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The Andromeda, or building a reference speaker
By Tony Gee
02 Apr 2000
The shopping-list
The quality of the crossover components used is very high. The Focal Audiom 13KX 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 with tin-foil anodes. All resistors are matched for minimum tolerance. Fig. 16. The filter scematic
- L1 = 4,7 mH transformer inductor, R = 0,14 (tolerance 5% - matched pair 1%)
- L2 = 0,33 mH air-core inductor 1,0 mm wire, R = 0,35 ohms (tolerance 5% - matched pair less than 1%)
- L3 = 0,56 mH air-core inductor 2,0 mm wire, R = 0,12 ohms (tolerance 5% - matched pair less than 1%)
- L4 = 0,82 mH air-core inductor 2,0 mm wire, R = 0,15 ohms (tolerance 5% - matched pair less than 1%)
- L5 = 0,82 mH air-core inductor 2,0 mm wire, R = 0,15 ohms (tolerance 5% - matched pair less than 1%)
- C1 = 3,3uF MKT polyester foil capacitor (tolerance 5%)
- C2 = 15uF MKT polyester foil capacitor (tolerance 5%)
- C3 = 33uF MKT polyester foil capacitor (tolerance 5%)
- C4 = 4,7uF MKP-Sn polypropylene-tin foil capacitor (tolerance 2%)
- C5 = 6,8uF MKP-Sn polypropylene-tin foil capacitor (tolerance 2%)
- C6 = 10uF MKP polypropylene foil capacitor (tolerance 5%)
- R1 = 6,8 ohms, 20 watts cemented resistor (tolerance 5%)
- R2, R3, R4, R5 = 3,9 ohms, 0,25 watts carbon film resistor (tolerance 1%)
- R6, R7, R8, R9 = 10 ohms, 0,25 watts metal film resistor (tolerance 1%)
- R10 = 10 ohms, 10 watts cemented resistor (tolerance 5%)
Wiring and connectors.
On my quest of minimalizing vibrations I decided to place the filters outside the cabinets in separate sand-filled enclosures. 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 though you can not 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. Fig. 12. A rear view of the system
One of the important things for a well defined bass is the ability of the amplifier to be able to use its damping factor as well as possible, therefore the resistance between the speaker and the amp must be as low as possible. The internal wiring for the Focal Audiom 13KX is made of double 2x4,0mm2 oxygen free copper (so 8,0mm2 for the plus and 8,0mm2 for the minus). The sub-woofer cabinet is connected to the amplifier with 2x 20mm2 oxygen free copper. This cable is designed to take up to 100A normally used in the high-end car hi-fi where high currents flow. Compared to a standard 2x4,0mm2 cable there is slightly more definition in the bass.
The Scanspeak 18W8545 and the Focal 120Tdx2 are wired directly to the connectors with Teflon insulated silver wire. This type of wire sounds more detailed than standard OFC, but you should only use it in combination with a smooth sounding amplifier. If used with a "bright" sounding amp the sound can get a little too clean some times.
Using thick cables means using large connectors. I used the ST-975GM by Monacor for each driver. They are gold plated and can take up to 20mm2 cable and have nice strong screw terminals. If necessary they can also take banana plugs. Fig. 13. The connectors
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