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Loudness Control For car hifi systems Most cars with big sound systems have loads of features but here’s one they usually don’t have – a loudness control. Now you can add a loudness control with this circuit which involves a quad op amp and not much else. Design by RICK WALTERS Why would you want a loudness control in a car? Well, con­ trary to what you might expect, not everyone with a big sound system in his or her car wants to cruise the boulevardes with the windows wound down and the levels wound all the way up all the time. For a start, it can give you a headache if you do it for long periods and the police tend to frown a bit . . . not to men­tion that it will ultimately send you deaf after a while. “What’s that?” you say. Using this loudness control will let you hear the highs and lows better without having to turn the wick up The prototype was housed in a standard plastic utility case. The knob controls the volume while the switch allows the loudness circuit to be bypassed. 54  Silicon Chip so far. It provides a similar function to the Loudness switch on many hifi amplifiers but does not rely on a special tapped volume control. But as often happens with articles of this sort, we’re getting a little ahead of ourselves and we need to explain the theory behind Loudness controls. Our ears are not perfect, funnily enough. While they re­ spond to an enormous range of sound levels, from whisper quiet to the roar of a jet engine, and with a frequency range from around 16Hz up to as high as 20kHz, we just don’t hear all frequencies equally well, unless the sounds are very loud. In effect, when sound levels are low, we don’t hear bass frequencies particularly well at all, and to a lesser extent, we don’t hear the treble well either. This has been well documented for many years and was pub­lished in October 1933 in the “Journal of the Acoustical Society of America” by H. Fletch­er and W. A. Munson. Fletcher & Munson produced a famous set of curves, shown in Fig.1. These are “equal loudness curves” taken at sound levels from very soft (0dB) up to very loud (120dB). As you can see from these curves, at the softer levels, our ears are far less sensitive to bass and treble fre­quencies. To partly compensate for this, some hifi amplifiers have Loud­ ness controls. Most of these just boost the bass at lower volume settings but do not boost treble. Whether these controls should be on hifi amplifiers is argua- ble but many people like this facility so that is why we are presenting this project. To understand what our Loudness control does, have a look at the curves in Figs.2, 3, 4 & 5. Fig.2 shows the frequency re­sponse at a low setting of the Loudness pot, with the control wound up 25% from the zero setting. As you can see there is about 10dB of bass boost compared to the mid-frequencies and about 8dB of treble boost. This goes a long way towards compensating for those hearing losses we’re talking about. In Fig.3 we have a similar set of curves but now the Loud­ness pot is at half rotation. You can see that the bass boost is slightly higher and the treble boost is slightly reduced compared with the curve in Fig.2. Fig.4 shows a similar story, with a reduction in the boost available. Finally, Fig.5 shows the fre­ quency response when the Loudness control is fully wound up and now you can see that the response is virtually flat across the whole frequency range; ie, no boost at all. The reason for having the boost cut back as you wind up the control is twofold. First, you don’t need lots of boost when the music is very loud and second, by cutting back the boost so that the frequency response is flat, there is less chance of overload­ing the amplifiers and loudspeakers. This is most important because if you consistently overload your loudspeakers they will not only sound horrible but there is a big risk of burning them out. Fig.6 shows how the Loudness control could be added into a typical car sound system. It is interposed between Fig.1: Fletcher & Munson “equal loudness curves” taken at sound levels from very soft (0dB) up to very loud (120dB). These curves demonstrate that our ears are far less sensitive to bass frequencies and somewhat less sensitive to treble as the sound level is reduced. Reproduced by courtesy of “Journal of the Acoustical Society of America”. the cassette/tuner and the electronic crossover. The line level signal from the cassette/tuner will typically be no more than 1V RMS. In use, you would first wind up the Loudness control to its maximum setting and then set the volume control on the cassette/tuner to give the highest setting that you are ever likely to want. From then on, you use the Loudness control to set the audio level you want and you can use the bypass switch to cancel the bass and treble boost if you desire. Circuit details AUDIO PRECISION SCFREQRE AMPL(dBr) & AMPL(dBr) vs FREQ(Hz) 15.000 Now let’s talk about the circuit 26 OCT 97 22:12:14 15.00 which is shown in Fig.7. This uses a TL074 quad FET-input op amp and not much else. Looking at the left channel, the input signal is fed via a 0.15µF capacitor to IC1b which is connected as a unity gain buffer. This gives a high input impedance to prevent our circuit from unduly loading the program source and a low output impedance which we need to allow the loudness control to operate properly. The buffered outputs are fed via 10µF capacitors to the top of a 100kΩ ganged volume control, VR1a. Ignoring the components associated AUDIO PRECISION SCFREQRE AMPL(dBr) & AMPL(dBr) vs FREQ(Hz) 15.000 26 OCT 97 22:12:55 15.00 10.000 10.00 10.000 10.00 5.0000 5.000 5.0000 5.000 0.0 0.0 0.0 0.0 -5.000 -5.00 -5.000 -5.00 -10.00 -10.0 -10.00 -10.0 -15.0 -15.00 -15.00 20 100 1k 10k 20k Fig:2: frequency response in both channels with the Loudness control wound up 25% from the zero setting. -15.0 20 100 1k 10k 20k Fig:3: frequency response in both channels with the Loudness control wound up 50% from the zero setting. December 1997  55 AUDIO PRECISION SCFREQRE AMPL(dBr) & AMPL(dBr) vs FREQ(Hz) 15.000 26 OCT 97 22:13:44 15.00 AUDIO PRECISION SCFREQRE AMPL(dBr) & AMPL(dBr) vs FREQ(Hz) 15.000 23 OCT 97 21:55:43 15.00 10.000 10.00 10.000 10.00 5.0000 5.000 5.0000 5.000 0.0 0.0 0.0 0.0 -5.000 -5.00 -5.000 -5.00 -10.00 -10.0 -10.00 -10.0 -15.0 -15.00 -15.00 20 100 1k 10k 20k Fig.4: frequency response in both channels with the Loudness control wound up 75% from the zero setting. with switch S1a for a moment, the signal from the wiper of VR1a is fed through a 0.1µF capacitor to the input of another unity gain buffer which feeds the output via an electrolytic capacitor. With S1a in the bypass Parts List 1 PC board, code 01111971, 102 x 46mm 1 plastic utility case, 127 x 68 x 42mm 1 100kΩ dual ganged linear potentiometer 1 knob to suit potentiometer 4 RCA chassis mount sockets 1 14 pin IC socket (optional) 12 PC stakes 2 6mm untapped spacers Semiconductors 1 TL074 quad operational amplifier (IC1) 1 1N914 or 1N4004 diode (D1) Capacitors 2 100µF 25VW PC electrolytic 4 10µF 16VW PC electrolytic 2 0.15µF MKT polyester 2 0.1µF MKT polyester 2 .033µF MKT polyester 2 .001µF MKT polyester -15.0 20 100 1k 10k 100k 200k Fig.5: frequency response of the Loudness circuit at maximum gain or in the bypass setting. setting, the frequency response is flat, as shown in Fig.5. Note that the components associated with the bypass switch have no effect on the frequency response when S1a is in the bypass setting. Even though we effectively have two capacitors, .033µF & .001µF, and two resistors, 15kΩ & 3.9kΩ, in series across the 100kΩ potentiometer, they have negligible effect on the response because of the very low AC output im­pedance of the buffer stage IC1b. But when the Loudness function is switched in, those four components across the potentiometer have a major effect, depend­ing on the volume setting. To explain how the boost works assume the volume control is set to mid-position. Now we see that the bottom half of the potentiometer is effectively shunted to ground by capacitor C2 and resistor R2. This means that frequencies above, say, 300Hz are progressively reduced which is another way of saying that the bass is progressively boosted. At the same time, the top half of the potentiometer is shunted by capacitor C1 and resistor R1. At the higher frequencies, say above 3kHz, the impedance of C1 will progressively reduce, allowing more high frequency signal to be fed from the top of the control to the wiper, giving treble boost. This interaction between the boost components and the wiper position is quite complex, and as noted above, the amount of bass and treble boost is progressively reduced at higher settings of the volume control. We have selected component values which we feel give satisfying results without going overboard. The circuit is powered from 12V DC which we assume will be from the battery in a car. Alternatively, if you wish to build the Loudness control into an amplifier or preamplifier, it could be run from any supply rail ranging from +12V up to +30V without any component changes. Diode D1 prevents any damage to Specifications Frequency response ������������� -0.3dB at 20Hz and 200kHz at maximum clockwise or bypass setting Resistors (0.25W, 1%) 4 330kΩ 2 10kΩ 2 100kΩ 2 3.9kΩ 2 15kΩ Bass & treble boost ................ +10dB at 90Hz and +8dB at 12kHz Miscellaneous Red and black hookup wire, solder. Input overload capability ........ 2.85V RMS with a 12V DC supply rail 56  Silicon Chip Signal to noise ratio ��������������� -106dB unweighted (20Hz to 20kHz) with respect to 1V RMS. Total harmonic distortion ........ less than .003% at 1V RMS the circuit if the supply voltage is connected the wrong way around. Normally, an op amp such as the TL074 is used in a circuit with balanced supply rails, eg, ±15V. In this case, we split the incoming 12V supply with a voltage divider consisting of two 10kΩ resistors. This provides a 6V supply to bias the op amps and this is fed to their non-inverting inputs via 330kΩ resistors. We should make one point about the dual-ganged potentiome­ter used in this project. Normally, volume control potentiometers have a logarithmic resistance/rotation characteristic but we have specified a linear pot. This has proved satisfactory and has a smooth and progressive action in this circuit. It also has the advantage of better matching between the two track sections. Putting it together We have assembled the Loudness Control into a plastic utility case measuring 127 x 68 x 42mm. This has the dual-ganged potentiometer Fig.6: this shows how the Loudness control could be added into a typical car sound system. It is interposed between the cassette/tuner and the electronic crossover. and bypass switch at one end and the RCA input and output sockets at the other end. The PC board measures 102 x 46mm and is coded 01111971. Some people may wish to delete the bypass switch and if this is so, the PC board may be mounted into an alternative case which is pictured elsewhere in this article. The wiring diagram for the PC board is shown in Fig.8. Before assembling any components onto the PC board, check for any defects such as shorted or open-circuit tracks or undrilled holes. Make any necessary repairs before installing components. Begin by fitting and soldering the three links, then the resistors and diode. Next fit the IC socket if you use one, followed by the PC stakes and the capacitors. Make sure that the electrolytic capacitors and diodes are installed the right way around. Then fit the potentiometer. We have made provision for conventional 25mm dia­ meter pots or the small 16mm diameter type. The wires for the inputs, outputs Fig.7: each channel of the circuit uses a FET-input op amp con­nected as a unity gain buffer. The loudness boost circuit itself is passive, reducing signal in the midrange to obtain bass and treble boost which varies with the control setting. December 1997  57 Fig.8 (above): this is the component layout and wiring diagram. Shielded cable is not required for the signal connections. Fig.9 (left): actual size artwork for the PC board. If you don’t want to include the bypass switch, the unit can be housed in this more compact plastic case which measures 120 x 60 x 50mm. and power should now be soldered on the PC board. The holes for the RCA sockets and power wires should be drilled in one end of the case while holes for the bypass switch and dual-gang potentiometer are drilled at the other end. The PC board has been laid out for either 16mm or 24mm potentiometers and the position of the hole for this control in the end of the case will depend on which one you use. We suggest that you use a 24mm potentiometer as the tracking bet­ween the gangs will probably be closer. Note that you will also need to drill two holes in the base of the case for two 6mm untapped spacers to support Table 1: Resistor Colour Codes ❏ ❏ ❏ ❏ ❏ ❏ No. 4 2 2 2 2 58  Silicon Chip Value 330kΩ 100kΩ 15kΩ 10kΩ 3.9kΩ 4-Band Code (1%) orange orange yellow brown brown black yellow brown brown green orange brown brown black orange brown orange white red brown 5-Band Code (1%) orange orange black orange brown brown black black orange brown brown green black red brown brown black black red brown orange white black brown brown The PC board is secured at one end by the pot terminals and at the other by 6mm standoffs and machine screws and nuts. The bypass switch can be considered optional – if you leave it out, the unit can be housed in the more compact case shown on the facing page. the PC board at the end opposite to the potentiometer. Trying it out To test the unit it will be necessary to connect it at the input to the power amplifier. Run your preamp leads to the input connectors and the amplifier input leads to the output connectors of the adaptor. Rotate the Loudness control fully clockwise and then adjust the normal level controls on the system so that the volume is the loudest you are ever likely to want it. From now on, you use the Loudness control to adjust the playing level. When you set the switch to the Bypass position you will notice that the overall sound level is higher but it will have less bass and slightly less treble. Now switch to the Loudness mode and you should immediately notice that the sound has more bass. As you wind up the Loudness control to maximum setting, you should notice that while the sound becomes much louder, the bass does Table 2: Capacitor Codes ❏ Value IEC Code EIA Code ❏ 0.15µF   150n   154 ❏ 0.1µF   100n   104 ❏ .033µF   33n  333 ❏ .001µF    1n  102 not become proportionately louder as well. This is as it should be because the amount of boost is progressively reduced as you wind up the level. There will be times when the Loudness does not suit the program you are listening to and that is when you switch the Loudness mode off, using SC the Bypass switch. THE “HIGH” THAT LASTS IS MADE IN THE U.S.A. Model KSN 1141 The new Powerline series of Motorola’s 2kHz Horn speakers incorporate protection circuitry which allows them to be used safely with amplifiers rated as high as 400 watts. This results in a product that is practically blowout proof. Based upon extensive testing, Motorola is offering a 36 month money back guarantee on this product should it burn out. Frequency Response: 1.8kHz - 30kHz Av. Sens: 92dB <at> 1m/2.83v (1 watt <at> 8Ω) Max. Power Handling Capacity: 400W Max. Temperature: 80°C Typ. Imp: appears as a 0.3µF capacitor Typical Frequency Response MOTOROLA PIEZO TWEETERS AVAILABLE FROM: DICK SMITH, JAYCAR, ALTRONICS AND OTHER GOOD AUDIO OUTLETS. IMPORTING DISTRIBUTOR: Freedman Electronics Pty Ltd, PO Box 3, Rydalmere NSW 2116. Phone: (02) 9638 6666. December 1997  59