by RICHARD J. KAUFMAN
For those of you who share my fascination with audio and my pleasure in tinkering, I'd like to suggest a simple, low-tech project. Although it can be completed in a few weekend hours, the results will add a noticeable degree of convenience to your system. This project requires no unusual level of skill; you needn't have an advanced degree in engineering or be especially handy.
I have one auxiliary input on my preamplifier. To plug in a CD player, a Hi-Fi VCR, and a digital recorder resulted in a chaotic tangle of wires that I had to ... unsort before I could plug in the correct one and listen. It would be possible to use the tape monitor inputs for these sources, but I have two cassette decks plugged into them.
A perfectly good preamp without enough inputs must be a widespread problem since so many new high-fidelity products are becoming available. The solution is obvious too. If you don't want to trade in your preamp or receiver, you need an outboard switchbox. This sounds simple, but try to find one. It's not easy.
It seemed that the only solution was to build one. This project is a Radio Shack special. They sell a six-position, two-pole rotary switch (catalog #275-1386), which should be more than adequate. Although the switch has six positions, I only used five. A sufficiently roomy box (such as their #270-253), some phono jacks (#274-346), a knob, and some 22-gauge wire (or larger) are all the parts needed.
By using a metal box, which provides adequate shielding, it is possible to use single-strand wire instead of shielded cable.
Drilling holes in the box must be done with care, as it will affect the look of the finished project. Use a ruler and felt-tip pen to lay out the locations of the jacks and the switch (a water-soluble ink will wash off with a damp sponge). Then use a metal punch, or a hammer and nail, to dimple the site of the intended hole. This makes it easier to drill accurately. Start drilling 1/8-inch holes and work up to 1/4 inch for the jacks. You will need a 3/8-inch bit for the switch. Remove metal burrs from the holes with a rattail file. You will need a second hole, approximately 1/8 inch, for the stop tab that prevents the switch from rotating. Mount the switch and the phono jacks.
You are now ready to wire the unit. The two input jacks go to the two inner solder tabs on the switch. Examination of the switch structure will show which pairs of tabs to use for an output, and in what order to connect them. If the rotating wipers aren't visible, use a continuity tester or an ohmmeter to determine this information. Note: The ground lugs of all the phono jacks for each single channel should be wired together, to ensure a good ground connection. The two channels' grounds needn't be cross connected. Only the inner or "hot" leads from the jacks need to be connected to the switch.
To improve the appearance, you will probably want to shorten the switch's shaft with a hacksaw. A professional look can be had by labeling the switch positions and the inputs and outputs with dry transfers (catalog #270-201), but I found it more convenient to write the labels by hand with a permanent, fine-point felt-tip pen.
All in all, for a small investment in time and money, this project can add considerable flexibility and convenience to a good preamp or receiver which would otherwise be rendered limited by the multiple-input demands of today's systems. A little extra effort will yield a switchbox as attractive as it is useful.
PARTS LIST
- Metal box, approximately 3 x 5 1/4 x 5 1/8 inches (Radio Shack #270-253 or equivalent); one required
- Six-position, two-pole rotary switch (Radio Shack #275-1386, Mouser 10WA125, or equivalent); one required.
- Mouser version is sealed, with silvered contact; for ordering information, call (800) 546-6873.
- Panel-mount phono jacks (Radio Shack #274-346 or equivalent); 12 required
- One knob
- Dry-transfer project labels (Radio Shack #270-201 or equivalent); one set required
- Hookup wire (#18 to #22 gauge); 8 feet required
(adapted from Audio magazine, Aug. 1990)
Also see:
Simple Construction Projects--Build an Active Filter (July. 1988)
= = = =