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Coral Beta 8

Pair of Coral Beta 8 - 8 inch Fullrange Drivers

With its unique sound signature Coral Beta 8 gained a status of legendary and highly desirable full-range driver in the audiophile circles.
Coral beta 8 features specially treated paper membrane with enforcing polymer fibers, copper coated aluminum voice coil and unique star shape phase plug.

Two versions of the Coral Beta 8 driver

They are two versions of the Coral Beta 8. The first production (70's) was rated 16 W, and the later (80's) was rated 35W. Except the labels itself, I found only two differences between the versions - the magnet of the later production has about 4 mm larger diameter and there is a slight difference of mounting the connecting wires to the voice coil, which is completely the same for both versions. The measured difference between the versions is the same as the difference between two drivers of a same version.

This is the manufacturer's datasheet in Japanese Link To Coral Beta 8 Manufacturer's Datasheet

Coral Beta 8 Datasheet in English

There is a misunderstanding concerning the sensitivity of this driver. The sensitivity, given in the datasheet has nothing mentioned about the measurement conditions - input voltage and a measuring distance. In fact the sensitivity does not exceed 94 dB/W/m

My Measurements

Driver Properties Thiele - Small Parameters
Chassis material Vacuum-molded aluminum alloy   Parameter Measured Datasheet
Membrane material Enforced Paper
 Fs 42 Hz 30-45 Hz
Magnet System Ferrite type FDX4 Re 6.6 Ω N/A
Surround Cloth Qms 3.60 N/A
Voice Coil Diameter 25 mm Qes 0.38 N/A
Nominal Impedance 8 Ω Qts 0.285 0.5
Sensitivity 94 dB/W/m   Vas 46.8  ltr N/A
Rated Power 30W   Le 0.27 mH N/A

                                       

This driver is suitable for back-loaded horn applications.
That's why I included a measurements in so-called "Universal" Horn.
More details about this project can be found here.

Coral Beta 8 responce at 1m on axis and 15 and 30 degrees of axis in 18 ltr sealed box Coral Beta 8 responce at 1m on axis and 15 and 30 degrees of axis in the Universal Horn

Frequency response at 1m with 2.83 V input RED - on axis, BLUE - 15 º off axis, GREEN - 30 º off axis


Second, Third and Fourth Harmonic of Coral Beta 8 in 18 ltr sealed box Second, Third and Fourth Harmonic of Coral Beta 8 in the Universal Horn

The second, third and fourth harmonic curves measured at 2.83V input RMS.



Burst decay Responce of Coral Beta 8 at 1m in 19 ltr sealed box Burst decay Responce of Coral Beta 8 at 1m in the Universal Horn

Burst decay response, measured at 1m on axis


The impedance of Coral Beta 8 in free air and in 18 ltr seled box The impedance of Coral Beta 8 in Universal Horn

The impedance curves


Coral Beta 8 crossover schematics

In some setups the trebles of the Coral Beta8 based system are too bright. Some listeners found this effect beautiful, but some prefer flatter response in the high-frequency range, specially when the driver is mounted in a bass reflex vented enclosure.
For an application like this a simple equalization network can be applied:
R1 = 10 Ω
C1 = 1.0 µF
L1 = 0.22 mH

Coral Beta 8 with crossover at 1m in the Universal Horn Coral Beta 8 with crossover in Universal Horn

The affect of the equalization network is shown on the graphics above.

Conclusion

Coral Beta 8 has a bright details at all the listening levels. It can be matched with a wide range of single-ended and push-pull tube amplifiers. Unfortunately nowadays is not easy to find a pair in NOS condition at affordable price.

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