My brother and I are pleased to present our Definition Gold 900 project for your reference.
For about three years I patiently reviewed many of the speaker postings on Audioasylum.com and several other audio forums. We 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 Tannoy 12" Definition series that are trapezoidal in shape.
These drivers are over thirty years old and are still appreciating in value each year. 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 aluminum 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 customize 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 which 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 those changes since we wanted to keep the drivers in their original factory condition. The Energy and Rolloff controls are used to flatten the measured on axis frequency response.
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.