application note 1 of 2 www.xicor.com june, 2000 AN49 digital audio ampli?r gain control using logarithmic xicor digitally controlled potentiontiometers (xdcps) by applications staff often the power ampli?r stage of a communications device is located some distance from the control panel. the typical solution is to mount the volume control on the front panel and run a pair of shielded cables back to the power ampli?r stage. a better solution is to use the xicor x9314 logarithmic taper digitally controlled potentiometer. this completely eliminates the need for shielded cables since the digital pot can be located on the printed circuit board near the low level input stages of the power ampli?r. the up/down volume switches can be located yards away as these are merely low speed ttl signals. circuit description figure 1 shows the x9314 with the low voltage lm386 power ampli?r. this circuit will produce 250mw of audio power with only a 5v dc source. figure 2 is a similar circuit that uses the higher power lm380 and is capable of delivering over 1w of audio if a 12v dc source is available. either circuit will provide plenty of sound for intercom, communications, or monitoring applications. the 1w circuit would be preferred when higher ambient noise is present, such as in an automobile. a 74hc132 quad schmitt nand gate (u2) plus a few discretes are all that are required to interface to u1, the x9314w 10k ? xdcp. when either pushbutton switch, s1 or s2, is depressed, u2a output will be high. this immediately brings u2b output low which enables the xdcp. after a short delay determined by r3 and c3, the astable multivibrator (u2c) will begin generating incre- ment pulses. the frequency of the oscillation (and consequently the speed at which the volume can be controlled) is determined by c1 and r1. the values shown will advance the x9314 through all 32 wiper taps in about 5 seconds. r1 can be changed from 5kw to 1mw to suit your preference. when the pushbutton switches are released, r2 and c2 delay the release of the x9314 chip select in order to ensure that the last incre- ment pulse is written. the x9314w needs little else to apply it to most audio circuits, but c9 has been included here to remove any dc component that may be present on the input signal. also, lowpass ?ter r7 and c8 removes residual high frequency noise from the digital pots internal charge pumps. the lm380 and lm386 power ampli?rs have ?ed gains of 34db and 26db respectively. the gain of the lm386 can be increased to 46db (x200) by connecting a 10uf capacitor from pins 1 to 8. the positive pole of the capacitor should be connected to pin 1. the output snubber network (r6 and c5) is required for stability and should be located close to the output pin of the power ampli?r. these ampli?rs have fairly high gain and wide bandwidths, so be careful with pcb trace lengths. all connections to these ampli?rs should be as short and direct as possible.
2 of 2 AN49 application note www.xicor.com june, 2000 figure 1: low voltage audio ampli?r v cc r5 10k r4 10k s1 decrease s2 increase r1 220k c1 1f c3 1000pf cr1 1n4148 c2 1000pf 4 5 10 9 8 6 r3 27k 1 2 3 inc cs u/d r2 27k cr2 1n4148 j1 jack audio input c9 1f 3 5 6 r7 7.5k - + 4 6 2 3 c8 470pf c6 .1f c7 .1f v cc c10 10f lm386 2.7 r6 .1f c5 c4 250f 2.7 4 ohm speaker u2d 74hc132 74hc132 74hc132 u2b u2a vh vw vl 2 7 1 5 x9134 u1 figure 2: 1 watt audio ampli?r v cc r5 10k r4 10k s1 decrease s2 increase r1 220k c1 1f c3 1000pf cr1 1n4148 c2 1000pf 4 5 10 9 8 6 r3 27k 1 2 3 inc cs u/d r2 27k cr2 1n4148 j1 jack audio input c9 1f 3 5 6 r7 7.5k - + 7 4 2 3 c11 .1f c6 .1f c7 .1f lm386 2.7 r6 .1f c6 c4 250f 2.7 4-16 ohm speaker u2d 74hc132 74hc132 74hc132 u2b u2a vh vw vl 2 7 1 5 x9134 u1 +v bat 6 u3 c10 10f out in gnd v cc 2 1 9v-16v dc lm78l05ac2 vr1 8 1 +v bat u2
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