GUITAR MIXER/ PRACTICE AMP/ PRE-AMP

Maybe you don’t want to cough up the big bucks for an amplifier and mixer. This little amp solves that problem. Switch to speaker mode and it is loud enough for you to hear but not loud enough to disturb anyone else. Flip the switch and plug a headphone into the output and only you can hear it. Plug a patch cord into the output and it acts as a pre- amplifier so you can connect it to your stereo, sound bar or other device for really big sound. It also has an internal passive mixer for up to 3 inputs allowing you to jam with some friends. The components are tiny so there is room in the case for cords and small headphones. The case also gives it a vintage look and its compact size makes it easy to carry.

This is basically a combination of two of my previous projects:

https://www.instructables.com/PASSIVE-3-CHANNEL-AUDIO-MIXER/

https://www.instructables.com/GUITAR-MINI-PRACTICE-AMP-HEADPHONE-AMP-PRE-AMP/

Materials:

LM386 Amplifier kit ( Available from Amazon or other suppliers online. About $10 CDN) Alternatively, there are many Instructables on building a similar amp. This one, however, comes with all components and circuit board so it was very convenient.

3 mono audio sockets (1/4”)

1 stereo audio socket (1/4”)

9V battery clip

9V battery

panel mount LED

1-1000 Ω resistor

3 - 5000 Ω resistors

SPST switch

DPDT switch

Hook up wire

Small speaker

4 small nuts and bolts

solder

Velcro tape

small utility case-8x6x3”

Tools:

Fine tip soldering iron

cutting pliers

needle nose pliers

wire strippers

1/16” drill bit

step drill bit and drill

metallic markers

The kit comes with no schematic but the circuit board is printed with the component locations so it is a simple matter of placing them in their proper orientations, soldering them in place and clipping the excess wire. Note: I eliminated the socket for the battery and soldered directly to the + and - locations. Also I eliminated the audio input socket and soldered the input leads directly to the 2 contact points. There are 3 resistors. 2 are the same. Just look at the codes (stripes) to identify the one that is different and place them in the proper marked locations. The speaker output is well marked so just solder the 2 leads directly to the board. Also, solder the 2 leads to the audio output. Once finished, test to see if it works. If not, check all your solder joints.

An easier build than it looks. Just be patient as the parts are extremely tiny.

Make a map of the case front on paper and mark the locations of all components. Transfer the locations to the case by drilling through the paper with the 1/16” drill bit. Drill out all holes to the proper sizes with the step drill.

Attach all components with their included hardware. Attach the speaker with the 4 nuts and bolts. Push the LED into place. Friction should hold it in place. If not, glue may be required.

Start with the mixer circuit. First connect and solder the grounds of the input jacks together. Solder one end of the resistors to the tip connections and connect the other ends together. This is the simplest of additive mixers. The resistor network is purely passive. It combines signals from various instruments. It adds up to 3 individual signals, giving out a composite signal that contains the frequency components of each of the source signals.

A circuit diagram is supplied in the photo. Follow it for the remaining connections. Keep all wires as short as possible.

Use Velcro tape to hold the battery in place.

*Note: This is a monaural amp but if you solder the two tips together on the stereo output jack you will hear through both sides of your stereo headphones or, both sides of your sound bar or stereo amp if you use a stereo patch cord.

After running tests, use the markers to label all controls.