Anyone who has ever tried to power a piece of high-gain audio gear with an ordinary department-store "wall-wart" knows that the results are usually awful. Because wall-warts are designed to be sold as cheaply as possible, they contain only the barest minimum of filtering. The ripple current that gets through shows up in an audio output as hum--unpleasant at best, and often intolerable. The solution for powering stompboxes and other portable audio devices is a power supply that is well-filtered and tightly regulated: a Small-Wart.
This article describes the design and construction of the Small-Wart 60. This Small Wart delivers up to 60 ma. and has a "dying battery" feature. It is ideal for experimenting with boosts and distortions, or for powering a few pedals on a small board. It can be built in a metal case that will take the abuse of a stage environment, or in a plastic case that is easy for a beginner to tool. Both versions are small and very physically rugged, and have a detachable AC power cable for easy storage in a gig bag. The outputs are physically isolated from the chassis, so different grounding requirements can be accommodated easily. I have incorporated a resettable fuse for short-circuit protection, and "semi-fixed" trimpots for the voltage level, set up so that they can be adjusted from outside the case.
Because they are powered from the AC line, neither Small-Wart is suitable as a first project for a complete novice. In particular, making correctly the connections to the primary of the transformer and insulating them carefully takes some skill and a lot of care. Especially if you do the metal version, you should have some experience with tooling and painting a case. If you have built a couple of simpler projects successfully and now want a line-operated power supply for them, a Small-Wart may be the ticket.
Before I go further, this is a note to builders of my earlier metal-cased versions that have the two-wire AC line interface: Modern, fail-safe, construction practice is to run a ground lead from the case to a known, solid, building earth. You can do this by adding an external ground lead. I modded one of my bench units in this way by first boring a 3/16" hole slightly below and to the right of the mounting screw for the power inlet and lining it with a rubber grommet. I made a knot an inch from the end of a length of stranded wire (can be #18 or #20) and ran this through the grommet. Then I bored a 1/8" hole just above the grommet and installed a solder lug and a 4-40 screw and nut. For mechanical stability, I locked the threads with solder (Loctite is also fine) before I soldered the ground lead to the lug. Call me paranoid: I knotted the ground lead on the outside close to the grommet so that it has strain relief on both sides. Connection arrangements at the grounded end will depend on your individual situation.
It is also possible, though much more difficult, to remove the two-wire AC inlet and replace it with a three-wire power inlet and line cord. If you bought a kit and want to do either of these modifications, e-mail me and I will provide the components.
A note to more experienced hobbyists: While I describe the construction in unusual detail to guide beginners, the methods and techniques shown are meant to spark your own creativity. I hope you find some of my ideas useful!
The model 125-B case shown above has a screw-on lid, and its sides are ribbed; this construction both makes it extremely strong and provides slots for mounting small printed circuits or perfboards. I used the slots to hold a piece of perfboard on which the transformer is mounted. The board is secured with a few drops of epoxy cement, and the resulting assembly will take considerable abuse if it's built carefully.
The clover-shaped hole for the 3-wire AC inlet must be cut accurately, both for safety and esthetics, so I have specified a particular part for the inlet and created an actual-size drilling template. You can download Figure 7 from here. Cut out the template, and tape it carefully to one end of the box. Drill two 3/8" starter holes. I used a Unibit, but ordinary twist drills and/or a reamer are also fine. Drill the third pilot hole to 3/16". Don't be tempted to make this one larger; you can lose control of the drill when it punches through! Cut or file out the center section.
How you proceed from here depends on the tools you have. It is possible to do the rest of the cutting by hand with round files, but a rotary tool does it much more efficiently. You can use a Dremel high-speed cutter, or the tool shown here is a EuroTool "inside-ring" burr #BUR970.00. The power inlet should fit snugly into the cutout, and you can then drill the two holes for the screws that secure it. To get a flat purchase for the hardware inside, grind or file down the ribs. My burr did this in seconds, but you can also use an abrasive wheel or hand files.
The Output Jacks
You have choices about: how many jacks you include; what types, and where they are located. You will want at least one for the +9 and one for the "dying battery" output, and the standard "BOSS-style" plastic jack is definitely the easiest to work with. Figure 13 download from here is an actual-size drilling template for mounting two of these on the end panel opposite the AC inlet. In the same general way that you did the cutout for the AC inlet, tape the drilling template to the end panel, drill the holes and grind or file down the ribs inside.
Some people just don't like the appearance of those plastic jacks. It's possible to use the ones that have a metal bushing, but for many applications you want the bushing insulated from the case. Here are the techniques I used to create a "platform" for mounting them using fiberglass auto-body filler (Bondo is a common brand). Figure 15 which you can download from here is an actual-size drilling template for up to three jacks on the end panel. Fasten this template in place, drill out the holes with a 1/4" drill and ream them to size. The holes are actually larger in diameter than the jacks, and you'll see why in a moment. Prepare the end of the box by sanding it thoroughly on both sides and cleaning up with acetone. Then seal off the fronts of the holes with transparent tape as shown in Figure 16. Mix a small amount of Bondo according to the directions on the can and, as shown in Figure 17, spread it over the entire area. It must fill the holes and also cover the back to a depth of about 1/16". Let it cure overnight and then sand smooth.
Using a 1/8" drill and then a reamer, create the new holes for the jacks (Figure 18). Make sure that the jacks fit snugly. Sand the rear surface thin enough so that the bushings show out front, and you can screw on the washers and retaining nuts (Figure 19). Use an ohmmeter to make sure that there is no continuity between the metal bushing of the jack and the box.
Once the holes for the output jacks are done to your liking, locate and drill the hole for the on-off switch. As shown in Figure 20, with the AC inlet screwed temporarily in place, visualize a vertical line connecting the second rib on top and the second rib on the bottom. With a marking pen, mark a point about 1/16" to the right of the center of this line, directly in front of the inlet. Use a center punch or scratch awl to mark the center of the hole, and drill it. I have specified a subminiature switch that requires a 3/16" hole, and I strongly suggest using this particular one to conserve space. Make sure that the switch fits the hole snugly.
Most people will want a pilot LED to indicate when the Small-Wart is on, so you should drill the hole for whatever bezel or LED holder you want to use. I located this on one side of the screw-on lid near the output jack. The chrome bezel shown (and included with the kit) requires a 1/4" hole. Here's a drilling template.
At this point, with all hardware removed, the case can be painted. I usually do it by sanding and cleaning up with solvent, applying a primer and then a couple of coats of spray enamel, and baking for an hour at 150 degrees in a small toaster oven that I keep for the purpose. Finish with an automotive clear coat. You can find detailed "recipes" for finishing metal in many on-line DIY references.
Securing The Power Inlet
The AC inlet goes in first. This fixture must be held rock-solidly in place and not have any "play" that would permit it to work its way loose when the power cord is inserted or removed. If you have a riveting tool, this is a good application for it. I didn't, so I created the equivalent of rivets by using solder to lock the threads of the two 4-40 mounting screws. Remember to add the solder lug for the ground connection under one of the screws before you lock them in! I have a 35-watt iron, and I heated each screw thoroughly with it before applying solder and letting it melt into the crevices between the threads. Figure 22 shows the soldering process. When I was done, I used a cutoff wheel on my Dremel tool to shorten the screw ends and an abrasive stone to remove sharp edges. WEAR GOGGLES FOR THIS! THE PIECES CAN AND DO FLY LIKE BULLETS! Make the solder connection between the solder lug and the ground pin of the AC inlet. Carefully fasten a rubber foot at each corner of the bottom of the case. Solder short connecting leads to the switch, insulate them with 3/32" heat shrink, and install the switch. Make the connection from the center switch terminal to the power inlet. Figures 23 and 24 show what the result looks like.
Now you can continue with installing the power transformer.
The Hammond model 1591C box has a screw-on lid, and its sides are ribbed; this construction both makes it strong and provides slots for mounting small printed circuits or perfboards. I used the slots to hold a piece of perfboard on which the transformer is mounted. The board is secured with a few drops of epoxy cement, and the resulting assembly will take considerable abuse if it's built carefully. This version is fairly easy to tool, and it uses a two-wire AC line interface, which is more convenient in many situations. However, if you build this way, to avoid any possible shock hazard, you MUST NOT have any metal parts penetrating the case. The retaining screws for the cover don't count, because the design of the box isolates them entirely from the circuitry. But the exposed parts of the power switch, LED bezel and power jack(s), and any screws that retain them, must be plastic. My kit for this version includes the case and appropriate components and hardware.
To provide a solid mounting surface for the AC Inlet and the output power jacks, I beefed up the end panels of the case by filling in the space between the ribs. Figure 37 shows how I did this with Bondo auto-body filler. I have also successfully used quick-setting clear epoxy, and some of the subsequent photos show a prototype that was done with that material. Before you fill, sand the area lightly to roughen it, and clean up with alcohol. Mix a small quantity of the filler and spread it thoroughly with a paddle. Use enough to just cover the ribs. Let the filler cure for a couple of hours.
When the filler has hardened, download the drilling template for the AC Inlet. Cut out the template, and tape it carefully to one end of the box. Create a 3/8" starter hole roughly in the center. I used a Unibit, but ordinary twist drills and/or a reamer are also fine. Work slowly and carefully to avoid cracking the case or the filler material. Enlarge and widen the hole to its oval shape using round files, or, preferably, a Dremel or similar rotary tool with a cutting burr (Figure 39). The AC inlet particularly must be mounted securely for safety reasons, so sloppiness can't be tolerated here. DO NOT USE YOUR SOLDERING IRON for this job! If you don't have the right tools, borrow or buy them! The back of the inlet should fit snugly into the cutout. You can then use a scribe or scratch awl to mark the positions of the mounting holes and drill them. Secure the inlet in place with 4-40 threaded studs and nylon screws. I tried regular nuts for this and wasn't happy; they don't have enough threads to properly grip the screws.
To lock the inlet in place, mix another batch of the filler material that you used on the end panels, spread it around the edges of the inlet and the studs, and give the whole thing a few hours to cure. If you do it carefully, the result is something that might have been custom-machined. Figure 2 above shows the result when using Bondo. Figure 42 shows another prototype that I did with epoxy cement.
The Output Jacks
You don't have a choice here; use the plastic ones. You can download from here an actual-size drilling template for mounting two of these on the end panel opposite the AC inlet. In the same general way that you did the cutout for the AC inlet, tape the drilling template to the end panel and drill the holes. A Unibit does this in seconds, or use twist drills and a tapered reamer (Figure 44).
The Power Switch
I used a particular miniature slide switch for the power switch. Other types are possible, but remember: NO exposed metal parts! The slot for the actuator of the switch is 3/8" long and 5/32" wide. I located it by visualizing a horizontal line connecting the second rib on each side of the case. Then I marked the center of this line and drilled a 1/8" hole as shown in figure 45. Using flat and round files, I lengthened and widened the hole to form the slot. Once the switch actuator could move freely back and forth, I used its mounting holes as a guide to mark and drill 1/8" holes for the mounting screws (Figure 46). You'll have to ream the mounting holes of the switch slightly larger with a file in order to pass the 4-40 nylon mounting screws. Once again, I used studs to secure them rather than nuts (Figure 47). Make the connection from the center lug of the switch to the AC inlet, and solder a short connection to the other switch lug for connection to the transformer (Figure 48).
Most people will want a pilot LED to indicate when the Small-Wart is on, so you should drill the hole for whatever bezel or LED holder you want to use. I located this on one side of the screw-on lid near the output jack. The plastic bezel shown (and included with the kit) requires a 1/4" hole. DON'T use anything metal here! Here's a drilling template.
With a couple of exceptions, this is done the same way whether in the plastic or the metal case. The piece of perfboard that holds the transformer can be cut with a sharp scissor. I cut it 23 holes long and 11 wide. If you are working in the plastic case, you'll need to file down the left and right edges a little so that the piece fits the width of the box correctly. Figure 25 shows where I drilled the mounting holes and the holes for feeding the leads. I prepared the transformer leads by splicing and soldering a 3 1/2" length of #24 wire to each bare lead and insulating each one with 1/16" heat shrink. Note that the center-tap wire on the secondary is not used, so it is cut off close to the body. To conform the heat shrink to the wire, I used a butane lighter that I keep available just for this purpose. Figure 26 shows the last splice before the heat shrink goes on. The insulated transformer leads feed neatly through the holes in the board. As I did with the mounting screws for the AC inlet, I sealed the nuts to the screws by soldering them (Figure 27).
The transformer assembly is mounted between the third and fourth ribs. I sanded in that area on the floor of the case and on each side and cleaned up with a Q-tip wetted with solvent (Acetone is OK on metal; use alcohol on plastic). Then I applied a few drops of epoxy cement in each mounting slot and a line of cement on the floor. I inserted the transformer assembly and left the epoxy to cure. When it is hard, you can finsh wiring the primary circuit. Insulate carefully with heat shrink the connection between the transformer and the switch! (Figure 28)
Once you are at this point, continue with...
Building The Filter And Regulator
From here, the construction is exactly the same whether you are working in the plastic or the metal case. I built the circuitry on a pad-per-hole perfboard, which I located with its front edge 3/16" forward of the transformer. I marked the locations of the mounting holes with a scribe as shown in Figure 29. The circuit board is held in place by four threaded aluminum studs that are secured to the bottom of the box with epoxy cement. To get good adhesion, I prepared the locations where the studs mount by sanding lightly and cleaning up with solvent. Then I screwed the studs to the board (Figure 30). I cleaned the bottoms with sandpaper and solvent, applied a drop of epoxy cement to the bottom of each one and gently set the assembly in place. When the adhesive had cured, I was able to remove the screws and reinforce the studs with more epoxy. The result looked like Figure 31.
To precisely locate the holes for the knobs of the trimpots, screw the board in place, and bore a hole right through the case with a #59 drill at indexes E6 and E11 as shown in the board layout drawing and the pic below. Drill these holes as straight and true as you can. Remove the board and enlarge the holes in the case to 1/4 inch diameter. If you want, you can stick the trimpots in place temporarily on the bottom of the board and see how they will fit and feel later when the board is wired.
Next, I built the circuit board. Figure 32 shows, in X-Ray view, the layout that I used. I suggest that you stick with this arrangement, because it puts the inputs and outputs where they need to be and gets all the components connected without jumpering.
Figure 33 and Figure 34 show the top and bottom of the board. When you wire point-to point in this way on pad-per-hole, do not wrap connecting wires around components; it wastes space and can cause shorts. Butt a connecting lead against the point to which you are soldering, hold it in place with locking tweezers or some other "third hand" and then solder. The points at which leads connect to off-board components are terminated with "flea" clips or other tie points. When done, the board is screwed down in the box and is ready for wiring and testing.
Testing
I did this before wiring the outputs. With the transformer secondary leads soldered in place, I connected a voltmeter between the +7.5 to 9 volt pin and ground. Then I plugged in the line cord and turned on the power. Rotating R1 gave me the proper voltage range. I also checked for 9 volts at the "Dead Battery" pin and made sure that I got illumination from an LED connected from the LED contact to ground. In my prototype, current-limiting resistor R4 is 10K. The kit includes a high-brightness red LED that is brightly lit even with that much series resistance. Ordinary LEDs can be used, but R4 will need to be lowered appropriately.
I wanted to test the current limiting, and I had a 500 ohm 2 watt control in my junk box. Using my ohmmeter, I set this to 150 ohms, which permits a current of 60 ma. at 9 volts. With the supply set to exactly 9 volts, I connected this load across the output. No change in the output voltage, so clearly the regulation is good! Then I started to very slowly lower the resitance. About 10% further in rotation, the voltage began to drop off and I stopped lowering. The voltage continued to drop, and within 15 seconds it had gone to almost zero. This shows that the resettable fuse X1 is working: When the current through it passes its "trip" value, it goes into a very high resistance state. When the overload is removed, it recovers quickly to a low resistance and permits normal operation.
Wiring The Outputs
This will vary some with your application. The output jacks that I specify have three contacts, because they include a normally-closed switch. When they are used in a battery-powered device, the switch disconnects the battery when a power plug is inserted. We are using the jack as an unswitched output, so only two contacts are used (Figure 35 and 36). For most applications, the center contact will be the common negative. The "Dead Battery" output is wired to the positive contact of a second jack.
Alternate Uses
To anticipate a question: Yes, you can drop the output voltage to the 6 volt range; just reduce R2 to 680 ohms. This lowers the voltage range to about 4.8 to 7.5 volts--makes the Small Wart useful for powering many devices that would normally require four of those always-getting-weaker AA penlights.
Patch Cords
You will want to have a few of these available, usually about 3 feet long. The specified plug has a 2.1 mm inside diameter, and it mates correctly with the output jack shown on the Small-Wart side. Depending on what you are powering, you may need a different type of plug on the equipment side. Since I use 2.1 mm power jacks on all my gear, I use the specified plugs and wire them shell-to-shell and center-to-center. The cable is a #22 zip cord that is commonly sold for connecting miniature speakers. Some brands ID the conductors with a stripe on one of the leads, while others have one tinned lead and one bare copper. Musicians will want to ID the plug barrels for easy tracing in stage environments, and this can be arranged either with colored heat shrink tubing or a liquid plastic/rubber coating such as is used to put handles on metal tools.
Here are some pictures of the assembly process using the heat shrink method. I used 1/4" heat shrink as an inner "jacket" and 3/8" material as an outer cover over the barrel. In order to pass the 1/4" material through the rear hole of the barrel, I had to ream the barrel out slightly.
To use the liquid coating, I slipped a 1" length of heat shrink over the strain relief of a plug. Then I assembled the plug (pulling the heat shrink out the end with chain-nose pliers), cleaned it with a little alcohol, dipped and hung it according to the manufacturer's instructions and let it harden overnight. Cut off the closed end, and you have a professional, close-fitting result.
Acknowledgements and References
R. G. Keen's article at www.geofex.com explains the theory behind the "dead battery" output. Many thanks to Mark Hammer, who looked over the original prototype and suggested a number of valuable design ideas. Howard Davis helped me with adding and testing the current-limiting feature, and the crew at www.diystompboxes.com have always been generous with support and feedback.
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Suggested
Mfr. Part Numbers |
| Common Electronic and Mechanical Parts | |
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All
Fixed Resistors 1/4 watt 5% Tolerance |
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R1
- 500 ohm trimmer potentiometer |
Mouser
323-409H-500 |
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R2
- 1.1K |
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R3
- 240 ohms |
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R4
- 1K to 18K (See Text) |
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R5
- 200 ohm trimmer potentiometer |
Mouser
323-409H-200 |
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C1
- 1000 mfd. 35 volt radial electrolytic |
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C2,
C3 - 10 mfd. 16 volt axial electrolytic |
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BR1
- 50 PIV Bridge Rectifier Module |
Mouser
625-W005G |
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VR
- LM317T adjustable voltage regulator |
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T1
- Transformer, 117 V to 12 VCT 60 ma. |
Mouser
41PG006 |
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LED1
- See text |
Mouser |
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LED
Holder or Bezel |
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DC
Power Output Jacks, 2.1 mm center |
Mouser 163-4302 or 4304 |
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Pad-Per-Hole Perfboard |
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1/4"
high x 4-40 threaded spacers |
Mouser
534-1450A or Jameco p/n 143360 |
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Perfboard For Transformer Support |
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Flea Clips |
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Heat Shrink Tubing - 1/16" 3/32", 1/8" |
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Hookup
wire |
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| If Building in the Metal Case: | |
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Case |
Pro's Kit #125-B |
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S1
- SPST toggle switch |
Mouser
#10TC405 |
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Square
rubber feet |
Mouser 517-SJ-5514BK |
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AC Power Inlet |
Schurter, Mouser 693-4300.0100 |
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AC
Power Cable |
Mouser 173-21103 |
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M |
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| If Building in the Plastic Case: | |
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Case |
Hammond 1591C-BK |
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S1
- SPST miniature slide switch |
Mouser 629-GF-123-3011 |
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Square
rubber feet |
Mouser 517-SJ-5023BK |
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AC Power Inlet |
Schurter, Mouser 693-4300.0096 |
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AC
Power Cable |
Mouser 173-21101 |
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| Patch Cord Materials | |
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#22
gauge zip cord |
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DC
Power Plugs, 2.1 mm center |
Mouser
1710-2131 |
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