BUILD YOUR OWN IC GUITAR AMPLIFIER

Battery-powered portable unit goes everywhere

By HERB GILL

YOU CAN BREAK AWAY FROM THE END

of the power line with this battery-op-erated guitar amplifier. Beach, boat or boondocks are now fair game for your electric guitar.

Most of the "components" are in the preamp, the RCA CA-3020 inte-grated circuit amplifier. This little "gem" in a single TO-5 transistor case with 12 legs is a complete 550-milli-watt push-pull amplifier. Its 58 dB gain is more than enough to work di-rectly from the guitar pickup. Distor-tion is less than 1%. The $2.80 price would warm the heart of a value engi-neer too; you couldn't buy the 7 tran-sistors, 3 diodes and 11 resistors as individual parts, let alone connect them up and make the circuit work, for this small amount of money. Just in case you haven't seen the silicon

SEPTEMBER, 1968

chip used to act as a piece of real estate for all these components, it's about the size of this small letter "w".

Outdoor renditions generally need more sound power. To get this power you can add an additional out-put stage. While there is no limit to the size and power of the amplifier you can add, you should find this 2-watt rig surprisingly adequate. Battery drain is very low; easily available 6-volt lantern batteries provide hun-dreds of hours of playing time.

How it works

Integrated circuit ICI performs five functions: voltage regulator, buff-er, differential amplifier and phase split-ter, driver, and power-output amplifier.

Now refer to Fig. 1. The signal from IC1 is transformer-coupled (through T1) to push-pull power am-plifier Q 1—Q2 and then fed to the

speaker through T2. Diode 131 pro-tects the IC against reversed battery polarity.

Construction

If you use the printed-circuit board ( Figs. 2 and 3) practically all connections are automatic. If you use point-to-point wiring, keep parts placement as close as possible to that of the PC board to minimize the pos-sibility of wiring error. Watch the key-ing tab on the IC; it is at pin 12. Watch the polarity of electrolytics C1 and C2. The negative terminal of C1 goes to the input jack, and the nega-tive terminal of C2 is grounded.

Diode D1's polarity is the only other critical connection; connect its anode to the junction of batteries BI and B2. This diode, incidentally, al-lows the use of a single-pole on—off switch. It blocks a reverse-current

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Close up of the circuit board shows how easily all the components fit. The tiny 1-µF electrolytic capacitor can be seen in series with the input lead, top left. Com-pare this photo with the printed-circuit board and parts layout on next page.

flow between the 6- and 12-volt leads when the negative lead is opened. If the diode is omitted you will have to use a double-pole switch to open the two positive leads.

The IC leads are on a 'A" circle but holes on the PC board are on a 3/8" circle to make the board easier to

construct and to wire. My speaker did not have a

mounting bracket for an output trans-former so I mounted the transformer on the wall of the speaker case. Its position is not critical. The amplifier was mounted in the box, well away from the speaker to minimize the pos-sibility of any mechanical feedback or vibration.

Figure 4 shows the parts of the speaker box. They are assembled with

glue and screws and then covered with a vinyl leatherette or upholstery material.

Contact cement of the type used for bonding plastic surface material to counter tops is ideal for gluing the covering on the speaker box. However, do nor let the cement dry—as you are instructed to do on the can—before applying the vinyl covering. Put the plastic on while the cement is thor-oughly wet so you can stretch and smooth the plastic into place.

You can use some of the same cement on the grille cloth. Cement it to the face of the speaker baffle board and let the glue dry. Fold the edges around the back of the board and tack or staple in place.

You have a choice of suitable 6-volt batteries. They come with several types of terminals, and some types are more readily available than others. The most readily available is the NEDA type 908 spring-terminal lan-tern battery. You will need a special retainer setup with a special connector board as in Fig. 5.

If you live in an area where al-most all types of batteries are avail-able, the NEDA 915 with binding posts, the NEDA type 6 with a Cinch No. 5A4 plug and the 917 with spring clips can be used. I used the NEDA type 6. If you use the amplifier a lot, two NEDA 918 batteries will last about two and a half times as long as

Fig. 1- The complete circuit of the IC guitar amplifier. Thanks to the IC a lot of wiring has become completely unnecessary.

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Fig. 5—You'll need a battery connector like this if you use lantern batteries with spring terminals such as the NEDA 908.

Fig. 4—Construction of the amplifier/ speaker case. All parts except the cor-ner braces are cut from 3/8" plywood.

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Fig. 2—Parts on the circuit hoard. In this view you are looking at the parts arrangement from the wiring side of the board.

Fig. 3—Exact size drawing of the circuit board. Follow it to make yours. You can trace it or use a photographic process.

C1, C4—I-µF, 12-volt miniature electrolytic or tantalum capacitor

C2, C5—0.1µF, 50-volt ceramic capacitor C-3—0.01 µF, 100-volt ceramic capacitor R1—5000-ohm miniature potentiometer with

s.p.s.t. switch R2—510,000-ohm, 1/4-watt resistor R3—1.2-ohm, 1/4-watt resistor R4—1000-ohm, 1/8-watt resistor

the ones listed above. The amplifier deserves a good

extended-range speaker, not just any old 8-incher. I used a Utah type D8-LA dual-cone unit. The SP8J8 is also

Parts List

R5—33-ohm, 1/4-watt resistor R6, R7—10-ohm, 1/4-watt resistor R8-960—ohm, 1/4-watt resistor T1—Transistor audio interstage transformer,

Pri. 500 ohms ct, sec 150 ohms ct. (Ar-gonne AR-163, Lafayette Radio)

T2—Transistor audio output transformer, Pri. 48 ohms ct, sec 3.2 ohms (Lafayette

good but the best choice is probably a speaker designed especially for gui-tar amplifiers.

The amplifier can be used for other purposes too; any high-imped-

Radio stock No. 33 H 8578) IC1—CA3020 integrated-circuit amplifier (RCA)

Q1, Q2—2N1183 transistor (RCA) BI, B2—6-volt heavy-duty battery (see text) S1-s.p,s.t switch (on R1) MISC.—(4) 1/4" spacers for No. 8 screws, 4

rubber feet, grille cloth, plywood, input jack for guitar cord

ance device such as a crystal micro-phone or phono cartridge will drive it. You can also drive it from the signal at the phone jack of a small transistor radio. R-E

SEPTEMBER, 1968