Intro: The ' Little Gem ' - Mini Amplifier / Guitar Amp -
This is a mini guitar amp based on the Little Gem amplifier, found at runoffgroove.com. The unit produces only 1/2 a watt, if you want to produce a full watt to drive a 12" speaker, then I suggest building the Little Gem MKII (PDF layout attached). The amp sounds pretty dang good for what it is, and clips can be found at the site I just mentioned above.
Listen to it here
What I want to show you (conceptually) is how to use one of these as a portable mini guitar or practice amp that already has some good sounding distortion.
My last instructable (attempted) to do this, but I think the sound quality of that amplifier doesn't compare to this one. The distortion tone is a huge improvement, and the same concepts (switchable tone capacitors) can all be applied to this as well.
You can build this with or without a Volume pot, I chose not to since it doesn't get extremely loud to my ear after playing through a Fender SS tube amp (last pic). So I mean this isn't going to 'take over' as your main rig, and you won't be able to play to 500 people - but it's great for taking your electric guitar camping. Seriously. Try it. It's fun, be a guitar geek. Blaaaack Beauuuuuty is my main axe ; I hope you like ooogling her as well.
Step 2: Gather the Parts
You're going to need the following
Speaker (or surround sound speaker / enclosure)
5K Linear Gain POT
25 Ohm POT (Volume) - [this is optional]
10ohm Resistor (brown / black / black / gold)
9V Battery Clip / 9V power source
ON / OFF Switch
1/4 Mono Jack
Drill / Bits
Guitar Player Mojo [optional]
I say the volume pot is optional, because this thing gets cranked all the time anyway (only a 1/2 a watt remember) and I left mine out on this version.
Step 3: Prepare Your Parts, Get Ready to Assemble
Start by gutting the speaker out of your enclosure . I highly recommend using an enclosure with a closed or open back, because your speaker will sound 10X better when it's inside of something instead of magnetized to your bench.
Pretty much any surround sound speaker and enclosure works for this. I know most of you don't have old surround sounds sets just laying about ; but they can be purchased pretty cheaply. I used old Kenwood surround sound boxes for my amps ; primarily because I liked the sound the speakers produced over sony's, etc... that I had. Try all of em on your circuit before you decide to build the unit around a particular speaker.
Gather your parts, which were outlined in the last step. Cut your perf board so it's about the size of a 9V battery.
Once the surround sound speaker is out of the enclosure, wire it up or use existing wire.
Step 4: Assemble
Ok, the best way to assemble this circuit is to follow the graphic. I can't explain how to do that part, but I can give you some advice. Don't follow my photo of the layout I used. I made a ' layout mistake' on purpose to showcase some common mistakes (too long leads).
Lay everything out on the board, and try to lay the parts out how you see them on the circuit below. I didn't follow their layout ; by the way but went astray with my 100uf cap putting on the other side of the board to showcase a common mistake. Bad move, because those connections are longer than they have to be which can = either feedback or radio stations coming across your signal.
POTs are numbered Pins 1, 2 , 3 looking at them with the knob face up, pin one to your left. When soldering them upside down, it's 3 2 1
Wire up your mono jack to the input, ground to the ground shown on the PCB layout. Also wire up the positive from your speaker to the 25 ohm pot, lug 2, and ground to the PCB. The gain pot is wired as shown in the PCB layout graphic below.
Wire your pots, a power on / off switch (just wire the 9v clip into the switch then switch to the PCB) , the 9V clip, and your mono jack into the input and ground to the jack's connection ground on the board. You'll also need to ground your speaker to the board that you took out of the enclosure.
Drill holes for a mono jack, on / off switch, and Gain / Volume pots into your enclosure / surround sound speaker.
Test the thing once it's assembled outside of the enclosure to assure you've got it all hooked up properly. If you don't hear anything, your mono jack or your volume pot are good places to start looking for a mistake.
Step 5: Drill On!
Time to drill some holes
Take the enclosure / gutted surround sound speaker and drill the holes for your Mono input jack, 2 POTs (or one), as well as your on/off switch. Depending on the components your drill sizes will vary, so I'll leave out that bit.
Install POTs, speaker, on/off switch, power jack, etc... then finally the entire PCB into the enclosure . Use zip ties on your lines to clean em up, otherwise they can rattle against the inside of the case when playing.
I removed the back of this speaker so you can see the internals of mine askew before the final assembly.
I like to use these self adhesive backed stand offs for attaching the PCB to the inside of the enclosure (away from the speaker magnet) . This is important.
Step 6: Play ON!!!!!
She's a pretty good sounding little amp. That's the best part of this instructable.
I'm really stoked with my latest attempt at the 'dirt cheap' portable amp project. This one really shines. Inexpensive, custom portable guitar tone to the masses! I'd post clips, but there are already some on runoffgroove.com so check em out. They also have a ruby circuit that follows along the same lines but adds a FET filter.
Of course if you don't like the way the distortion sounds / breaks up or whatever, you can always make some value changes on those capacitors, or install a two way switch (on / on) with two capacitors wired to it so you can switch between two distortion capacitors. I'll probably do something like that later, but I like how it sounds.
....and plus, it only cost me about $10 in parts and an old surround sound speaker. Bonus!
Well, I build custom pedals so I added my own flavor of the LPB-1 (linear power booster) circuit to mine that I can also use plugged into (any amp), and it sounds more like modern heavy on the mids distortion now. Simply put, this circuit design gives you even more of a volume and distortion boost, and it's scalable depending on what transistor you use.
Pay attention to H-F-E values when it comes to your transistors. The higher the value, the more distortion / gain of the transistor.
The circuit layout for LPB-1 is attached , I use a lower hFe valued transistor (in the 60-80hfe range) to couple with the gain that's already in the amp circuit.
You can find a larger layout picture of the LPB-1 circuit here :
Update: OCT 9, 2010 -
Decided to tear out the LPB-1 ; and installed my own version of the Dallas Rangemaster. Sounds pretty good because it's light on distortion / more of a treble boost for the amp... Gets louder and crunchier when it's almost max'ed. A few layouts of the Dallas Rangemaster are attached. Please don't ask me how to make this work ; if you need to brush up on some basic guitar pedal building technique then I suggest you start with the LPB-1 layout also below.
jfidalgo rodriguez made it!