Horn/dipole speakers and amplifiers

By Thomas Dunker
01 Jan 1996

Hello all!

I have scanned a few pictures of my current projects (which have been built and partially conceived at Omega Verksted, the hobby workshop for the EE students at NTH, Norwegian Institute of Technology) and I'll briefly describe the pictures in this file. All the pictures are color jpegs, scanned on a SHARP JX-300 300 dpi scanner, converted from 24-bit TIFF to 8-bit jpegs with a compression factor of 75%.

Fig. 1. april94.jpg shows the experimental setup I am using right now. The amp on the floor with all the glass on it is my 6AS7G OTL (Bruce Rozenblit design from GA) which powers the bass dipoles only. This is a stereo amp!! To the left of the OTL amp is my Pass Zen single ended Mosfet amp which you can see better in the file named "zen.jpg". The system is too big (or is the room too small?) to fit in the picture. I live on the fourth floor and didn't want to climb out the window for the sake of a photo. So you see most of the speakers, with the dipoles on the bottom and the tractrix horns on top. To the right of the rack is an ugly line conditioner I built out of an old modem enclosure and a Schaffner 30 amp industrial filter. Visible above the amps is my cheap and slightly modified Sony CDP-797 CD player. Above that is a pile of junk that I hesitate to call a preamp. It has a noisy, microphonic 6CW4 phono stage designed by Eric Barbour and built by yours truly, and a PAS-style cascade 12AX7 line stage with 20 dB of gain in it. From here the signal goes into the next junkpile which is on the shelf above the 'preamp'. This junkpile, resting in a black box (!) is filled with a number of unspeakable devices which take care of the 500 Hz 3rd order xover job. The low pass signal goes to the OTL amp and the high-pass via a stepped attenuator and a buffer into the Zen amp which powers the JBL 2461 drivers on the tractrix horns. This system plays music! With SE amps and a new tubed xover and a new Siren Song phono stage it can only get better. Yesterday I moved the system across the room, but essentially it looks the same and sounds the same as it did as I took the picture.

Fig. 2. dipole01.jpg is a rear view of one of my bass dipole arrays. Twelve out of the 16 modified Audax 8" woofers are visible. The drivers work in a push-pull sort of way to create a Heil-style loading effect. My friend Torbjoern Lien came up with this idea, and he also sold me the drivers very cheaply. Two drivers work into each of the eight cells. The system can be configured for an impedance of either 2, 8 or 32 ohms by rewiring the four four-in-series strings in different ways. Efficiency is some place between 95 and 100 dB/1W/1m. The plan is to cover the back of the speaker with some nice cloth to keep dust away from the exposed voice coil gaps.

Fig. 3. fullflip.jpg shows one of the dipoles, with horn on top, with both the foldable sections fully 'flipped out'. Then I have a baffle which is 94 cm tall and about 115 cm wide. The divisions between each of the eight cells are visible through the cloth. This is where the front radiated sound comes out.

Fig. 4. halfflip.jpg shows one of the dipole woofers with the first foldable secton folded out and the rest pointing backwards. Now the flat front baffle is 70 cm wide. I made the hinge arrangement so that it's possible to play with different arrangements for different rooms and different setups. The sonic differences are not dramatic as the 'flaps' are rearranged. I get wonderful bass out of these systems using an EQ incorporated in the crossover, which has a corner frequency of 66 Hz and lifts bass amp output by 1 dB/octave from this point on downwards. I have good output down to something like 35 Hz and the rolloff is comfortably gradual.

Fig. 5. horn01.jpg shows one of the horns from the rear. Each horn is made up of a sandwich of fifteen 20 millimeter particle boards (not MDF, it was plenty of work with coarser particle board and each horn weighs 20 kg. less driver) Turning 30 pieces of particle board into two tractrix horns in this manner is no small job, believe me. The results literally speak (sing!) for themselves, however. I used an adapter (which actually was part of a lens assembly called 2031) to mate the JBL 2461 1" driver with the circular throat of the wooden horn. This is visible on the picture. There's a pretty serious time alignment problem in this setup, but when I thought of the WE 15A 'ram's horn' in the L'Audiophile systems I decided to chill out and hope for the best. I was not disappointed.

Fig. 6. horn02.jpg shows one of the horns close up from the front. The mouth measures 24 cm x 48 cm, so the mouth cutoff is approximately 230Hz. The tractrix is purely Edgar style all the way back to where it measures about 12 x 12 cm, where it begins to approach a circular shape towards the mouth. The wooden part of the horn is 30 cm deep, and at this point it has a 6.5 cm diameter circular cross section which creates a smooth transition the 2031 adapter. This part of the throat was a lot of work, but I'm very happy with the final result. After I was happy with the smoothness of the bare particle board, I used latex spackle, then polyurethane varnish and finally black spraypaint.

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