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The Sunshine
By Holger Kraft
09 Aug 1997
First thoughts about choosing the right drivers
To reach my goals I began playing around with the new (at the moment they are not very new any more) Audax HD-A loudspeakers. My first choice was the HM 170 Z0, the 6,5" HD-A Driver. Here in Germany there is a quite successful 2-way design with this driver available on the DIY-audio-market. It is the Esprit, which was developed by the German DIY-audio-magazine Klang&Ton. In this design the Aduax driver is combined with the HRA 531, a ribbon tweeter from Expolinear which is very similar to the Jordanow ribbon tweeter HRA 531. I auditioned this speaker in a local speaker shop but I was not to impressed. There are also a few features in that design I don't like at all. First is they use a very large serial inductor for the low pass of the crossover (they choose 1,8 mH) which causes that the sound of the speaker is a little bit boomy like the sound of all the other speakers on the market. Speakers with a boomy sound are the worst choice you can make for SE Triode Amps. Because of there low damping factor the bass response will be gained and the resulting sound will be even more boomy than with a normal solid state amp. The second property of the design I dislike is the use of a very critical 12 dB filter for the high pass. Because of this, the impedance drops to nearly 4 Ohm in the region of 4 kHz and I think this is far to low. I wanted to use a 6 dB/6 dB crossover because over the years I discovered that this sounds best to me. That means I have to use a tweeter which is suitable for a 6 dB high pass. I decided to use a Dynaudio tweeter because they recommend their drivers for the use with 6dB filters and I like the sound of the Dynaudio tweeters (I own a pair of Dynaudio Contour 1.3's and I am very satisfied with their sound, but the heights , the mids and the bass...., I could imagine a little progress in every frequency range). There are two tweeters that sound really good to me:
- first is the Esotec D 260
- and second the famous Esotar D 330-T
Every tweeter of this two has its special problem. The Esotec still has little problems with the sibilance. I hear this problem when I listen to my Contour 1.3 which has a tweeter that is on a midway level between the Esotec and the Esotar according to Dynaudio. The Esotar has only a "minor" problem: It is very expensive, especially here in Germany (around 670,- DM / piece). But the Esotar has a few advantages over the Esotec:
- First of all the sound quality. I heard this tweeter 3 years ago at the High-End in Frankfurt. The Dynaudio people made a presentation of the new Confidence 3 speaker and I am still very impressed by this presentation.
- The second advantage is, that the efficiency of the Esotar is 4dB higher than that of the Esotec and I need this gain in efficiency for the design ( I used a special circuit in the high pass crossover to correct the phase between woofer and tweeter and this circuit wastes 2 dB of efficiency).
After simulating around with the HM 170 Z0 and the Esotar D 330-T I recognized, that this two drivers do not cooperate in the best thinkable way. By this time I discovered the parameters of the HM 210 Z0 looked even better then those of the 170 Z0. The bigger chassis gives me an increase in efficiency around 2 dB and the frequency response showed a very good-natured behavior. After simulating for a few rounds, it seems that the 8" Aerogel driver only needs a very small inductor around 1 mH to give a reasonable frequency response. In connection with the Esotar tweeter the final result of the frequency response looked like this Fig. 1. Frequency response.
There has to be some bass too !!!
By now you only got a guess about the frequency response in the higher octaves. I have to mention here, that I am no bass freak at all. But every speaker should have some fundamental sub tones. Therefore and because the parameters of the Audax chassis point in this direction, I constructed a bass reflex cabinet. I know, this is not very creative and new, but I think a well done bass reflex cabinet performs better than a bad closed one. Because of the low damping factor of the famous single ended triode amps I decided to stay a little bit on the lean side of life with my bass reflex design. The result looks like the picture in Fig. 2. Bass response. As you can see on the graphic, the bass response reaches down to approximately 43 Hz. The netto box volume is around 47.5 liters and the tuning frequency is 35 Hz. This tuning frequency corresponds to a port with a diameter of 6.6 cm and a length of 13.0 cm.
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