Switchbox

A new project has finally made it up the list to be put on my site! This one was something a friend asked me to make for him. I'll call it a "switchbox" for lack of a better word; it's an input selector for your stereo system. Once built, it gave me a concept I'd like to use for some future equipment which I'll add in once I have pictures.

The idea here was to have a very high quality phono-level line switcher. Five turntables were to be switched into a single phono preamp.

The person asking me to build this had been looking at some ready-made items and wanted my opinion on them. The main problem I saw was the type of switches used: power relays for high current AC loads. One might think that such relays will handle switching low-level signals but there's another problem: there isn't enough current being commutated to keep the contacts clean in the long term. An unfortunate facet of most relays is that their contacts degrade with use no matter what. To ensure long life the manufacturer specifies certain conditions for use which normally include environmental requirements, the type of line and load to be switched, physical orientation, switching rate, voltage, current, etc. The current is a particularly important specification for open-contact relays as oxidation eventually builds and causes a loss of conductivity. The contacts are usually designed so that when switching sufficient current for normal operation they will "renew" themselves. Many who have had the speaker protection relay in an audio amplifier fail has noticed this effect and will find that turning up the volume during a dropout seems to restore operation. The voltage across the contacts breaks down the oxides and causes a sufficiently hot arc to restore a connection although not necessarily reliably.

In comes my choice of switches: I could have used solid state switches (bilateral for example), plain manual panel switches, etc. but instead I chose to stick with a special kind of relay. The solution to the bad contact problem mentioned above comes with Mercury wetted reed relays. Such a relay is constructed within a sealed glass vial filled with inert gas and a small amount of mercury. The mercury pools away from the contacts but also condenses on them enough to form a fresh contact surface for switching. No matter how small the current being switched, the contacts consistently renew themselves. The lifetime of a mercury relay is rated in the tens of millions of switch cycles, as opposed to open-contact relays which can be as low as tens of thousands.

The design of my first switchbox in the mercury-relay vain is inherently simple: five relays, sets of RCA jacks, a power supply, and something to control the relays. A nice touch was had by using capacitive touch-switches (excuse the pun). To produce an elegant form I chose a simple rectangular chassis and plexiglas faceplate. The faceplate is suspended away from the chassis a small distance via brass standoffs to give it a suspended look. Yellow LEDs behind the transparent face give a very appealing effect without being overstated.

With the layout arranged, I needed to come up with some kind of paint; all I had on hand were black and blue along with special primer I acquired specifically for this kind of work. I settled on a full coat of black for the entire chassis and then a coat of blue only on the top. I accidentaly neglected to mask all the way to the bottom on the sides so there is a faint blue overspray "perimeter" about the lower end of the chassis: it was left in place as it seemed to have some artistic merit.

Below you can see the end result of my work on this project. After delivering this unit to a very pleased customer I set out to make something similar for myself and came up with a pre-amp design I'm quite fond of.

 


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