Altec Lansing A7 VoTT Redesign+Build
NOTE: Complete drawing compilation is in progress. During this time, drawings will be moved to this page where they will be able to be downloaded.
The latest and current project is the redesign and build of the famous Altec Lansing A7 loudspeaker. This loudspeaker exercise will focus heavily on the efficient production of sound with the use of horn-loaded drivers.
The CAD model and design of the low-frequency speaker cabinet is still in progress, the picture below shows the current state. Once completed, the rendering will be made available on the Downloads page. First the original speaker will be modeled and then refinements will be applied to a copy of the model so that the two models can remain separate. Just like the original, the model of the modified version will also be made available. At right is a picture of an original A7.
Low Frequency Horn
The cabinet choice for the LF portion of the A7 is the 828B. This is modeled to the specifications as found on the original 1972 Altec 3D438-03 kit drawing.
Current state of model (Updated 24 March 2013). Click picture to enlarge.
Since this is a redesign of the original it is time to start distinguishing the improvements of my version over the standard 828B cabinet design.
The biggest improvements one can make to a loudspeaker enclosure, especially one of this size, is to make the structure as rigid and damped as possible. There are many ways of doing this and this redesign of the A7 will employ many of these practices. This project will employ a material change as well as several modifications to the structure of the cabinet. The common objective of these changes should be to reduce cabinet excitation by the drivers. Ultimately, coloration of the sound by the cabinet will detract from the overall listening experience.
The exclusive material used in the original A7 is pine hardwood for all components. This build will convert all components except for the cleats to MDF (medium density fiberboard). Due to MDF's weak pull-out strength the cleats will need to remain a hardwood material for fastening strength. The gain made by this material change is an increase in damping properties of the cabinet. The density of the material will increase from approximately 500 kg/m^3 to 1000 kg/m^3. Additionally, the overall thickness will change from 0.625" to 0.750", an increase of 0.125".
The structural changes that are being applied are numerous. The most notable change is being made to the internal baffle upon which the driver is mounted. While this component completely serves its purpose of holding the woofer, it does not contribute much in terms of cabinet rigidity. This component is of great interest in the case of solving cabinet rigidity as it lies almost at the exact center of the side panel, making for a great opportunity to expand the structure outwards to strengthen the cabinet.
Coloration is most likely to come from unsupported, large expanses of material used to form the cabinet. The more uninterrupted surface area the worse. In terms of the A7, the biggest offender is large side panels.
In addition to the side panels, efforts in increasing cabinet rigidity have extended themselves to the horn flare. Like the sides of the cabinet, they are expanses of thin material that are not braced and supported to the proper degree. Reasoning behind this is likely due to the fact that the flare brace is a very complicated part to produce. With the help of a CNC router, this problem of horn flare vibration can be easily fixed. My design calls for an increase of flare braces, totaling 8 per speaker.
High Frequency Horn
For this project, the JBL 2360A will be used with the A7 bin. Crossover will be active and set at 1,200Hz with an 18db/octave slope. The driver will be a Selenium/JBL compression unit - the D220Ti. Using the Selenium flange adapter and a custom tapered throat, the D220Ti will be able to connect to the 2360A horn.
The custom HF horn development has been put on hold and will be resumed at a later date.
Since the geometry of the original Altec Lansing 511b and 811b high frequency horns is not available and I will not be using original drivers, I will be designing an exponential horn flare from scratch. While it won't be an exact replica like the low frequency bass-bin, it will be a close match to the original. For example, I will employ a low frequency cut-off of 800Hz and it will use a exponential horn flare constant. I will also employ a horizontal dispersion angle of 90 degrees. Below are pictures of an original 811b sectoral horn.
This portion of the speaker is very critical, the high frequency system of this speaker (like many) is responsible for a majority of the sonic information delivered to the listener. With a lower cutoff frequency of 800Hz, its output will be over 95% of the total range of the speaker system. Getting this wave guide correct is paramount to the performance of this speaker. With that being said, the geometry of different flare constants, volumes and various other factors will be studied and simulated extensively in CAD and acoustic analysis programs such as Hornresp. Simulations will be posted as they are generated.
This is a basic model of the wave guide. This is used primarily to calculate cross-sectional areas of the 5 required sections of Hornresp. Additionally, this model will be used to develop the throat adapter for best shape, volume and the like.
Driver choice is difficult with the constraints in place. Constraints of this project are largely based on cost. The chosen driver for the HF section is the Selenium D220Ti compression driver. For the best results, this will also need to be modeled both physically and analytically. All of the parameters required to model the performance of the driver are not published by the manufacturer and will need to be obtained. This will be done by either contacting the manufacturer or performing the measurements on the driver themselves.
Threaded (1-3/8" 18tpi) to Flange-Mount adapter. This adapter will allow modern compression drivers to be retrofitted to older horns and wave guides that use a flange mounting system. This adapter is designed to be 3D-printed. However a part of this volume makes for an expensive pair of adapters.
The alternative to the above adapter is a combination of tapered billet aluminum adapters and flange adapters from Selenium.