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Construction pictures |
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The Opal speaker cabinets can be customised
The system on this page is an example of a commissioned Audiophile Opal system with 27in bass speakers. Yes you read correctly --- four 27in bass speakers --- The system is designed to fit within room corners, and surprisingly has minimum effect on consuming living space. The system height is designed to fit floor to ceiling 2.5 meters 8ft.
Each structure is made up with 3 cabinets. Two Bass boxes, one inverted on top of the other.
A 3-way centre box fits within the 2 bass boxes, as above.
Original Concept
From the original concept the architectural CAD drawings are created and a computer graphic of the final outcome as in the pics at the top of the page.
Cabinet Construction
Centre box for the JBL components. The mid-range horns for the 2in compression drivers are turned from a solid block of MDF. Small horns are also made for the hi-frequency slot radiators, to improve acoustical directivity. The mid and tweeter horns are then permanently fitted within the centre box baffle. This procedure insures that secondary acoustical resonances are zero, and only the music is heard.
Assembly of the large bass cabinets
These cabinets are painted with Mercedes metallic-grey auto duco. The centre box can be rotated 180deg, allowing the mid horn to be at the correct listening height, when listening from a seated position. Special commissioned Opals are completed and installed in stages. The first stage is the design and construction of the basic sections as above.
The second stage the system is shipped, installed and fitted with speaker components similar to the final classic components. This is done to avoid speaker damage, and check system performance for unseen cabinet vibrations etc. During the second stage the system is initially powered and tested with 4-way active solid-state amplification.
The third stage the classic driver components are bench-mark matched and fitted. This final stage also includes the change over from solid state to 4-way active valve amplification, including final detailed acoustical calibration.
Systems individually commissioned at this level are installed and functioning at stage 2, within a few months. However the final stage is often dependant on the time taken to obtain and assemble the specialised speaker components, and complete construction of the valve amplification.
Cabinet Construction
Speaker Components
The three way centre box contains the original 1970s classic JBL components. These components are now collectors items, and were made when engineering craftsmanship was at its height. The application of these components, honours the earlier engineers who's life passion was dedicated to excellence.
JBL 2220 15in speaker 100Hz - 800Hz.
JBL 2440 2in compression driver and horn 800Hz - 6kHz.
JBL 2405 slot radiator 6kHz - 20kHz.
The musical performance of these original components is outstanding. The design and impedance of these components was created for achieving the best performance with valve amplification, which was the standard for monitoring till late 1970.
Most Opals are assembled with modern high quality speaker components. However the opportunity to receive a commission to create a system with original JBL classic components is a dream come true. There are many variations of how Opal systems can be configured, and each application will give varying results which are tailored to the clients musical interests and of course the listening environment.
This system is capable of delivering the full dynamic experience of a symphony orchestra, as though it is actually present, including a 16Hz note of a 64ft organ pipe.
Comparisons of smaller systems can be argued. However the laws of physics govern us all; there is simply no right or wrong. But 'sad to say' for those who's unshakeable beliefs are attached to the latest model numbers of small audiophile 2-way speaker systems with magical cables, the musical experience of a hand crafted large 4-way active Opal system with classic 70s speaker components, will have no meaning.
Two 27in bass speaker cabinets 16Hz - 100Hz per side.
The above picture shows comparison between a 15in and a 27in Bass speaker
Large diameter speakers improve the coupling efficiency to the air at low frequencies and can easily reproduce the lowest note from a 64ft organ pipe 16Hz as sounding real. The 27in were made by Rick Randell 1980s and are capable of producing extreme sound levels which is not the purpose for this application The 27in bass speakers are very efficient 102db/mW, and they achieve the maximum level required with 10 Watts collectively. However 400W is available.
Valve Amplifiers
Each amplifier cabinet consists of 5 five 100 Watt Ultra-linear valve amplifiers. The amplifiers are mounted vertically. There is a separate amplifier for each speaker. The 2 two lower amplifiers drive the bass speakers and top amplifiers drive the higher frequency speakers; the order is logical to follow.
The amplifiers for the cone speakers (bass and lower voice) are in class AB which enables 100 Watts of power to be achieved. However the amplifiers for the upper voice 2in compression driver and tweeter are in class A which limits power to 40 Watts.
The efficiency of the compression driver and tweeter is approx 100 times greater than the lower frequency cone speakers and only requires 1/100 the power to achieve the same level.
The high frequency drivers are very sensitive to detail and it is essential for the amplifiers to be as close to class A as possible to achieve the lowest distortion figurers.
The 4 four way active crossover is at the top of the cabinet and the controls easily accessible. The crossover frequencies are calibrated to achieve the best performance from the speakers.
The level controls of the crossover are calibrated for free field which means zero room reverberation. The controls can be easily re-adjusted to give the best performance to suit the room environment and music.
The crossover is Linkwitz-Riley 4th order time aligned
Each 100 Watt amplifier is a separate module and can be changed easily if required. Each valve bias (temperature) can be easily externally adjusted New valves take approx 2 two weeks to stabilise.
The 2 two red LEDs (light emitting diodes) in front of each valve in the above pic are a comparator. When both LEDs are at the same light level the valve bias is correct. The amp modules are vertically aligned in the cabinet and therefore the LEDs are in the vertical position as in the pic on the right.
The middle LEDs in-between the valves indicate by flashing if a valve has failed and needs to be replaced. Replacement valves are readily available and each system is provided with spare valves.
The driver circuitry is solid-state which is absolutely silent and non-microphonic. This is essential for very efficient active speaker systems that use compression drivers. The microphonic and thermolic noise (hiss) generated by small driver valves would be audible and intolerable. The unique sound quality of valve amplifiers is created by the output stage (power valves and output transformer). This unique quality is not created by using small pre amp valves.
Valve amplifier construction
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Right pic shows output transformers and Electronic crossover →
Hi-frequency small transformer (500Hz - 50KHz) is at the top →
The 3 small toroid chokes mounted vertically above the large bass output toroid transformer are for filtering the 600V HT supply. ← Left pic shows the power supply opposite side of chassis. ↑ The small top toroid power transformer (+-20V and +-100V) is for the electronic crossover and driver circuits of KT88 output valves. Below the top toroid are the two medium toroid for the 12.6V DC valve heater supply of 8 Amperes. The DC heater polarity + - is reversed each time the amplifier is turned on. This insures valve filaments and cables are free from electrolytic deterioration. ← The 2 large toroid transformers create the 600V HT supply. The HT supply is constructed from 3 X 200V supplies in series. This enables a 400V supply for the 2 high frequency amplifiers in class A push pull. ↓ The power turn on management system is at the bottom of the chassis. All power transformers are toroid, to insure zero magnetic interference, and are also electrostatic shielded. Total mass approx 60Kg |
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Questions are often asked about valve amplifier class Push pull Single ended A AB B
Single ended refers to the 1 single output valve in small cheap radios and radiograms that were made before 1960. A single output valve is only capable of small power and produces a non-synchronous second harmonic distortion. Some audiophiles claim this distortion has spiritual qualities similar to magical crystals and crop circles.
Class A single ended refers to the 1 output valve needing to be at maximum quiescent temperature to obtain the small power available. This is similar to a one legged person riding a one pedal push bike. As it is not possible to pull up and push down on the pedal with identical energy, the output is non-synchronous.
Push pull refers to 2 two output valves, similar to a 2 cylinder auto engine, or a push bike with 2 peadls. Push Pull produces 4 times the power than single ended and has minimal distortion, due to being fully synchronous balanced. The correct way to make valve amplifiers, and push bikes.
Push Pull 2 two output valves is naturally efficient with minimum distortion. There is no scientific evidence that push pull achieves lower distortion by running the output valves at the maximum allowable temperature. However greater academic linearity is achieved by running the valves, with lower voltage and higher current, resulting in less power, insuring zero distortion. Sometimes referred to as class A push pull.
Class AB refers to push pull for audio application. The valves are run at medium temperature insuring there is an overlap where one valve takes over from the other for each half of the wave form. Increasing the temperature increases the overlap, referred to as Class AB1 and AB2. The latter has the least overlap.
Class B refers to push pull but not for audio application. In radio transmitters the valves are run at the minimum quiescent temperature (zero overlap) to obtain maximum power . If class B were used for audio there would be a gap between the top and bottom half of the music signal, which would be clearly audible as 3rd harmonic distortion.
Many of todays audiophile valve amplifiers are constructed similarly to the original designs that were made in the 1950s and 1960s. This is done for romantic nostalgia similar to grandfather clocks. However advances in modern technology and transformer design (toroid) has since enabled valve amplifiers to be vastly improved as in the Lenard designs.
Happy Opal Owner
A review of the Opals Canberra Opals PDF in Australian HiFi March/April 2007
written by Stephen Dawson www.hifi-writer.com
Opals and airships are number one in the book of dreams