A Fun DIY Amplifier- the Monstrocity- the Organ Donor Project Part 3

In this instructable I'll explain how I designed and built a fun little battery powered amplifier from recycled materials and a handful of electronics components. It's a catalog of failures and solutions that really paid off. There's nothing more satisfying than playing an instrument you built through an amp you built... unless you're doing it in a boat you built! (hurry Summer!)

It's funny how projects just happen. After taking everything out of the organ from The Organ Donor Project, I was concerned with what to do with the cabinet. I've seen some cool desks made from old organs, but this one was in rough shape and not worth the effort. I decided to disassemble it, save the usable wood and dispose of the rest. That's when I saw the cool little speaker grille on the side.

Hmm... it was kind of cool and it popped out easily enough so I set it aside and kept going on the cabinet. When I went back inside I put the grille on a shelf and had one of those 'Aha!' moments. I had inadvertently laid the grille next to an old camera case I had left over from this project. The grille was the perfect size for the camera case! I had just found a few 386 op amps so I suddenly knew how I was spending the rest of my afternoon.

It's funny how projects just happen. Sometimes it all falls together and works on the first try. This instructable is about what happens when it doesn't.

The first step was to prepare the camera case. First I removed the small storage compartment from the inside. I left the little pouch and the elastic straps in the lid. Next I traced the grille onto the back of the case. I drilled a hole to start and then carefully cut out the hole for the grille. I tested the fit to insure a snug fit and went on to mount the speaker.

I also installed a recycled handle on top of the case. I drilled two holes and mounted the handle with some recycled hardware.

OK so I'll admit it... I got a little over zealous. It's just that the six inch speaker with the huge magnet looked so cool. Oh well... live and learn.

I Used a scrap piece of Luan to make a plate to mount the speaker to the grille. The original idea was to mount the grille in the hole, add a wooden spacer on the inside of the case and screw the speaker plate to the grille, mashing everything in place. I traced the back side of the grille slightly larger than actual size and determined the center. I used a compass to draw a circle for the speaker and marked the mounting holes. I drilled and cut the plate. I also cut a piece of tweed grille clothe from the piece I recovered from the organ to put between the grill and the speaker.

As I was trying to figure out how to mount everything it became apparent that this approach wouldn't work because there wasn't enough room inside the case to allow the mounting screws to be tightened with the speaker in place. The next option was to trim the speaker plate down to the same size as the grille with a bit of a lip on top. This would allow me to put the entire grille assembly together, slide it through the hole and secure it.

I used nails to attach the speaker plate to the grille with the cloth sandwiched in between and some recycled hardware to mount the speaker to the speaker plate.I slid the grille and speaker sandwich through the hole in the case. The lip on top of the grille assembly held it in place loosely. A wooden spacer would be used to wedge in the gap and a screw on the bottom will hold everything in place. I cut a triangular baffle to cover and protect the speaker inside the case.

The next step in my process was to build the circuit and install it. I've built a few 386 amps before and they are pretty straight forward so I didn't foresee any hassles. Well... the problem was that the 386 didn't have enough power to push the big old speaker I'd wedged into the camera case. I breadboarded several solutions and switched out every component I could with other values and nothing worked. It was just too weak. I ended up having to cut another, smaller speaker plate to hold a 4 1/2 inch speaker and mounting this to the existing assembly. It worked quite well but it was an extra step that I could have skipped with a bit more planning. Same goes for the original speaker plate and having to trim it down. Remember- measure twice and cut once and always breadboard a prototype before you start mounting things.

Lesson learned- we'll do better on the next steps, I promise. let's build the panel.

Now it was time to make the control panel. This was simple a piece of luan to go in the opening of the bottom part of the case to protect the speaker and hold the controls. Measure twice and cut once, right?

I cut another piece of luan to fit the case's opening. Since the wooden spacer fit so well in the gap between the case and the grille I decided to attach it to the edge of the control panel to hold it in place. I attached another strip of wood along the opposite edge of the panel. I would mount this edge in place with two screws from the outside of the case.

After the strips were attached I drilled the holes to mount the controls. I needed a power switch, an indicator LED, two input jacks and two volume control pots. I laid out the controls and drilled the appropriate size holes. When I slid the control panel in place I realized the error of my ways. The lip on the grille assembly that held the wooden spacer strip in place was blocking the holes for the LED and the power switch. Arghhh!

It wasn't that big of a deal since I was planning to cover the whole panel with 3M reflective sign film, but the holes would still show through. Oh well, I'll think of something. I covered the panel with the reflective film and installed the pots, jacks, switch and LED holder. Unfortunately, the luan was a bit too thick for the nuts and washers to fit on the volume pots. I left them loose for now to make wiring the amp circuit easier.

I'm not going to go into too much detail here as this has been cover a million times. This was mostly straight from the LM386 datasheet- an amp with a gain of 200 and a bass boost loop on the output. See the full schematic above.

I wanted to be able to run two separate signals into the amp and control their individual volumes. This is really easy with a passive mixer. You can use either resistors or diodes to match the attenuation of the two signals. I've done it a million times so ... yeah.

It buzzed like a nest of hornets. No matter what I did the little 386 amp just didn't like passive mixing. I tried resistors and diodes. I tried various values of pots and added floor resistors. Nothing would make the buzz go away. Now I probably could have figured it out eventually, but drove me nuts, so I decided to go with a single input. Of course, this worked just fine.

Now there were two more holes to deal with. As I've said before- stickers make everything better. I had some cool monster stickers left so I covered the holes and the rest of the control panel with awesome monsters and gave this amp it's name- the Monstrocity!

I added a wire from the power switch through a 560 ohm resistor to a slow color changing LED mounted in an LED holder and then tied it to ground. This will serve as a cool power indicator lamp.

With everything wired up all I had to do was secure the volume pot, add a knob and battery and rock out. Since the luan was too thick for the nut to catch the threads on the pot shaft I decided to use hot glue to fix the pot in place. I let it dry and it seemed to work great. I installed a battery and reassembled the amp.

It seemed to work fine- until I turned it all the way up. Suddenly it howled like a hurt beast. When I turned it down it cut out completely and suddenly about 1/4 of the way from the bottom of the pot. WTF? I opened it back up and looked closer. It seems that some hot glue had flowed into tiny gaps at the edges of the pot where the case met the board. It was doing not nice things to the sweep of the pot at the ends of its travel. %$#@!&^!

I removed the pot and desoldered the connections. I used a tiny scoop chisel to remove some wood from around the hole so that the pot would sit a slight bit flatter. This gave me just enough room to get a washer and a nut onto the pot's shaft and tighten it down. I soldered the connections to the new pot and tried again.

Success! I was the proud new owner of an awesome looking portable practice amp/retro boombox.

Time to plug in the Lapful of Scrap and have a go!

So here's the moment of truth- how does it sound? Pretty good, actually. I'm kind of surprised hoe clean it sounds. I've played music and several instruments through it and it has a good frequency response. Judge for yourself- here's a quick video of me playing the Lapful of Scrap DIY lap steel guitar through the Monstrocity amp-

monstrocityamp from chuck stephens on Vimeo.

Rock on!

( https://vimeo.com/121443163 )