thick wire for low Rdc and no saturation and are matched within 1/100 of a millihenry using a LCR meter. The capacitors use metalled polypropylene foil and the metal film resistors are also matched for minimum tolerance. The tweeter is connected in reverse polarity. Fig. 11. Image 11

The non-D' Appolito' Appolito version.
This is designed as a 2-and-a-half-way system. The upper Seas CB17RCY/P and the Seas Excel T25-001 are wired together using a first-order series-filter giving slopes of 9dB's per octave and the lower Seas CB17RCY/P is connected using a first-order parallel network. Both woofers have an extra parallel RC-network to compensate for the rising impedance due to the voice-coil inductance. The lower woofer is filtered using an E-core transformer type inductor for low Rdc and high power handling. All other inductors are air-core type using thick wire for low Rdc and no saturation and are matched using a LCR meter. The capacitors parallel to the woofer use metalled polyester foil all the others use metalled polypropylene foil. The resistors use metal film with 1-% tolerance. Depending on your equipment and taste the series resistor can vary from 1,5 ohms to 2,2 ohms. I had the best results with 6 10ohm resistors in parallel giving a total of 1,67 ohms. Fig. 12. Image 12

Wiring and connectors.
I decided to place the filters outside the cabinets in separate enclosures to keep them away from the woofer magnets so that the inductors will function properly. This also has the advantage that you can choose the right cable to run from the loudspeakers to the filters that are placed near to the amplifier. Even thou you can't bi-wire a series-filter, this way you can still have the advantage of bi-wiring - a long section of wire from the speaker to the filter and a short little bit from the filter to the amp.
Each driver has separate connectors on the rear of the cabinet. I used the BP-500G by Monacor. They are gold plated and can take up to 6mm2 cable, spades or banana plugs.

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