Undercolor/840102/Monitor Audio

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UnderColor, Volume 1, Number 2, December 25, 1984

• Title: Monitor Audio
• Author: Mark Haverstock
• Synopsis: Giving voice.
• Page Scans: Link

Article

Do you have a silent monitor? The monitor you own may have a great picture, but probably no sound capabilities. Until a few days ago our 13 inch color tv was a fixture on an already cluttered computer table, providing audio for game and educational programs. Fortunately, there is an easier and smaller solution.

Let's look at two sets of plans to make your own audio amplifier. The first is for the electronic hobbyist who enjoys building circuits. The second is a "quick and dirty" method for the person who has a passing acquaintance with the soldering iron and doesn't care to spend much time on projects. Both can be put together for $10 or less, depending on the state of your parts bin.

All the components are common, and could be purchased at most electronics stores. (Radio Shack part numbers are included for your convenience.) The projects were designed with the Video Plus monitor driver in mind; other drivers may require different audio cables.

Project 1—Integrated Circuit Amplifier

This amplifier is built around the LM386 integrated circuit (IC). It's an expensive, low voltage audio amplifier that requires a minimum of external parts. Power requirements are not critical; it can be powered by voltages ranging from 4-12 volts. This makes it an ideal candidate to use existing power supplies inside the Color Computer or a monitor.

Here's the parts list:

RS part number

R1 47 kohm resistor 271-042

R2 10K potentiometer switch 271-215

C1 4.7 uF electrolytic capacitor 272-1012

C2 10 uF electrolytic capacitor 272-1013

C3 47 uF electrolytic capacitor 272-1015

C4 220 uF electrolytic capacitor 272-1017

IC1 LM 386 audio amplifier 276-1731

IC board 276-024

Speaker 3 inch 40-248

Audio cable 42-2370

Battery clip 270-325

Optional

Plastic project case 270-222

Tools Needed: Soldering iron, needlenose pliers, wire cutters, screwdriver, and drill (if project case is used).

Assembly

The IC board was chosen for its small size and large solder pads. Note that there is space for a 16 pin IC; since the LM 386 only has eight pins, I decided to center it on the board to leave free space to connect other components. Following the schematic diagram in Figure 1, mount and solder the components together. Leave the volume control and speaker off the board, as shown in Photo 1.

Parts placement is not critical, but all the ground points on the schematic should be soldered together at one location to prevent a ground loop. This will discourage feedback

and reception of local radio stations. R1 acts as an attenuator to limit the maximum amount of volume. It may be removed if increased volume is desired.

The next steps depend on how and where you plan to mount your amplifier. The finished unit was designed to be placed inside a project case and operated from a 9V battery. This unit could be placed inside the case of the Color Computer, under the keyboard, or in the housing of the monitor.

Listed below are locations in the Color Computer and Color Computer 2 where supply voltages can be obtained. As for the monitor, you’ll need a schematic for your particular monitor to determine where supply voltages between four and 12 volts can be found.

Color Computer

TP9 +12V

TP12 +5V

R59 +9V

Color Computer 2

U1 pin 3 +5V

R2 +5V

The Quick and Dirty Solution

If the first project didn’t inspire you, maybe this one will. This amplifier is based on a transistor radio. A transistor radio? Well, think for a minute: A transistor radio is basically a tuner and an amplifier in a small plastic case. The trick is to disconnect the tuner so you’ll hear Pac-Man munching instead of Boy George singing. Fortunately, this is a fairly simple task and requires only a few simple tools, a capacitor, and some wire.

Here’s the parts list:

RS part number

C1 .47 uF capacitor 272-1433 or 272-112

Transistor radio 12-166

Audio cable 40-2370

Tools needed: small phillips screwdriver, soldering iron, PVC tape, and wire cutters.

Procedure

Actually, any transistor radio would work, as long as the audio section is in good shape. I chose the Flavor Radio because it was the only transistor radio available in our house. The directions and photographs are for this radio, but the procedure is basically the same for others. The schematic is shown in Figure 2.

First remove the rear portion of the case to expose the innards of the radio. Be sure to remove the battery if there is one inside. Look at the circuit board: there are two screws

holding this board to the plastic case (on the left and right center). Remove these carefully, using the phillips screwdriver. Lift the board carefully from the case and turn over. Don’t pull too hard—the speaker wires are still attached.

Next, locate the volume control opposite the tuning knob (the one without numbers). Remove the volume control knob with the phillips screwdriver; this will expose the volume control. After this we'll need to do some trace cutting and wire-tapping.

No, we aren’t going to do any subversive or covert activities here! We’ll disconnect the tuner from the amplifier and provide a means to connect the finished product to the computer.

The volume control has three leads coming from it which are soldered to the board. Orient the radio as shown in Figure 3. We’ll number the leads clockwise from top to bottom—1,

2, and 3. The center lead will not be used. Use a single-edge razor blade or small hobby knife to cut the uppermost circuit trace, which is connected to the volume control at 1. Be careful not to cut adjoining traces, and to cut through completely until the plastic circuit board shows.

The next step is to prepare the wire for soldering. Solder the 0.47 uF capacitor to the inner conductor of the audio cable. This acts as a coupling capacitor between the computer’s audio output and the amplifier in the radio. Solder the other end of the capacitor to lead 1. The shield will be soldered to lead 3. After soldering, wrap all exposed leads in PVC tape to prevent shorting.

Before assembling, test the unit. Install a battery and attach the cable to the computer. Turn on the radio. Load a program that generates sound, or test using this short program:

10 FOR X=1 TO 255

20 SOUND X,1:NEXT X

If you don't get sound, check for incorrect wiring, loose connections or cold solder joints. Most radios I've seen follow the same wiring pattern, but some have 1 and 3 reversed.

Your newly built amplifier could be left in its plastic radio case or removed and placed inside the monitor or computer; however, a suitable 9V supply will have to be furnished.

Now the game players in your family can enjoy sound, and the the tv can be put to other uses—maybe with a second Color Computer?

Listings

None.