RAY MARSTON
THIS THIRD ARTICLE IN A SERIES ON active filters focuses on audio signal processing and ampli- tude control circuits. The last article, in the February 1995 is- sue of Electronics Now, page 65, discussed popular passive control networks including high- and low-pass filtering cir- cuits. This article picks up where that article left off with a discussion of active tone-con- trol circuits, and it goes on to explain amplitude-regulating circuits. Active tone controls An active tone-control circuit can be made by connecting a passive tone-control network to the negative feedback loop of a linear amplifier, typically an op- erational amplifier. This circuit provides signal gain rather than attenuation. Tone-control networks can be simplified versions of Fig. 15 in the February 1995 article. How- ever, they are more likely to be based on the alternative passive tone-c~ntrolcircuit shown here as Fig. 1. This circuit's perfor- mance is comparable, but re- quires fewer components, and it includes two linear control uo- tentiometers. rf the input signals to the cir- Learn about active filters in audio cuit in Fig. 1 are low enough so that capacitors C 1 and C2 act signal processing and control, and apply like open circuits, the output signal amplitudes are con- that knowledge to your circuit designs tr6lled entirely by resistor R5. This occurs because resistor R6 is isolated from the output by to 20 decibels (dB) of boost or capacitor C2. cut to bass or treble signals. However, at input frequencies The circuit shown in Fig. 3, that are high enough so that the although similar to that of Fig. two capacitors act like short cir- 2, is more versatile. It has an cuits, the output signal ampli- additional filter control network tudes are controlled entirely by that is centered on the 1-kHz resistor R6. In this situation, rnidband of the audio spec- resistor R5 is short circuited by trum. This network permits the $ C1. midband to be boosted or cut by The low-frequency (bass) cir- as much as 20 dB. cuit cutoff is determined by the g values of Rl and C1, and the Graphic equalizers m(D high-frequency (treble)cutoff is The more sophisticated 2 determined by C2 and the val- graphic equalizer tone-control a ues of R1 to R3. system consists of a many paral- 5 Figure 2 illustrates how the lei-connected, variable-response 5 network in Fig. 1 network is in- filters that overlap and have nar- circuit that includes two potentiorne- tegrated into an active tone con- row-passbands to cover the en- ters. trol circuit that can provide up tire audio spectrum. This cir- 87 are in parallel, and their out- puts are summed in the output stage that includes the ICll op- erational amplifier. The operational amplifiers in this circuit can be the industry- standard LM741 or comparable dual versions such as the LM747. Stereo amplifier sys- tems visually contain two com- plete circuits of the kind shown in Fig. 5.
RIAA equalization Phonograph records are no longer the preferred media for storing and reproducing music, having been replaced years ago by tape cassettes and compact discs (CDs).Nevertheless, many people still own turntables for playing 33 RPM long-playing record (LPs) or 45 RPM records. The pickup arms of those record players include either ceramic, crystal, or magnetic cartridges and needles that are in direct contact with the grooves in the records. The ceramic and crystal car- tridges were inexpensive, but they produced large-amplitude and fairly linear outputs suit- able for low-priced record play- ers. However, magnetic car- tridges were preferred for high- performance, high-fidelity ster- eo systems. Although their out- put is low, and they have FIG. 3--THREE-BAND ACTIVE TONE-CONTROL control (bass, midband, treble). nonlinear frequency response characteristics, their corrected equalizers. output provides more faithful Figure 4 is the schematic for a music reproduction. typical octave (graphic) equal- The characteristics of any rec- izer section. It is similar to the ord playback system can be de- active tone-control circuit termined with a test record shown in Fig. 2, except that tre- containing a three-decade span ble control network consisting of sinewave tone signals with of C2, R3, and R4 is fixed rather constant amplitude from 20 Hz than variable, and the bass and to 20 kHz. A quality magnetic treble cutoff frequencies are cartridge should generate a spaced closely. As a result, the nonlinear frequency response two response curves overlap. that rises at a rate of 6 dB per Consequently, the circuit in octave (equal to 20 dB per de- Fig. 4 circuit acts as a narrow- cade). Thus the output signals FIG. 4-TYPICAL OCTAVE (GRAPHIC) band filter with a center fre- would be weak at 20 Hz, but equalizer section. quency response that is fully would be one thousand times variable between + 12 dB (full greater (equal to 1-60 dB) at 20 cuitry permits tailoring an am- boost) and - 12 dB (full cut) by kHz. plifier circuit's spectral re- adjusting potentiometer R5. This nonlinear frequency re- sponse to suit individual needs. Figure 5 shows how the cir- sponse is an inherent charac- The filter center frequencies are cuits of Figs. 3 and 4 are inter- teristic of all magnetic pickups typically spaced at one octave connected to form a high because their output voltage is intervals. As a result, those sys- quality, ten-band graphic equal- directly proportional to the tems are also called octave izer. The ten equalizer sections pickup needle movement rat? most low-end record players. Figure 6 shows a plot of a typ- ical phonograph record play- back frequency response curve as a solid line and dotted line superimposed on the solid line that represents the ideal re- sponse curve. The ideal (dotted- line) curve is flat to 50 Hz where it rises at a 6 dB1octave rate to 500 Hz. It remains flat from 500 Hz to 2120 Hz, then rises again at a 6 dB1octave rate to about 20 kHz. When a record is played through a magnetic pickup in a high quality hi-fi system, the output of the pickup is pre- amplified before going to the power amplifier. The pre- amplifier must have a frequency equalization curve that is the exact inverse of that shown in Fig. 6, so that an overall linear response is obtained. Figure 7 shows the RIAA equalization
FIG. 5--TEN-OCTAVE (GRAPHIC) equalizer circuit. FIG. 7-RIAA PLAYBACK equalization which, in turn, is proportional curve. to recording frequency. Phono disk recording equip- curve. It is the inverse of record- ment usually did not provide ing curve shown in Fig. 6. truly linear frequency response. To enhance the effective dynam- .RIMpreamplifier ic range and signal-to-noise Magnetic pickup cartridges ratio of records, frequencies be- are low-sensitivity devices that low 50 Hz and those in the 500- give typical midband outputs of Hz to 2.12-kHz midband range only a few millivolts. Con- were recorded nonlinearly in ac- sequently, their output signals cordance with a standard curve must be preamplified by a dedi- defined by the Recording Indus- cated, low-noise preamplifier in- try Association of America tegrated circuits rather than IRIAA). general-~urposeo~erational
FIG. B-TYPICAL PHONOGRAPH record pickups, but this decrease was includes an LM381 ~ow-IIo~S~, playback frequency response curve. too small to be objectionable in dual preamplifier IC. 89 aged in an 8-pin DIP) Because nals form the magnetic pickup two of these preamplifier cir- cartridge are AC coupled to the cuits are needed in a stereo au- LM381 by C1. dio system, both sections of the dual preamplifier ICs are used. Nonlinear amplifiers The LM381 has a high, open- An operational amplifier will loop gain of 112 dB, and a total act as a nonlinear amplifier if a input noise rating of 0.5 micro- nonlinear component is in- volts. Its output voltage swing cluded in its negative feedback equals the supply voltage minus network. Figure 9 shows two 2 volts and it has a wide power square-law response (non- bandwidth of 75 kHz, 20 volts linear) feedback elements, a peak-to-peak. The LM387 is pair of diodes connected back- similar to the LM381 except that to-back in the feedback loop. it has an open-loop gain of 104 When small signals are ap- dB and a total input noise rat- plied to this circuit, the diodes ing of 1.0 microvolt. The ampli- act as infinite resistance (open fiers in both ICs are electrically circuits), so circuit gain is high. independent of each other. However, when large signals are FIG. &LOW-NOISE MAGNETIC ~hono The LM381 in Fig. 8 is config- applied, the diodes act a low re- cartridge preamplifier that iniludes ured as a noninverting ampli- sistances, SO circuit gain is low. RlAA equalization. fier. Negative feedback is ap- The gain follows a semi-log- plied from the output to the arithmic function, and circuit inverting input terminal. The sensitivity can be varied by al- voltage divider consisting of re- tering the value of resistor R1. sistors R3 and R4, and the net- Table 1 summarizes the circuit work formed by resistor R6 and performance with two different capacitor C2 determines AC sig- values of R1-1 and 10 kilohms. nal gain. For example; it can be seen that At the audio frequency mid- a 1000:1 change in input signal band (centered on 1 kHz) capac- amplitude causes a change as itors C2 and C3 have low smallas 2:l in output level. This impedances and C4 has a high characteristic can be put to impedance. As a result, AC gain practical use in single-range is determined principally by the bridge-balance detectors and value of resistor R5 divided by signal-strength indicators. Volt- R6, and it equals about 400. At age measurements can be made fier based 0" the 741 operational amp[[- lower frequencies, the imped- with an AC millivoltmeter. fier. ance of C3 becomes significant When a sinewave input is ap-
TABLE 1 TABLE 2 NONLINEAR AMPLIFIER PERFORMANCE CONSTANT - VOLUME AMPLIFIER PERFORMANCE / Millivalts Rl=lK R1= IOK / (input, RMS) V,,, VGAtN Vom VouN mV RMS mV RMS 1 .o 11 0 x 110 21 x 21 1 lo.0 330 x 33 1 70 x17 100.0 450 x 4.5 360 X 3.6 7000.0 560 x 0.56 470 x 0.47 j 10,000.0 600 x 0.07 560 x 0.56
3 As an alternative, a National and it causes AC gain to.in- Semiconductor LM387, an- crease until, at very low fre- other low-noise, dual pre- quencies. it is limited to 4000 $ amplifier will work in the cir- by the ratio of the value of re- cuit. Both the National LM381 sistor R3 with res~ectto R6. and LM387 are suitable as am- By contrast, at high frequen- plifiers in audio-tape playback cies, the impedance of C4 falls plied to the circuit, the two di- preamplifiers. significantly, shunting R5. This odes limit the output voltage The pin numbers shown in causes the AC gain to decrease swing to about 1.4 volts peak-to- Fig. 8 are those of the first half until, at very high frequencies, peak by clipping the waveform. of the LM381, packaged in a 14- it is limited to 10by the ration of The output approximates a pin DIP (The LM387 is pack- the values of R4 to R6. The sig- Continued on page 93 AUDlO FILTERS FET gate from the network con- sisting of Dl, R5, R6 and C3. continued from page 64 The FET functions as a resis- I*Ail-80 ROM WRITER KIT Nit: $90.00 I I .Avaiiable IoruseanhlBM PB OOSN~ersonalco tance with a value of several CanespondstoROM2764.27128.27256 InteioalZB0developng lunct,on square waveform, and it is rich Contents hundred ohms. The voltage di- Dedcated board
The operational amplifier is signal level. (This is equal to configured as an AC amplifier 500 to 10 millivolts.) It does this with its gain controlled by the without generating audible sig- ;Please send me free facts. color brochure and full information ; ratio of the values of resistor R1 nal distortion. The circuit's per- with respect to K2 and by the AC formance is summarized in voltage divider formed by R4 in Table 2. BUS.Mgmi.4nance opmn Electrical Engiieeringiechnoiogy series with the internal imped- The value of resistor R1 deter- I 0BUS. Mgmt.--Marketing Optlon EIectronlcsTechnology I Applied Computer Science D Industrial EngineeringTechnology ance of Q1. mines the sensitivity of the cir- CAREER DIPLOMA COURSES I High School MotorcycleRepair This FET functions as a volt- cuit. It is selected to accommo- , Auto Mechan~cs 0 CaterlnB/Gourmet Cooklng Surveying and Mapping Computer Programming age-controlled resistor. Its con- date the maximum input signal 0 Personal Computer Specialist 1 !%,"!d'lioning & Refr~geration Computer-AssistedBookkeeping trol voltage is obtained from the amplitude that the circuit is ex- W~ldlife/ForestryConse~aiion PC Repalr 1 Fashion Merchandising 0DESH0p P~blIshinQ& Design output of the operational ampli- pected to handle. This is deter- Diesel Mechanics Secretaw wdh Computer fier with a network formed by mined on a basis of 200 kilohms ' 0Electrician Ftness & Nutrdlon I Small Business Management NNCR Repair . Gun Reoa~r Animal Care Soecialist diode Dl in series with resistor per RMS volt of input signal. 1 Electronics o Photography ' 1 x I 0HoteVRestaurant Management Journalism/Sholt Story Wining 5 R5 and resistor R6 in parallel For example, to accommodate Child DayCare Florist 1 2. Legal Assistant Teacher Aide with capacitor C3. a maximum input of 50 volts, R1 0Interior Decorating MedicalTranscr~ptionist When a small signal is applied should have a value of 10 Medical Oflice Assistant 0Professional Locksmithing 'Z ! Bookkeeping Appliance Repa~r !O to the 741, its output is small. megohms. Capacitor C3 deter- Name Age- I2 Consequently, very little nega- mines the automatic gain control I Adlress Apt. #- I tive bias is developed on the time constant of the circuit. R I CiiyBLate Zip I 93 I Phone ( )