Cracker box amp and Picaxe pictures

Cracker Box Amp gone wrong

Cracker Box Amp gone wrong

I was looking through some old file folders on my desktop. I finally arranged the icons by name and moved all of the things on my second desktop to the first. I found a folder that I haven’t looked at it in years.  January 2008 to be exact. I found the photo of my first Cracker Box Amp circuit. I am looking at the circuit, and I realize that I didn’t follow the schematic very well. I used this circuit to power speakers in a tool box. Not my best work. :) It’s actually some of my first work.

I also found some Picaxe kits that I put together. This is about a year after receiving my first Make Magazine. I wanted to get into microcontrollers, but I never really understood it. It was about this time that I found Picaxe. I bought these kits from HVWTECH.COM. They are part of SOLARBOTICS.COM. They are extremely nice and helpful folks.

PICAXE 08M start kit

PICAXE 08M start kit

The photo below is my first use of Altoids cans for a project. I still have this thing somewhere.

PICAXE 08M start kit in an Altoids can

PICAXE 08M start kit in an Altoids can

I know now where I have two nice speakers to build a few more cigar box amps with. The tool box circuit is stuffed in the attic of my barn/workshop. I’m going to pull them out next week and mount them in something. I love finding old stuff and re-purposing it, and I love recycling!!! :þ

What’s happening this week @ tech-tut.com? 7-23-2010

This is a quick post to update any followers of the site. I spent the beginning of the week installing laminate floors in my house. I neglected most of my college course work, and now I’m trying to catch up.

I’m working on a 4 speaker Cigar Box Amp. It’ll make it to eBay sometime next week. It’s a working prototype. The electronics are basically the same except that I’m adding an external power jack, a hard battery clip, and a front cover that may or may not look good. I covered most of the front with fabric to cover the speaker holes. I’ll have pictures up next week in the Cigar Box Photo album. I’m also removing the power switch and using a stereo input jack as the switch, and I’ve added an LED to signify that the power is on.

The suitcase amp is still under construction. I do not like version 1.1, and I’m probably going to scrap the 2 watt version for now. It has a horrible “hummmm” when used with wall-warts, and I probably need to add some smoothing capabilities to the incoming power. I am going to test it out with the LM386 circuit and see how that sounds.

That’s about it for Tech-tut.com’s latest updates. I haven’t said this in a while but remember that “it’s more than just a project.”

LM380 2W Samsonite suitcase guitar amplifier

I built this amplifier using the LM380N IC. It was an easy amp to build, but I used a Radio Shack circuit board to do some tinkering. I wanted to have a switchable transistor boost pre-amp, but without using a 3PDT switch and true bypass, I couldn’t think of a way to do it without making the whole thing unusable.

I settled for a clean, 2W amp. The great thing is that it can be used with effects pedals to achieve any sound that could be desired. One aspect of the LM380 chip is the input voltage. The maximum input is 100mVp-p, which has been observed by using a standard electric guitar. The input voltage is reduced using a voltage divider consisting of a simple potentiometer. I soldered the pot directly to the input jack and took the output to the circuit from the wiper. This gives the end user complete control over the amp. Hotter guitars will need to be trimmed down, and hobby guitars built with Cigar boxes can be trimmed up, if needed. I set the amp to work with a Tele-style guitar to work with minimum noise.

This is a springboard project. Version 2.0 is in the works to have distortion capability. I want to have the LM386 as the pre-amp and the LM380 as the power amp. This same circuit will also become the basis of future Cigar Box Amps.

2W amplifier schematic [Version 1.0 - Scrapped]
2W amplifier Schematic [Version 1.1 - added trim pot/removed input cap]

Adding a power jack to the Cigar (cracker) Box Amp

<August 6, 2010> This power supply configuration should only be used if you can find a positive tip power supply. Negative tip is more common in the music scene (effects pedals). General Guitar Gadgets sells a negative tipped power supply for $11. Check out this post for more info…Adding a power supply to a Cigar Box Amp (REVISED)
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Cracker or Cigar Box Amp as seen on Makezine.com

This is a quick demonstration of how to mod a 1k ohm potentiometer to be able to work in a region between 0 and 25-30 ohms. The Make schematic for the cigar box amp calls for a 25 ohm rheostat, which is either hard to find or much more costly than it needs to be.

Cigar Box Guitar Amp [LM386; as seen at Makezine] (Currently up for bid until July 12, 2010)

http://makezine.com/09/crackerboxamp/

The amplifier circuit from the pages of Make is a very simple one, but I had some issues finding the 25 ohm rheostat that the schematic calls for. This is the only modification to the circuit that I made. I used a 1k ohm pot for the volume, but I modified it with a 33 ohm resistor to get it as close to the schematic as I wanted to. The volume doesn’t completely cut out, but who wants to use an amp at zero volume anyways?

modified amp schematic

modified amp schematic

The trick to this mod was easy. I just used the parallel resistance formula where total resistance equals the inverse of the sum of the inverse resistances (or the inverse of the sum of conductances). Either way, I know that when the volume pot is at 0 ohms, most of the current will flow through the pot as if it were just a wire. If the pot were to go to infinite resistance, the current path would be through the shunt resistor.

I used my graphing calculator to graph y = 1\((1/1000)+(1/x)). Then I traced the line to find the Y-value (my shunt resistor). I chose the point where x = 25. Y was so close to 25 that I just rounded up. Well, I didn’t have a pile of 25 ohm resistors so I used a 33 ohm resistor.

The volume pot now works a little better. It’s not perfect, but it provides a better range than just a 1k pot.