Non-Metalic Chassis
NON-METALLIC CHASSIS MATERIAL
Conventional construction of our electronic devices built with the metal chassis and metal transformer containers are now being closely looked at as to why they came to be and the results of using these materials. Metal and labor were inexpensive after the war. Many uses for metal chassis were created as electronics transitioned from war to home electronics including TV’s, Radio’s and Hi Fi equipment. Metal containers for power transformers were used in industrial and military equipment because they stood up to harsh environmental situations. Metal chassis was used in the same way and so were considered the only way to go. They were strong, conductive (common ground) and helped with shielding.
In this era of high end audio with all its sonic improvements breaking away from the historical conventional construction techniques of audio equipment we are reviewing materials that are not metallic. Metal seems to be used for cosmetic purposes to increase the perceived value (and cost) of the equipment. So, the use of nonmetallic construction for the chassis and heavy components needs to be explained a little more.
Problems with metallic construction in audio equipment:
Transformers should be explained first due to it leading into the metal chassis problems. When you cover a power transformer with a metal container (ferrous or nonferrous), it will increase the primary current due to it’s becoming a shorted turn. This increased power is now in the container creating many other magnetic fields plus increasing the transformer heat rise.
This was known by the manufacturers during the war, who built industrial/military transformers. They compensated for it with special design considerations. Electric power was inexpensive and heat was of no real problem so acceptance prevailed. The 60 Hz field generated by the power transformer increases with its VA (power). A filter choke is also a transmitter; it generates a 120 Hz field. Both of these transmit via the metal containers and chassis.
One would think that the field generated would be shielded but in reality it is only redirected onto the metal container and redistributed in many complex shapes. If measured, they would be the sum of the original field. The metal chassis further redistributes these fields also. Audio amplifiers all have the same problem once the device is built - a hum problem. Even when the transformers are properly arranged in an uncoupled alignment on a metal chassis, there will always be some hum. Some hummers never get fixed correctly but become a number in the specifications.
All metallic construction will never cancel all the hum fields:
Using non metallic materials for transformer containers and the chassis will lower the overall hum problems measurably with proper alignment of all transformers. An undistorted transmitted field is cancelled much more efficiently (about 10db) at the audio transformer provided it is a like core mass as its opposite transformer. Fortunately, the audio output is usually the same core size as the power transformer so that the fields are identical in shape and magnitude.
This material can be tempered hardboard or similar materials. It is easily cut and fabricated. Using epoxies and other glues will add reinforcement to the physical integrity of the design. Your house is not a metallic container but it’s very strong. It is wood and nails and is constructed in such a way that an elephant could go upstairs (don't try this at home). One precaution is to reduce heat in such a construction with proper circuit design with a minimum of power resistors and proper fusing. This will also result in better and cleaner operation.
Last updated: Sunday July 8th, 2007 at 01:13:15 PM