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The Sunshine
By Holger Kraft
09 Aug 1997
The crossover construction
I mentioned above, I am a fan of 6 dB/6 dB crossovers. Because of this I decided to build one Fig. 3. The crossover. The major advantage of this filter type is that everything, like phase, frequency response and complex impedance is very smooth and the whole filter behaves very good-natured. One big disadvantage is, that the two drivers work in parallel for a long frequency interval . Therefore you have to take special care about the phase and because of this, I added some phase control circuit. I used a circuit very similar to the phase correction circuit used by Dynaudio. But take care: I connected the tweeter in the reversed polarity than Dynaudio! (This means that the tweeter in my approach is not reversed at all!!) The second big difference is the corner frequency of the circuit. Dynaudio uses in all their designs a corner frequency around 2 to 3 kHz. In opposition to this I use a corner frequency around 500 Hz. The only problem with this is, that it leads to bigger and more expensive component values. The advantage is, that the phase behavior of the tweeter and the woofer are nearly the same. This means, the impulse answer is better than the approach with the reversed polarity tweeter used by Dynaudio.
How the phase control circuit works
I will try to explain how the phase control circuit works (together with the whole design of the Sunshine). As you can imagine, when you are a experienced speaker builder, the cabinet and especially the placement of the drivers on the cabinet is very important for a perfect addition of sound between the two drivers. If you look at the picture below, you can see that the kind of placement used in the design of the Sunshine provides, that the way from the tweeter is approximately 2 cm longer than the way from the woofer to the ears of the listener. This works for approximately 2 meter listening distance Fig. 4. Distance from drivers to ear.
As you can imagine now, the crossover of the Sunshine is optimized for a listening distance of 2 meters and a ear level of 1 meter. This should be average values for most conditions. Now to the phase control circuit. As you might know from school or university there is a special way in AC-technology to display current and voltage. This is called a pointer diagramm. Two alternating voltages are displayed as two pointers rotating counterclockwise. The phase between the two pointers corresponds to the angle between the two pointers. You can translate this directly to the problem of two drivers in a speaker. Of course you would like the two pointers in a speaker to point in the same direction for a perfect addition of sound. Nature is cruel! A low pass always gives you a shift in the clockwise direction. A high pass gives you a shift in the counterclockwise direction. If you now add some phase shift from the drivers itself, a woofer always tends to add some more clockwise shift and a tweeter with ferrofluid tends to give you zero or counterclockwise shift. This means, the situation gets even worse. If you now consider, that in most designs the tweeter is located on top of the woofer, you have to add some more phase shift in the crossover region, corresponding to my little drawing. This means the situation gets even worse than before. In the design of the Sunshine I tried to get rid of this problems as good as possible. I did this with two main design features:
- First of all I optimized the Sunshine for a listening distance of 2 meters. This covers the problem of aligning the acoustical centres of the woofer and the tweeter. It works in the way explained in the drawing above.
- Because I do not like tweeters connected in reversed polarity to restore the phase alignment of the drivers, I use a phase control circuit. The tweeter connected in reversed polarity does not really restore the phase alignment of the drivers anyway. Only the average sum of the tweeter and the woofer, the amplitude answer will look OK. In reallity the tweeter will begin a half period earlier than the woofer to radiate a special frequency and, of course, the woofer will end to radiate a special frequency a half period later. The phase control circuit slows down the tweeter. This means, that there is no phase shift at very low frequencies and 180 degree phase shift at very high frequencies.
As you can see on the schematic Fig. 5. Response of the phase control circuit I designed the circuit to have a corner frequency around 500 Hz. That means a phase shift of 90 degree ( 1,57 ) at 500 Hz. Like I mentioned before I slowed down the tweeter. Therefore the real phase shift is in the negative direction ( The possibility with the to big and to expensive condensators and inductors described below will alter the shape of the curve, but not the corner frequency ).
Dimensioning the phase control circuit
If you look at the crossover schematic you will find the phase control circuit behind the filter condensator (L2, L2', C3, C3'). To provide perfect working conditions for the circuit, you have to take care for a linearized tweeter impedance. Also the 6dB filter will love the linearized tweeter impedance. The linearized tweeter has an average impedance around the DC resistance of the tweeter voice coil. This DC resistance will be the starting value for calculating the phase shift circuit. First step now is, to choose a corner frequency: Lets take 500 Hz like in the design of the Sunshine. The second step is to calculate an inductor and a condensator which have the same resistance like the DC resistance of the tweeter at the corner frequency. The calculating formulas are:
C = 1 / (2 * PI * corner-frequency * DCR-tweeter)
L = DCR-tweeter / (2 * PI * corner-frequency)
With this formulas and a DCR of the tweeter from around 5,3 Ohm you end up at a value of 60 uF for the condensator and a value of 1,68 mH for the inductor. For the Sunshine I chose 60 uF and 1,5 mH for the part values. You can design the circuit with the inductor and the condensator having only the half value of the DCR of the tweeter, but then you will end up with 120 uF condensators and 3,4 mH inductors. This seems to be a little bit to expensive, even for me, and a little bit to big too.
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