An electric guitar is as good as the amplifier that is connected to it. Without an it, the guitar is not very useful to play any music. Amplifier comes in many form and can be powerful enough to make a full stadium shake. Here, we will be building one of the smallest amp you can make: a jelly jar amp. You can find various examples of this amp online and you can even buy it already built or in a kit. But here, I will show you how you can make it on your own for a couple of bucks, with few tools and components.

The design of the Jam Jar Amp is very simple but also easy to personalize simply by changing a couple of components. The main electronic component of the amp is the LM386 integrated circuit which does all the amplification work for us.

The LM386 in details

The LM386 is an integrated circuit that contains a mono low voltage audio power amplifier. It can drive loads from 4 Ω to 32 Ω. It is design to be used in low power consumer applications such as radios or portable sound amplifiers. There are various models of the IC with different input voltage and output power.

Here we are using a LM386M-82

The LM386 has been design for battery operation as it has a quiescent power drain of only 24 milliwatts when using a 6V supply.

Thanks to its internal configuration, the LM386 can be used in a very low part count system.

The LM386, is small, cheap, and very simple to use. The drawback is that it does not produce high quality audio. But despite the distortion and noise of this chip, it is sufficient to build a fun little guitar amp with plenty of character.

Pins and circuit description

Since the LM386 is the main component of this amplifier, here is a description of the role of each pins of the IC and the role of the component connected to each pins.

Pin 1: Gain setting.

Pin 2: Inverting input. This pin is grounded

Pin 3: Non-inverting input. The signal to be amplified is fed into pin 2 through a 100 nF capacitor used to eliminate any DC signal. The capacitor is known as a DC blocking capacitor. Its value will affect the final sound and can be tweaked according to taste. A 10 kΩ variable resistor is also used between pin 2 and the ground to act as a volume control. It is used to reduce the amount of signal presented to the LM386 amplifier negative input which directly affects the output – if there is less signal at the input there is less signal at the output. An audio tapered potentiometer works best here.

Pin 4: Ground.

Pin 5: Output. This is the terminal in which the amplified sound signal comes out. The output of the IC goes through a RC snubber for stabilization. A 100 nF ceramic capacitor placed in parallel to the speaker acts as a current bank for the output. This capacitor fills with electrons when demand for current is low and drains when sudden surges of current occur. Without it, the LM386 would probably oscillate at about 1MHz. The other 100 μF capacitor placed in series with the speaker is a coupling capacitor. It removes any DC offset from the output of the LM386 amplifier and only passes the (AC) sound signal through.

Pin 6: Power supply voltage (V+). The IC receives the positive DC voltage through this terminal so that the IC can receive the power it needs to amplify signals. The LM386 can use an input voltage between 4 and 12V. Here we use a 9V battery as a power supply. As every little variation in the power supply will be amplified by the amplifier, the power supply has to be “cleaned”. We added a 1000 μF electrolytic capacitor in parallel to the power supply to reduce the low frequency oscillation and noise in the amplifier. This capacitor will also handle any major output load variations. When large current peaks are drawn, the capacitor supplied the surge in energy. A smaller ceramic capacitor can also be added in parallel to the power supply to limit high frequency fluctuations.

Pin 7: Bypass decoupling path. The pin 7 gives an option to control the power supply noise depending on the gain selected. For large gain values, a decoupling capacitor between pin 7 and the ground is generally required, depending on the quality of the power supply (for example a 10 μF electrolytic capacitor will work well).

Pin 8: Gain setting.

Gain

The gain of the IC is internally set to 20 (26 dB) but can be increased to any value between 20 and 200 (46 dB). This gain value is defined by the 1.35kΩ resistor between pins 1 and 8 and the 15kΩ resistor between pins 1 and 5. These two resistor in series form the feedback loop of the amplifier. Placing external resistor in parallel with these resistor will change the gain.

Without any external components, the predefined gain is:

G = 2 x 15’000 / ( 150 + 1’350 ) = 20

Installing a 10 μF capacitor between pins 1 and 8 bypasses the 1.35 kΩ resistor the gain will go up to 200:

G = 2 x 15’000 / ( 150 ) = 200

Adding a resistor in series with the capacitor, the gain can be adjusted to any value between 20 and 200. Finally, to control the gain,

For a guitar amp, the gain chosen will shape the character of the output sound. To control the gain, a potentiometer can be added between pins 1 and 8, in series with the capacitor.

When using the LM386 with higher gains (bypassing the 1.35 kΩ resistor between pins 1 and 8) it is necessary to bypass the unused input, preventing degradation of gain and possible instabilities. This is done with a 0.1 μF capacitor or a short to ground depending on the dc source resistance on the driven input.

Bass boost

Another advantage of the LM386 is that it has a frequency shaping capability. Adding a resistor and a capacitor in series between pins 1 and 5 increase the low frequency output to compensate poor speaker bass response.

A resistor of 15 kΩ lead to about a 6 dB effective bass boost. The lowest value for good stable operation is R = 10 kΩ if pin 8 is open. If pins 1 and 8 are bypassed then R as low as 2 kΩ can be used. This restriction is because
the amplifier is only compensated for closed-loop gains greater than 9.

A 0.033 μF capacitor and a 10 kΩ resistor are suitable for a good bass amplification.

“Jam Jar” fabrication

Preparing the housing

As you can guess with its name, the Jam Jar is made in a glass jar. Thanks to the two part cap, it is easy to fit a small cone speaker on the top of the jar. However drilling holes for the input and output connections in the glass jar is a bit more complicated. The jar I used has a diamond pattern on it which made it easy to place the holes. Drilling glass properly is not a simple task and a diamond holesaw is the best way to go. Using masking tape can helps prevent chipping of the glass. I used a 4mm bit to drill a first hole. I then increased the hole size by progressively grinding the glass with the same. Only a minimal pressure and time are necessary to drill or grind glass. It took me 10 to 15 minutes per holes but the results was worse it.

Assembling the electronic

There are plenty of kits using the LM386 IC that are available for around a dollar and that are a great starting point for this project. However, you might need to tweak the components values or add extra component to get a sound that you like out of the amp. I found the kit for this project on eBay. Most of the other component can be recycled from discarded electronics. If you have an old radio, chances are that you can find most of the component inside. For this amp, I wanted to have switched jack plugs for both the input and the output. The input switch allow to connect the battery only when a cable is plugged to the amplifier to preserve the battery life. The output switch is used to disable the speaker when headphones or another external source is connected.

The kit is supply with a hefty 1000 μF capacitor to filter the power supply. This is design for a wall transform which can introduce a large amount of noise in the circuit. For a 9V battery, since no buzz from a transformer is present, a smaller capacitor should be sufficient (220 μF for example). The circuit is also design for a maximum gain of 200 which is probably far too much to get a descent amplified sound. There are two way to improve this. The first option is to lower the overall gain by adding a resistor in parallel with the capacitor between pin 1 and 8. A 1.2 kΩ will give a gain of about 50 which should not give too much overdrive. If hi gain is needed, adding a bypass of a few μF from pin 7 to ground will isolate the high gain input stage of the LM386 from power supply noise. A 4.7 μF capacitor should work well. To add a variable gain option, one can add a 5k linear pot between pins 1 and 8, still in parallel to the capacitor.

If you use a kit, the assembly is quite straightforward. Solder the component in the order of heat sensitivity. In particularly, if you are not planning on using an IC holder, install the LM386 last. Be sure to check the orientation of all the polarized components.