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This loudspeaker, which I ended up calling STYLE, was originally designed because I needed a small speaker that would play with my 300B SE amplifier. One of the areas where SE amplifiers excel is in the small details and the decay of the sounds. Sometimes this can be appreciated even better in small rooms at low listening levels. Therefore it seemed natural to design a small speaker to work with SE amplifiers in small rooms.

This speaker was one of the speakers I have demonstrated at the VSAC 97. It has around 90dB of efficiency with a very, very easy input impedance.

I believe that the very best performance with SE amplifiers can only happen when the whole system of amplifier, output transformer, speaker driver and box is very well matched. This requires the loudspeaker be designed for that particular amplifier, or less frequently, the other way around. This includes taking in account the amp output impedance, distortion levels, the transformer primary inductance and who knows how many other interrelated things! This may explain why most of the time it is more effective (and a lot more fun!) to let past experience, some intuition and mostly the ear do the whole job and find this correct match. After all, the ear must be the final judge

But although this speaker was designed to be used with my amplifier it should work very well with most typical SE amplifiers. The bass alignment is targeted for amplifiers with an output impedance of 3.0 ohms, and it will work well with amps with Zout in the range of 2.0 to 4.0 Ohms. At the frequency range this speaker is expected to reproduce, any primary inductance bigger than 10H (for a 3K primary impedance) will work well. This means you may use good quality small transformers and get good sound. This speaker was designed according to some ideas that are presented in Glass Audio (3/97 and 6/97) and that were the subject of my seminar at the VSAC 97. For its size and efficiency you will have surprisingly deep and controlled bass when used with a typical SE amp.

THE BOX

The box is very simple. It can be made with MDF. I have used 5/8″ all around and 3/4″ for the fascia. This fascia should have rounded sides. After some tests I found these materials just right for this box size, with no need to further bracing. If you want to try, thicker materials and bracing may be used. Remember to keep the internal volume constant. If you decide to use thicker materials please change only the depth of the box. The frontal dimensions should remain the same. The box should be lined with 1″ foam of low density in all sides except the front baffle. Of course you can also experiment with this until you get the best results. The duct should be made of a 5cm (2″) curved PVC tube about 18cm (7″) long. It must be a curved duct otherwise it will not fit. Using a smaller diameter will allow you to use a straight tube. This will work but it is not the ideal. Always keep the duct as free as possible. Do not allow the foam to block the area close to the duct.

THE DRIVERS

I have seen no projects with the Audax HM130X0, but this speaker with a TPX cone is a remarkable mid/woofer for our application. It has a very smooth mid range and just the right parameters to be used with high output impedance amplifiers. It probably has not been used frequently because the calculated ideal box for commom low Zout amplifiers will be ridiculously small and the speaker could only be used as a mid range. But this driver has the Xmax and power handling to be used as a small woofer. The Seas T25-001, with its silver coil and very good subjective sound was my choice for the tweeter. It has complemented the Audax in a seamless way. Although not cheap these are not terribly expensive units each one being around $60.

THE CROSSOVER

This is a mix of the minimalist approach with some impedance correction. The Audax unit is connected straight to the amp.. The impedance correction network keeps the impedance curve in the mids and highs very smooth.with 8 ohms being just about the average. The measured frequency response is really very flat being within ± 2.5 dB at most of the range.

I have used no diffraction compensation in the crossover. This makes this speaker sound better placed closer to the rear wall, as it is expected to sit in a small room. The best position is something that only experimenting in your own surroundings can determine. I should also recommend that you put them on 24″ to 28″ stands placing the point halfway between tweeter and the woofer at ear level. This is not very critical, but certainly helps. The frequency curve shows the response from 400Hz up, without any smoothig. This response changes very little with high output impedance amps. At frequencies below 400Hz the response is affected by measurement setup limitations and by the value of the output impedance of the test amplifier.

CONCLUSION

I believe you should built this speaker with all the normal care that high performance equipment begs for. The drivers and crossover components should be very well fixed. The cabinet should be very well made and the drivers should be mounted flush with the front surface.

I should say that this speaker is intended to be used with high output impedance amplifiers and that it will be bass shy with the average transistor amplifier or tube amps using lots of negative voltage feedback.

I hope you try this little speaker. It has been the source of a lot of great listening for me and I believe it fulfills its intent very well.

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Crossover components:
R1=1R5 L1=0.30mH C1=2.2 uF
R2=1R L2=0.44mH C2=5 uF
R3=15R

(c) 1997 Electronic Tonalities/VALVE and Eduardo B. E. de LimaOne pair of these loudspeakers may be built by the reader for non
commercial purposes only. No parts of this article may be reproduced
without permission.

WARNING: The Audax HM130X0 has been discontinued and is not
avaiable anymore. The HM130Z0 has been suggested as a
substitute but requires a different crossover and other changes. We
intent to make the plans for a revised version avaiable whenever it
is ready.

Eduardo B. E. de Lima
(VALVE 12/97)

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