Intro: STEP 1
A lot of hi-fi buffs believe that a volume control made from a resistive ladder gives the ultimate in reproduction. Trouble is they are expensive and hard to build. This project is neither. It can be easily built in a couple of nights, all you need is a drill and a soldering iron. And if you need at kit check out my website which features this and other diy audio designs you might find of interest.
The only compromise made in this design is to use 5db steps rather than the standard 3db. This means that the whole thing can be fitted around high quality 12W rotary switches. 3db steps are, in any case rather small. 5db steps are rather more definite nad when you get used to it more satisfying.
The parts you'll need. The resistors specified are 0.1% tolerance types. Although more expensive than the usual1% type the accuracy between channels is a direct function of resistor tolerance. Using the 0.1% types gives a channel balance of 0.01db between channels. About 50 times better than the most expensive audio grade volume pots!
Step 2: STUFF YOU'LL NEED
RESISTORS, 0.1%, 2 OFF EACH VALUE REQUIRED FOR STEREO
R1 = 11k5 R6 = 357R R11 = 47R
R2 = 6k65 R7 = 200R
R3 = 3k65 R8 = 121R
R4 = 2k05 R9 = 68R
R5 = 681R R10 = 33R
S1A(B) = 1 OFF 4P3W ROTARY SWITCH*
S2A(B) = 2 OFF 1P12W ROTARY SWITCH*
SK 1-8 High quality phono sockets, gold plated. 4 red, 4 black.
ABS CASE MEASURING 180mm x 155mm x 40mm
3 OFF 35mm PLASTIC KNOBS
- Make your life easier by selecting rotary switches with plastic shafts. After all you have to shorten them to fit the knobs.
Step 3: DRILL THE CASE
Having got all the above parts together the next task is to assemble the project. This starts with adding legends to the case. I used rub down lettering fixed with transparent nail varnish ( but dont tell the other half!) Next task is drilling out the case. ABS is a great material for this because its soft and is very easily worked. The cutting detail of the project case is shown here. Of course this can be used as a guide if a case with different dimensions is used. As you can see from the photo there's plenty of space in the box used.
Phono sockets come in all types and sizes. The ones used on the prototype required 10mm diam holes. These are the professional variety which generally means chunky and gold plated. Of course some people might consider this an expensive conceit, well I'm not going to argue the toss. Fact is they look good and I reckon that alone makes the extra expense worthwhile.
Having drilled the holes the next task is to screw all the fittings into place. At this stage though leave both 12way rotary switches until after you have wired them up. Otherwise profanity and burnt plastic might become the order of the day.
Step 4: WIRING UP
The accompanying diagram shows the wiring up of the device. As mentioned previously it's a good idea to wire up the 12way rotary switches before mounting them in the case. The resistors are daisy chained between the switch contacts.Having wired these up mount in the casein the case as shown. The whole device can now be wired up. OFC(Oxygen free copper) screened lead was used in the prototype. Straightforward OFC hookup wire (16/02) was used between S1 and the input sockets SK1/2/3.When finished the project should look like the photo.
Lastly the shafts of the rotary switches will need to be trimmed down to about 12mm long. The plastic types can be easily cut with a mini hacksaw. Now the task is done. Screw up your case and enjoy using your new vol control.