Thứ Tư, 15 tháng 6, 2011


Đối với các loại máy dân dụng (Hifi), hay các thùng loa đơn giản, chúng ta thường áp dụng mạch LC (cuộn dây và tụ điện) để chia công suất phát ra từ ampli thành hai hay nhiều ngõ, mỗi ngõ áp lên một loại loa phù hợp. Tổ hợp này gọi là Mechanic Crossover (bộ chia loa cơ học). Tuy đơn giản và tiện lợi, nhưng nó có những khuyết điểm lớn: Không chính xác và bản thân nó cũng đã làm tiêu hao một phần công suất của ampli phát ra. Chính vì điều này, người ta phải nghĩ ra cách khắc phục những khuyết điểm nêu trên, và Electronic Crossover (bộ chia tần số điện động) và sau nữa Digital Crossover (bộ chia tần số kỹ thuật số) ra đời.
The Crossover
Due to the smooth rolloff of the drivers, the crossover can be made quite simple. Basically 2nd order filters are used except for the tweeter that needed a 3rd order filter to match the mid-driver and to produce a good phase profile. As can be seen from the lower graph above, the phase tracking between drivers is unusually nice. As shall be shown later, this was followed by actual measurements, where inverting polarity of the mid-driver produces broad and deep suck-outs in the FR response. Due to the 2nd order topology, the mid is connected with negative polarity and the tweeter has same polarity (negative) as the middriver.
LspCAD predicted response.
 The crossover is divided into two sections and the mid + tweeter is placed on the inside of the front panel.
The crossover for the two bass drivers are placed on the cabinet plinth as far away as possible from the mid-tweeter section.
All internal cabling is done with rigid, silver plated and teflon coated cable, Teflar 2, from 
Crossover for bass drivers. Place the coils as seen above to minimise interaction.
Glue the coils to the board by dumping them in a load of silicon glue or Superfix, the
latter my preferred glue for making crossover. In this way you are sure that nothing will
rattle inside the box once in place. Sourcing a rattling noise inside a finished box can drive you nuts!
Crossover for midrange and tweeter. The placement of coils may look a little peculiar,
but again the layout was chosen to minimise coil interaction.
Crossover components

Left: Impedance of all drivers in cabinet. Purple/black = 8543 in parallel. Green/yellow = middriver. Red/blue = tweeter. The impedance for the bass drivers are before the vent tuning was changed to 33 Hz. 
Right: Red = summed response of bass and mid. Nice suck-out at 850 Hz. Blue = bass and mid same polarity.
Left: Red = mid and tweeter with opposite polarity. Blue = mid + tweeter with same polarity. Right: Significance of mid attenuation. 0R, 1R0, 1R5, 2R2. 1R5 is preferred. The sonic impact of this is hard to ignore and don't miss the opportunity to hear what happens to the overall sound from such a small changes in driver attenuation.
Looking at the frequency response graph, you may wonder if this speakers isn't balanced to have an overall dark sound. It isn't. Despite the significant tilt downwards - a little more than the BBC dip - the speakers have a slightly bright and forward sound. In an absolutely non-aggressive way I rush to say. Due to the dispersion characteristics of the individual drivers, the on-axis response may look different from speaker to speaker despite my attemt to tonally balance the speakers in a similar way. The 13M driver has a much better sound dispersion in the 1-3 kHz region compared to a 6" going up to e.g. 3 kHz, thus needs a little further attenuation to produce an overall balanced sound. The tweeter, as always, has an attenuation until it only "dissapears", i.e. if you can still spot the tweeter in the overall sound, be sure it's playing too loud. The 9800 is very sensitive to this and changing the tweeter attenuation resistor from 2R2 to 1R8 is clearly audible.
Left: Step response from finished speaker. As can be seen: tweeter and mid with negative polarity. Bass with positive polarity. Right: Impedance of finished speaker. Vent tuning still 33 Hz here. 33-34 Hz in final construction. This is a 4 ohms speaker but an overall very easy load on the amplifier.