Home > Electronics, Lutherie > The Noisy Cricket Guitar Mini-amp – Part 2

The Noisy Cricket Guitar Mini-amp – Part 2

Over the long Labor Day weekend, I was able to get in a little shop time along with getting some chores done. As a change of pace, I decided to finish up my “weekend project” I started a year and a half ago. Although I hadn’t taken recent inventory, I was pretty sure I had all the components I needed to finish the build and this proved to be true. Take a look at my post of June 23, 2014, “The Noisy Cricket Guitar Mini-amp” for more information on the project. Unfortunately, the Beavis Audio website where I obtained much of my materials is not longer online. I was going to have to make do with the images and printed copies that I had on hand.

Where to Start?

Previously I had started by soldering components to the Radio Shack small projects proto-board. At this point I sorted through my large stash of purchased components and picked out what I thought I was going to need to continue the build. Let’s see, two 1/4″ guitar jacks, a DC power jack, three potentiometers, two LED’s and two switches all needed to mount in the plastic project box. I was a bit overwhelmed so I began by laying out what needed to fit in the box and begin drilling holes to receive the bits. I would say the project case was awesome but I wasn’t too happy with the grooves on the walls inside the case. It would appear these could accept vertical dividers (or circuit boards) but they prevented my components from mounting flush. I attempted to work around this by mounting components in between the ridges which limited my creativity with respect to layout. When something didn’t quite fit, I used a chisel to shave off plastic until it did! With most of the holes drilled I called it quits for the day. I was at least warming up….

The PCB

I picked up my work on the PCB board and quickly realized that I didn’t have very much left to solder. Methodically I reviewed what had already been done and worked my way around the board. I finally located a source for the MPF102  JFET transistor and removing one from the packet, bent the legs to fit and soldered it home. I also soldered in 3 or 4 capacitors that I didn’t have before. There really wasn’t a lot to do here, most of the work was going to be wiring in the external components. Note to self – If you build this again, use sockets for the LM386 and MPF102. That way if something goes amiss, you can easily pop in a replacement.

noisy_cricket

The Potentiometers

The design uses 3 pots, one each for volume, tone and overdrive which they call “grit”. Each of the three calls for a different value so the first thing I did was to label the case, V, T, and G to keep them straight during the build. Even though I’m supposed to be a guitar geek (student really) it occurred to me that I didn’t know how to wire these guys. I knew the theory, being two points with a resister in between and a wiper that moves to vary the resistance. Great, but which is pin 1, 2 and 3? The Internet and my multimeter proved to be invaluable. I came to the conclusion that pin 1 is the pin on the left with the pot turned fully counter-clockwise. These pots also had a prong that stuck out on that side. Placing my ohmmeter between pin 1 and pin2 and turning the pot clockwise, I observed the resistance increasing until the wiper was fully open displaying the full resistance as rated on the pot. Seems right, now I can wire according to the plan.

The Switches

Deceptively simple, even switches can confuse. Many have multiple lugs so again the multimeter proved quite useful in my quest to get things wired. My power on/off for example can be wired to be connected in either the on or the off switch setting. Something I managed to do during my troubleshooting times on the last day. The DPDT (double pole, double throw) switch was also new to me but using my meter and a lot of logic I was able to figure out how to wire and LED indicator to the overdrive switch. The DPDT is essentially two separate switches that both operate with a single actuator. Wow six terminals, again I am thankful for my multimeter. So one switch closes the overdrive circuit, the other switch provides POS voltage for the LED.

Note: I see the Sparkfun site has great tutorials for many of things I learned over the weekend. This one of switches in particular.

https://learn.sparkfun.com/tutorials/switch-basics/poles-and-throws-open-and-closed

The Power Source

Again I was thankful for the Internet while trying to figure out how to wire in the 2.5mm DC jack and battery. It turns out the jack has also three terminals. Obviously one of the terminals is for NEG (ground) and the other two somehow provide POS voltage either 9V if from the battery or 12V if supplied from a wall mounted transformer. So I knew the theory, power is supplied from the battery unless the external source is plugged in which case the battery is disconnected. I get that but for the life of me I couldn’t figure out how to make it work until I realized the external power needs to provide NEG on the tip and POS on the sleeve. All the power adapters I had looked at or had on hand all supplied positive power on the tip but reversing this was the only way I could make this work. As it turns out, the YouTube video I watched also did it this way so I figured I was on to something. Even after I had “figured it out” I still managed to misidentified the terminals and wired it all up wrong. My first clue was that the power LED didn’t light up. DOH!

The Test

Once I was able to get the power indicator light to glow, I figured I would see if the thing made any noise. I hooked up my guitar and…. nothing. Hummmmm. Let me turn some knobs and flip some switches. Finally when I flipped on the “grit” switch the noisy cricket made sound and it was definitely broken up as overdrive should be. Turning pots yielded a lot of snap, crackle, pop and hum. I was a bit deflated, time for some trouble shooting. I basically went over the entire board, looking for shorts and opens. Any solder joint that didn’t look sound, I reworked and tested. After some time (really not all that much time) I noticed a jumper that I had neglected to solder on the pcb. This jumper was underneath a resister and was something that I would have presumably done 18 months ago but had missed. The encouraging thing was the jumper linked the 386 to the transistor so it was definitely going to be important.

Like magic the amp sparkled to life with crystal clear sounds. DAMN!!! I went from deflated to well encouraged. Other than a some RF interference, everything seemed to be working. I played for a little while, turning knobs and testing. I could definitely overdrive the system using volume alone or turn volume down and use the “grit” setting. I was a happy camper.

Finishing Up

With this positive feedback, I decided to rework the “grit” switch to add in an LED indicator. For this I needed a different switch, which I was able locate buried in my stash. After figuring out how the DPDT switch worked I know only needed to find ground (NEG) and a POS source for the light. After looking over my options I decided to pull POS by wiring in to the power switch. It worked and just in time to mount all the components into the enclosure.

Assembly

I had already drilled holes for all the parts except for the second LED for the overdrive indicator. Measuring the hole size of the other LED I made short work prepping this one. As I began to stuff wire and components into the enclosure I began bolting up one by one until reached the LEDs. Oops, they have to be bolted from the rear BEFORE you hook up the wiring.

LED

No option but to snip them and solder them back after insertion. Now I do like this package because you get the LED, the panel mount holder and the needed resistor all ready for 12V service. I was now a soldering iron ninja so this didn’t take long and finally I had the project assembled.

noisy cricket - front noisy cricket - rear

Tweaking

It felt good to finally put this together and have an actual working example but there are a couple of final touches. First off, the internal wiring is definitely “spaghetti and meatballs”. I’m surprised there isn’t more RF hum than I’ve actually experienced. For this project, I’m not even sure how one would go about shielding but I will give it much more consideration on future builds.

noisy cricket - inside

A couple other last tasks include, securing the battery holder to the case and making rigging up a power adapter. I was thinking of just using hot glue to mount the battery holder. I have a surplus transformer that I think will work but I need to purchase a 2.5mm plug for it.

The neatest thing about this project was hearing it come to life and being able to button it up in the enclosure right before dinner time on my last day off. Priceless……

Thanks for dropping in— Sparky aka The Turtlecovebrewer

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  1. September 9, 2015 at 5:23 am

    Awesome post. I’ve always wanted to build a similar box.

  2. September 9, 2015 at 8:56 am

    Hi Randall. The Radio Shack centric build version probably isn’t the most efficient way to wire up the circuit rather it was to be a quick (fix) weekend project. In retrospect ordering from a reputable electronics supplier up front would have been less painful in the long run. The slickest way would be to etch a circuit board but the next coolest option would be a strip board. An experimental breadboard would also be a great way start and that was how I was going to start over if this RS proto-build never fired up.

    The source I used Beavis Audio appears to be defunct but fear not, I’d suggest

    http://runoffgroove.com/

    As the best and most informative source for pedals and mini-amps. Of course it’s cheaper to buy off the shelf parts or even kits but I look at this is as educational and recreational spending. I learned a lot and kept my mind busy.

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