HOME

Practice Amps

I wanted a little practice amp for my electric guitar so I could play it early in the morning, or so I could play it outside in my philosopher's chair. This page describes some experiments.

Half-Watt Amp 1

I made a half-watt amp from scratch based on the LM386 audio amplifier IC from Radio Shack ($1.29). Most of the other parts came from broken stuff lying around my electronics workbench. The case came from a computer power supply. The speaker and most of the caps came from an old broken cassette player. I pulled the 1/4" phone jack off a dusty old mixer I built I don't even know when. I got the circuit from this web site:

Electronic Hobby Kits

There are other amp circuits here:

Practice Amp Designs

Here is the inside of the amp. The battery pack is a 7-cell NiMH pack I used for my electric plane until I switched to a lighter pack for the plane.

Here is the finished amp:

Using an adapter, I can amplify my mp3 player:

I tried miking the amp using my tube instrument microphone. I was first very impressed with the sound, but that feeling wore off.

Cassette Player Amp 1

Not satisfied with the volume and tone of the half-watt amp, I experimented with other pieces of various broken cassette players. One player is based on a Rohm BA5406 3-watt stereo amplifier IC. I found a data sheet for it on line that included an application circuit. A similar circuit can by found on the hobby kit site listed above.

The Amp Board

I traced the circuit by taking a digital picture of the circuit board held up to a light. Here is the board, pictures, a partial schematic diagram.

Once I determined the portion of the board that contained the amp and required components, I sawed it off using my handy band saw:

I had to add a little piece to hold some parts I sawed off earlier :-)

The Battery Connections

The pins of the IC are marked "1" and "12" on the circuit board. The + and - points are pins 1 and 12 on the BA5406 so I soldered the battery connections to the copper traces leading to those pins. In the picture below I have labeled pins 1 and 12. The biggest meandering copper trace is ground or negative in this circuit. Pin 1 goes to (+). There are jumper wires on the other side of the board that link pin 1 to a bigger copper trace as I have labeled below.

The Speaker Wires

Pins 2 and 11 are the output pins and go to big capacitors, easy to find. You can see them in the picture of the sawed off board above, at the bottom of the picture. One speaker wire goes to the other end of the big capacitor, the other speaker wire goes to the - connection (ground). Since it is a mono circuit with one speaker, I soldered a 3.5 ohm equivalent resistor to load the unused amp output.

I only had 10 ohm resistors in my resistor boxes so I put three resistors in parallel to make one equivalent 3.33 ohm resistor. (The speaker is 3.2 ohms) Here is the equation, to make me feel like a physics teacher:

1/R = 1/R1 + 1/R2 + 1/R3

Signal Input

I guessed at the input connection. I soldered an input wire to a disk capacitor before the tone control:

You can see one white input wire next to a hole. Since I am using this as a mono amp, I only need one input.

Mounting

I mounted the circuit board on a metal bracket in the power supply box.

Here is the finished amp. Sounds great.

Cassette Player Amp 2, the sawed-off amp

I took apart a Sony cassette player and found a similar circuit board. The board was well-labeled and the speaker wires were simply attached with plug and jack. The stereo amplifier in the Sony is based on a TA7282AP IC. This chip has different pin assignments but I found that the signal input was at the same place in this circut, at the tone control disk capacitors. I traced the connections from the battery compartment to determine where my battery pack wires should go. Easy! When I tested it, there was some preamp hiss, so I cut off the preamp section on the circuit board with my band saw :-) The amp was much quieter after that. I used the original case to mount the parts, but hacksawed it off at the cassette section since it is a mono amplifier. Here is a picture:

In this elegant package, the on-off switch, headphone jack, volume and tone controls still work

Half-Watt Amp 2

I couldn't stand to throw the half-watt amp into my junk circuit board bin. I had a small speaker from an old Ryobi charger/radio combo. I used it to make a smaller amp for my guitar bag. Here is a picture of the back showing all the parts.

I closed it up with some scrap wood and duct tape.

April 2006

Deck Amp

I wanted a cheap little amp for the deck so I could play my travel guitar while grilling. Here is a small amp based on a 7 watt amp kit. The kit came from HobbyTron, for about $10 plus $6 for shipping. The circuit board has all the parts conveniently mapped out. Here is a picture of the board with some parts mounted:

The instructions are simple and clear. The parts are taped together in the order of assembly. Easy.

It took about 20 minutes to mount and solder all the parts:

I tested it with junk parts. Works great:

I have an 8" speaker mounted on a metal grill. I drilled holes for the circuit board, input jack and volume control:

Here is the amp in place, with philosopher's chair and travel guitar:

It's powered in this picture by an old 12 volt car battery. Good for a test, but I want a maintenance-free power source. I have a collection of wall wart power supplies. I chose a Ryobi 12 volt, 500 milliamp supply because that's what the amp needs and because it fits in the outdoor glass light fixture by the back door.

I spliced several pieces of 2-conductor wire together to reach the amp. I also added a light-emitting diode to the amp panel so I could see when it was on.

The light switch by the back door is my deck amp switch.

I made a cover for the amp out of various pieces from the wood pile. Oak and mahogany top, pine and poplar front, pine in the back. It's coated with linseed oil to protect it from the weather: It slides easily along the deck rail.

August 2006

Final Note

Before, when I wanted to play my electric guitar, I could a) boot up my computer, load any kind of software amp simulator/plug-in, or b) patch in my hardware amp simulator, effects box, etc., or c) drag out my heavy amp and plug my guitar into it. Now I can use my handy practice amps. I find it refreshing and satisfying to cobble some junk together that instantly makes my guitar sound clean and sweet. Instead of punching up various simulated amps, I can choose from a selection of real home-made amps.