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Build a three-band parametric equaliser If you are interested in musical instruments, public address systems or any application where you need fine control of the audio spectrum, then this three-band parametric equaliser could be just what you want. It is a very quiet, low-distortion circuit that is easy to use. Design by BOB FLYNN T HERE ARE many audio appli- cations where simple tone con­trols or graphic equalisers just can’t do the job. For the most precise control of the audio spectrum, a onethird octave graphic equaliser is the best but it is a complex unit. Such a graphic equaliser will have 30 or more sliders to cover the full audio range but its capabilities may be wasted in many situations. For example, you may only have two or three troublesome peaks or dips in the response and these could possibly be fixed by nudging only three of the sliders – all the rest would be unnecessary. 70  Silicon Chip By contrast, a three-band parametric equaliser can do many of the tasks of a graphic equaliser and it is a much simpler unit with considerably less active circuitry. Our parametric equaliser has three frequency bands, with their centre frequency adjustable over the nominal ranges from 40Hz to 160Hz, 320Hz to 1.3kHz and 2.2kHz to 5kHz. While they do not overlap, these ranges have been selected as a good compromise between overall circuit complexity, minimum interaction between ranges, ease of use and audible effectiveness. We could have added more bands but since each band needs a minimum of three potentiometers, the number of knobs on the control panel rapidly gets out of hand. With three bands we end up with 10 controls in all. The controls for each band are frequency, boost/cut and Q. The frequency control is self-explanatory – it tunes the centre frequency for each band; ie, the frequency at which the boost or cut setting is at a maxi­mum. The boost/cut control is same as a bass or treble control. In its centre setting the frequency response for the band is flat; when rotated clockwise, boost is applied and when rotated anticlockwise, the frequencies are cut. The third control is labelled “Q” and this knob determines whether the boost will be applied as a sharp peak or over a much broader range of frequencies. Similarly, when cut is applied, the Q control determines whether the cut will result in a deep notch or a much broader “valley” in the response. Let’s look at a few examples to see how the parametric equaliser works in practice. Have a look at the response curves in Fig.1. There are actually three response curves, all with the Q control set for maximum. The top AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 15.000 17 MAY 96 11:21:36 AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 15.000 10.000 10.000 5.0000 5.0000 0.0 0.0 -5.000 -5.000 -10.00 -10.00 -15.00 17 MAY 96 12:37:47 -15.00 20 100 1k 10k 20k 20 100 1k 10k 20k Fig.1: these boost and cut response curves were taken with the Q control set for maximum. The top curve shows the effect when maximum boost is applied in all three bands. This results in three sharp peaks centred at about 64Hz, 490Hz and 3.3kHz. Each one of those peaks can be moved back or forward within its respective frequency band, by rotating the relevant frequency control. Fig.2: this set of response curves was taken with the Q controls set for a medium value; ie, with the control centred. The first curve shows the low band set for medium cut while the other two bands have medium boost applied. The second is the reverse, with medium boost applied in the low band and medium cut applied in the middle and top bands. AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 15.000 AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 15.000 17 MAY 96 11:29:54 10.000 10.000 5.0000 5.0000 0.0 0.0 -5.000 -5.000 -10.00 -10.00 -15.00 17 MAY 96 11:33:38 -15.00 20 100 1k 10k 20k Fig.3: one of these curves shows the low and top bands boosted while the centre channel is cut. The other curve shows the low and top bands cut and the centre channel boosted. curve shows the effect when maximum boost is applied in all three bands. This results in three sharp peaks as you can see, centred at about 64Hz, 490Hz and 3.3kHz. Each one of those peaks could be moved back or forward within its respective frequency band, by rotating the relevant frequency control. The bottom curve shows the same frequency and Q settings as for the top curve except that the boost/cut control is now set to maximum cut. Meanwhile, the third curve which is between the top and bottom traces shows the overall flatness of response 20 100 1k 10k 20k Fig.4: this pair of frequency plots shows the low band set for a flat response, while the centre and top bands have either modest boost or cut. when the boost/cut controls are all centred. The response is less than 1dB down at 20Hz and 20kHz. As shown by the above curves, the maximum boost and cut which can be obtained at any frequency within the band ranges is ±10dB. Note that you can have any combination of boost & cut, frequency and Q settings so the number of response curves you could obtain is virtually infinite. It means you can compen­ sate or “equalise” the frequency response for many “real world” applications. Fig.2 gives another set of response curves, this time with the Q controls set for a medium value; ie, with the controls centred. The first curve shows the low band set for medium cut while the other two bands have medium boost applied. The second is the reverse, with medium boost applied in the low band and medium cut applied in the middle and top bands. Fig.3 is another variation on the theme, this time with the low and top bands boosted while the centre channel is cut and then with the low and top bands cut while the centre channel is boosted. Finally, Fig.4 is a pair of frequency plots with the low band flat while the July 1996  71 AUDIO PRECISION SCTHD-HZ THD+N(%) vs FREQ(Hz) 5 17 MAY 96 12:42:53 1 AUDIO PRECISION SCTHD-HZ THD+N(%) vs FREQ(Hz) 5 17 MAY 96 13:36:10 1 0.1 0.1 0.010 0.010 0.001 0.001 .0005 .0005 20 100 1k 10k 20k Fig.5: total harmonic distortion versus frequency with all the boost/cut controls centred (ie, with a flat response), at a level of 1.5V RMS. AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 15.000 20 100 1k 10k 20k Fig.6: total harmonic distortion versus frequency, taken with the three bands set for maximum boost and high Q, as in Fig.7. 17 MAY 96 12:54:11 Performance Frequency response ............... (see graphs) 10.000 Signal-to-noise ratio ............... 99dB unweighted (22Hz to 22kHz); -103dB A-weighted, with respect to 1V RMS (with boost/cut controls centred) 5.0000 0.0 Harmonic distortion ................ see graphs -5.000 Maximum output level ............ 9.3V RMS Maximum boost & cut ............. ±10dB -10.00 Range of Q ............................. 0.45 to 5 -15.00 20 100 1k 10k 20k Fig.7: response curve with all bands boosted; this is the test condition for the distortion measurement of Fig.6. other two bands have modest boost or cut. Fig.5 is a plot of total harmonic distortion versus fre­quency with all the boost/cut controls centred (ie, with a flat response), at a level of 1.5V RMS. As can be seen the distor­tion is very low, averaging about .002%. Fig.6 is another plot of total harmonic distortion but this time with the three bands set for maximum boost and high Q, as in Fig.7. This time the distortion is somewhat higher but still satisfactory for the applications in which the circuit is likely to be used. Circuit description Fig.8 shows the complete circuit of the three band paramet­ric equaliser. It is based on three “state variable” filters, one for each of the bands. Each of the state variable filters is identical 72  Silicon Chip Supply current ........................ 30mA (typical) at ±15V apart from the capacitors which determine their fre­quency ranges. All the op amps are LM833 dual low noise types. Eleven op amps out the total of 12 are used and IC2b is unused. To simplify the discussion of the state variable filters, let’s confine ourselves to band 1, the low frequency band. It employs IC1a, IC1b and IC2a. The latter two op amps are integra­tors with their frequency cutoff determined by the 0.12µF ca­ pacitors and their tuning controlled by the 25kΩ dualganged pot VR3a & VR3b. State variable filters have three useable outputs: high-pass, low-pass and bandpass (ie, low-pass and high-pass in combi­nation). The bandpass output is the one we want and this is taken from the output of IC1b, via the 6.8µF non-polarised (NP) capaci­tor. The Q of the filter is controlled by IC1a, in conjunction with the 100kΩ dual-ganged pot VR2a & VR2b. VR2a is in the input to IC1a while VR2b is in the feedback loop from IC1b to IC1a. Both pot sections are wired as variable resistors. Notice that the wipers of VR2a & VR2b are shown with an arrow to show clock­wise rotation of the knob; maximum clockwise rotation gives maximum resistance for VR2a & VR2b and this corresponds to the maximum Q condition. The three state variable filters are Fig.8 (right): the parametric equaliser is based on three “state vari­able” filters, one for each of the bands. Each of the state variable filters is identical apart from the capacitors which determine their frequency ranges. July 1996  73 Fig.9: follow this layout diagram when installing the parts on the PC board. In particular, check that the ICs are correctly oriented and don’t get the pot values confused. 74  Silicon Chip Fig.10: check your board carefully for etching defects before installing any of the parts by comparing it against this full-size pattern. There are quite a few links on the board and these should be installed before any other components are soldered in. Take care to ensure that all polarised parts are correctly oriented and note that the ICs all face in the same direction. effectively in paral­lel and connected into the feedback network of op amp IC6b on the input side and into the input circuit of op amp IC4b on the output side. When all the boost/cut controls are centred, the gain of the circuit is unity over the whole audio frequency range. When one of the boost/cut controls is set to boost, the signal from the accompanying state variable filter is increased to IC4b, while the feedback to IC6b is reduced. Hence, the gain is boosted for that particular band. VR1 provides an input volume control for the whole circuit. We assume that for most applications it will be set for maximum input signal to the circuit and thereby give an overall gain of unity; ie, 1V in gives 1V out. The circuit is designed to run from TABLE 1: CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.12µF   120n   124 0.1µF   100n   104 .015µF   15n   153 .0022µF   2n2   222 ±15V supply rails and these will normally be supplied by 3-terminal 15V regulators in the main amplifier or mixer. The rails are heavily bypassed with 100µF and 0.1µF capacitors to ensure good stability. Assembly We are presenting this project as a PC board which can be installed in a case together with a suitable power supply or incorporated into a larger piece of equipment. The PC board measures 230 x 72mm and is coded 01107961. To make the board size manageable it has been designed around 16mm diameter pots. Actually, we could have made the board a good deal smaller but in practice, the knobs need to be spaced so that typical male fingers can operate them comfortably. By itself, the PC board is difficult to use unless you also have the control panel; otherwise you don’t know where the pots are set. We have designed a control panel which measures 249 x 59mm. The completed PC board and control panel have been designed to fit neatly into a plastic    TABLE 2: RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 6 10 6 3 1 Value 100kΩ 20kΩ 10kΩ 8.2kΩ 4.3kΩ 100Ω 4-Band Code (1%) brown black yellow brown red black orange brown brown black orange brown grey red red brown yellow orange red brown brown black brown brown PARTS LIST 1 PC board, code 01107961, 72mm x 230mm 1 front panel label, 249 x 59mm 10 knobs to suit 16mm pots, 15mm in diameter 2 metres 0.71mm dia. tinned copper wire (for links) 7 PC pins 6 LM833 low noise dual op amps (IC1-1C6) Potentiometers (all 16mm dia.) 3 100kΩ linear dual-ganged pots (VR2,5,8) 3 25kΩ linear dual-ganged pots (VR3,6,9) 3 10kΩ linear pots (VR4,7,10) 1 50kΩ logarithmic pot (VR1) Capacitors 4 100µF 16VW electrolytic 4 6.8µF non-polarised electrolytic 1 2.2µF non-polarised electrolytic 2 0.12µF 63V MKT polyester 6 0.1µF 63V MKT polyester 2 .015 63V MKT polyester 2 .0022 63V MKT polyester Resistors (0.25W, 1%) 1 100kΩ 6 8.2kΩ 6 20kΩ 3 4.3kΩ 10 10kΩ 1 100Ω 5-Band Code (1%) brown black black orange brown red black black red brown brown black black red brown grey red black brown brown yellow orange black brown brown brown black black black brown July 1996  75 Running The Circuit From A 12V Supply Fig.11: this full-size artwork can be used as a drilling template for the front panel. Fig.12: use this power supply arrangement if you wish to run the parametric equaliser from the 12V supply in a car. 76  Silicon Chip While the parametric equaliser has been specifically de­ signed to run from balanced ±15V rails, it is also possible to run the whole circuit from a single 12V supply, as would be the case if the unit was used in a car. The distortion, signal han­ dling and signal-to-noise ratio will not be as good but for car applications its performance would still be more than adequate. To run from 12V it will be necessary to split the supply to effectively give ±6V rails. This can be done by wiring two 4.7kΩ instrument case measuring 259 x 65 x 180mm (W x H x D). This has space for a power supply and is available from Jaycar Electronics with plastic front and rear panels (Cat. HB-5974) or with aluminium panels (Cat. HB-5984). The full wiring details for the PC board are shown in Fig.9. Start construction by checking the PC board against Fig.10. Fix any shorts or broken tracks that may be evident. There should not be any of these faults but if they are present it is better to fix them before any parts are soldered in. There are quite a few links shown in Fig.9 and these should all be installed before the other components. This done, fit the resis­tors. Table 2 shows the colour codes for all the resistor values specified. Use your multimeter to check the resistor values if you are not sure of the colour codes. Next, fit all the capacitors, making sure that the electro­lytics are correctly polarised; ie, connected the right way around. Now fit all resistors across the 12V supply, as shown in Fig.12. However, the input and output signal earths will no longer be tied to the centre rail; instead, they go to the 0V rail. This means that input earth, the grounded side of the input pot VR1 and the output earth must all be isolated from the earth system (supply centre tap) and connected instead to the 0V line of the incoming 12V supply. If this is not done correctly, there will be a short across the -6V rail and the circuit will malfunction. six ICs; they are all oriented in the same direction. Last, fit the pots and make sure you don’t get the 25kΩ and 100kΩ pots swapped around. Check your work carefully against the wiring diagram when you are finished. Power up When the board is complete, connect a DC supply set to ±15V and check the voltages. +15V should be present at pin 8 of each LM833 while -15V should be at pin 4 of each IC. Then, if you check the output of each op amp, pins 1 or 7, the voltage should be close to 0V. The exception is pin 7 of IC2b (unused) which is likely to be at -15V; this does not matter. Further testing cannot be done until you make input and output connections to the board via shielded cable. You can then use an audio oscillator and an oscilloscope (or an AC milli­voltmeter or DVM with a wide frequency re­sponse) to check the effect of SC each control.