I was looking for an electronics project I could build after teaching myself to solder (by building alot of Velleman 3D Christmas trees) when I came across Ross Hershberger’s Monobox amplifier project. Right away, I knew I had to build this. I tried to find a cigar box that would work, but didn’t have much luck. But then I remembered that I had some old ammunition cans laying around, and inspiration struck. It was a fun project that came out great, and I continue to enjoy it as it makes a great companion for my iPod.
This project was featured on the Make Blog: MonoBox Mods: Same Circuit, Two Builds. Read on to learn what parts I used, some of the difficulties I ran into, and to see a video of the finished ammo can amp.
- Ammo can – the project calls for a an enclosure with between .125 and .25 cubit foot of space. I had a hard time finding a cigar box that met that criteria, so I decided to try some ammo cans I had laying around. The .50 caliber can has right around .25 cubit foot of volume, which is perfect. It’s a little rough looking, but I think that works for this project. And best of all, I already had it. If you don’t have one, don’t be discouraged – .50 cal ammo cans are relatively common and inexpensive. At the time I’m writing this, Army Surplus Warehouse has them in excellent condition for right around $20, and as low as about $13 if you don’t mind some rust.
- Components – The project is sponsored by Radio Shack, so it includes a handy reference with RS part numbers which makes getting everything really easy. I was able to get many of the necessary components at my local store. Several additional parts were ordered from Parts Express, including the grill cloth and polyfill. For the film capacitors, I bought some vintage NOS caps from an Ebay seller who specializes in guitar parts. Not because I think they’ll give my amp “sweet vintage tone”, but because they were cheap, the seller was in my state (so I got them quicker) and what the heck, they look cool. For the cork gasket, I bought some cork tiles from Walmart. I got 4 12″x12″ tiles for $6.
- Speaker – The speaker is an Aura NS525-255-8A purchased from Madisound for less than $4. I chose it because it’s cheap, has good sensitivity (89db) and the resonant frequency meets the suggestion from the Monobox project page. If you look at the photos on the project page, you can see that this speakers shows up in the photos, which led me to believe that Ross may have tried it himself. As it turns out, some time after building this project, I heard from Ross and he told me had had also bought some of these from Madisound. One problem he pointed out is that frequency response drops off above 6khz. So a tweeter should be added, which I detail below as a possible improvement.
Building and testing the circuit
The circuit is fairly simple and easy to build, but I did run into a problem. In a couple places, it’s necessary to insert two wires/leads into one hole on the PCB. The vintage caps I used have pretty thick leads, so inserting the .010µF and .022µf capacitors together that make capacitor C1 was not possible without modification. It’s a cheap trick, and probably not a good idea, but I ground down the leads using a Dremel to make them fit. I had to do the same thing in another spot where a lead and a jumper are inserted into the same hole. I assume that if the “official” parts listed on the project page are used this won’t be an issue.
With that said though, I was pleasantly surprised to find that the circuit worked on my first test using a small 3″ speaker. Success!
Building the enclosure and mounting the components
The ammo can needed a 5.25″ hole cut out for the speaker and several other holes drilled for mounting bolts and the power and signal jacks. The hole for the speaker was the most work, but it came out okay. I took my time, running two passes with a Dremel and a cutting wheel (actually, 4 cutting wheels, they wear out fast on the thick metal.) After cutting, I cleaned up the edge with the Dremel using a grinding wheel followed by a sandpaper wheel. It’s not perfect, but it came out much better than I expected.
The circuit board is mounted at the bottom of the can upside-down using some 1″ nylon washers to hold it up. A nice thing about the ammo can is that there is a large indentation in the bottom to make the cans stackable (so the handles have room.) This is nice because there’s enough room so that any screws installed in that area are sunk in enough to prevent them from interfering with the can sitting level on a table or other flat surface.
The speaker is mounted with the cork gasket and some nylon washers to ensure it’s secure. I also mounted a toggle switch which allows for selecting between the external power and an internally mounted 9v battery. The switch is mounted inside to prevent it from being damaged as the ammo can amp is hauled around.
Rattle, rattle, rattle
With everything in place, I stuff the box with Polyfill, closed it up and it was ready to go! Or so I thought. I found out quickly that it had quite a rattle when the bass hit. This was occurring in two places – between the lid overhand and the bottom portion of the can, and between the lid and top carrying handle. To stop the rattle, I cut up some scrap sweeper belts which are about .06″ to .07″ thick. i belt the lid a bit to make room for them to fit. They’re glued in place with E-6000. If you try this, keep in mind that belts are generally smooth on both the ID and OD – they need to have the surface roughed up a bit to get them to adhere to the glue. In my case, I was lucky as I had obtained some ground belts from work.
I’m real happy with the finished ammo can amp. It has a vintage feel to it which I like, I think due in part largely to the slightly rough condition of the ammo can and the retro-looking speaker cloth (Parts Express #261-810). The sound is good, not great, but it’s better than I had expected and there is a possible solution to making it better which I get into below. The main goal of this was to create a fun, portable speaker with lots of DIY charm. In that regard, it exceeds expectations.
There’s a few things I’d like to try to make this project better.
- Add a tweeter – this one comes directly from Ross. He left a couple comments on my monobox test video on Youtube. As he points out, the Aura speaker is great, but it doesn’t have much high-end. It drops off at around 6khz. He suggests adding a a cheap tweeter with about 3uf capacitance in series with it. Then add that in parallel with the Aura speaker. To do this, I purchased a JAMO 2″ paper cone tweeter from Parts Express ($2 clearance item) and some 3.3uf capacitors.
- VU Meter – From the front, the amp is too plain – it’s an ammo can with a speaker. I have a couple old VU meters made by Rauland in 1959 and I think one of those would look great in this amp. They’re not “real” VU meters though, so I need a circuit to drive one. JLM Audio’s VU buffer kit looks like my best bet as it runs from a single rail. Unfortunately, shipping is prohibitively expensive unless I buy multiple items (they’re in Australia). I can’t justify spending $18 to ship a single $25 kit. So I’ll probably wait until I have a reason to buy more than one.
- Pilot light – Sticking with the retro theme, I think a large, jeweled pilot light would look great on this amp. Most every jeweled pilot light I’ve seen is designed for AC power, but they’re easy to convert using an LED. I had the process documented on the Make Projects page, but I can’t find it since they redesigned the site. I’ll post it on my blog at some point.
- More power – This thing actually gets pretty loud for having just 325mw typical output power from the LM386. Based on the datasheet and other information I’ve seen, it appears that the LM386-3 could be used instead with no other changes to the circuit, which would result in a typical output of 700mw. I don’t expect it to make a huge difference, but it might help, especially if I add the tweeter.
- Stereo – a stereo version with two monobox circuits using the LM386-3 would be cool.