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Definition Gold 900by Don McDonald 
My brother James and I are pleased to present our Definition Gold 900 project for your reference. For the last three years I have patiently reviewed many of the speaker postings on this site and at Audioasylum.com. James had a pair of 12" Tannoy Monitor Gold speakers that were not in suitable enclosures. We combined my research with his excellent cabinet building skills to create these speakers. The name given to our project was meant to include reference to the famous Gold series and the current 12" Definition series that are trapezoidal in shape. These drivers are over thirty years old and are still appreciating in value each year. As a bonus service is still available from TGI in Kitchener, Ontario. The dual concentric driver includes a multiple throat horn loaded tweeter diaphragm that is located at the rear of the alnico magnet structure. This high frequency driver voice coil is 2" in diameter and is wound with aluminium wire for low mass. The woofer cone also driven by a 2" voice coil has a high compliance large extension butyl surround that results in a 49.7 Hz free air resonance. The crossover is a physically separate sealed unit that provides a crossover at 1,000 Hz. It has two controls that allow the owner to customise the characteristics of the driver. The first control labelled "Energy" can raise or lower the relative output of this system above 1,000 Hz by plus or minus 2.5 dB. The other control "Rolloff" is a four-position switch that affects the slope and frequency response above 2,000 Hz with a total reduction of about 5 or 6 dB above 10,000 Hz. The objective is to tailor the speaker's response over a moderate range to suit room acoustics or personal preferences. Based on a published article from High Fidelity who measured the driver, it has a response plus or minus 4.5 dB from 65 to 15,000 Hz. Between 1,000 and 10,000 Hz the response was extremely smooth varying by only plus or minus 1.5 dB. There are a number of published driver modifications that have been performed by other audiophiles that include removing the dust cap, replacement of capacitors in the crossover, taking out the variable crossover controls and changing the internal wiring. We did not make any of these changes since we wanted to keep the drivers in their original factory condition. The Gold series was released in 1967 and produced until around 1974. The gold painted magnet cover and cast chassis basket can identify them. They were available in 10", 12" and 15" models. 
The cabinets are constructed from high quality baltic birch plywood with an internal volume of approximately 75 litres. There are 2 flared ports 50 mm in diameter and a 100 mm long that provide a port frequency of 48 Hz. The front panel is 1.5" thick and all other panels and braces are 0.75". The corners were fashioned from 2" x 4" pieces of red oak. A table saw was used to cut the compound angles and a router was employed to dado the edges. The exterior panels, driver opening and braces were all dadoed so that the cabinet formed a rigid shell even before being glued together. The software Quark Express was used to complete the designs and calculate the angles. The first cabinet took almost 2 months to design and build with the 2nd taking just one weekend to assemble. The driver is mounted from the front and the crossover is fastened on the bottom of the cabinet. The ports were assembled by gluing together 3" square pieces of plywood and routing the front using a V groove bit to achieve the flare. The cavity behind the driver is lined with 0.25" felt pads with all other interior walls covered with polyester fiberfill. Finally the back panel was designed to be removable and is attached using large head bolts with cross dowels. 
The exterior has been finished in four coats of black stain. A picture of the finished speaker will be forwarded at a later date. Baltic birch was selected over MDF since it is known for adding warmth to the sound. The trapezoidal shape and internal volume was both recommended by Alex Garner (Engineering) from Tannoy. Total cost of materials for the project amounted to $530.00 Cdn. (Oct, 2001). 
When we auditioned the finished product the speakers literally disappeared in the room. This large imposing cabinets could not be located with eyes closed. There was a large spacious and relaxed sound stage created in front of us. When we compared the new speaker to the older compact version there was a veil lifted from the midrange and the bass was not bloated. All interior sound reflections were adequately dispersed by the rigid braces and multiple angles. Since the drivers are now at ear level the high frequency horns are perfectly in balance with the lower frequency woofer. The overall sound is very sweet, smooth and balanced even with solid state amplification. They have a strong lower mid range response that imparts warmth and body to the sound without ever becoming unnatural. We could listen to the sound without any hint of fatigue and sometimes there was even an emotional response to the music playing. In conclusion, we would rate the success of this project very highly based on our subjective evaluation. We would highly recommend it to any other music lover that appreciates the Tannoy sound and has access to a pair of vintage Tannoy dual concentric drivers. The Thiele-Small Parameters are as follows:
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