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Get an extra octave of bass with this . . . Bass Extender This Bass Extender circuit can give you as much as an extra octave of bass response from your existing hifi speakers, as long as you are not running them near full power. Design by RICK WALTERS T HIS MAY SOUND like black magic. Just how is it possible to get an extra octave of bass response from a hifi loudspeaker? Well, the theory supporting this idea originates from Neville Thiele’s 1961 paper (1) on loudspeakers and vented enclosures. He postulated that the response of a loudspeaker in a vented enclosure was similar to a fourth-order high-pass filter, rolling off in the bass region at -24dB per octave. For a sealed enclo- Fig.1: the response in a vented enclosure is similar to a fourth-order high-pass filter, rolling off in the bass region at -24dB per octave (red trace). Similarly, the response in a sealed enclosure rolls off at -12dB per octave (green trace), much like a second-order filter. This graph plots the response of hypothetical speakers with a cutoff frequency of 70Hz. 60  Silicon Chip sure, the response was similar to a second order high-pass filter, rolling off at -12dB per octave. Fig.1 shows this for hypothetical speakers that are -3dB down at 70Hz (the cutoff frequency), in each type of enclosure. Now if we apply bass boost with an amplitude of +3dB at 70Hz, rising to a maximum boost of around 11dB or so (for a sealed enclosure), it will partially compensate for the speaker’s rolloff and thus extend the bass response by as much as an octave. As we’ll see later, the Bass Extender can be tailored for either type of enclosure, applying less boost to a vented enclosure than a sealed enclosure. This is the opposite of what you might expect but is necessary because the speaker cone in a vented enclosure has little loading below the box resonance. There is a limit to the amount of bass compensation we can apply anyway. A speaker’s cone excursion increases as frequency decreases, so large bass boost levels would test the mechanics of the speaker as well as the damping ability of the enclosure. Also, it is likely that some power amplifiers would run into clipping. Even with all these limitations, we can usually gain an extra octave without major problems. This is much more precise than merely boosting the bass with your amplifier’s tone controls, as it’s compensating for the loudspeaker’s natural rolloff. Note this does not mean that the overall bass from the speaker will increase for all music. Since the bass response will be extended to a lower siliconchip.com.au Fig.2: the circuit includes two identical channels, each consisting of an input buffer followed by an equal component Sallen-Key filter. As shown, the circuit is configured for vented enclosures but will also work with sealed enclosures by changing the indicated resistor values. frequency (say, 35Hz instead of 70Hz) you will only hear the difference if the music signal includes bass content at these low frequencies. Incidentally, if your loudspeakers have a response down to 50Hz or better, there is no point in building the Bass Extender. Speaker specifics The catch in this process is that you need to know the rated cutoff frequency for your speakers. Once you know this, you need to calculate a particular resistor value for the bass boost circuit. Apart from that, the circuit is simple and foolproof. So, what is the rated cutoff frequency for your hifi loudspeakers? If you have the manufacturer’s original specs, it is easy. They should give a frequency response curve and you just look to see where the bass response is 3dB down siliconchip.com.au Fig.3: the cutoff frequency of your speakers can be determined from the manufacturer’s data sheets. Here, the frequency response curve from a VAF DC-7 G4 shows a -3db point around 25Hz. In this case, there is absolutely no point in building the Bass Extender! April 2005  61 Fig.4: the performance of the prototype when set up for speakers with a 70Hz cutoff frequency. The green trace shows the boost curve for a sealed enclosure, whereas the red trace is for a vented enclosure. with respect to the output at a higher reference frequency, say 200Hz. An example frequency response curve is shown in Fig.3 (this example has a very good low-frequency response). Failing that, have a look at the speaker’s impedance curve, if you have it. For a bass reflex (vented) enclosure, the impedance curve will have a double hump in the bass region. The -3dB point is usually to be found in the dip between the two humps. Similarly, if you have a sealed enclosure, the impedance curve will have a single peak (the system resonance) in the bass region and the -3dB point will be about 10% below that. For example, if the system resonance for a sealed enclosure is at 80Hz, the -3dB point will be around 70Hz. If we wanted to compensate a vented enclosure, we need to boost the bass by 3dB at 70Hz, rising to a maximum of 6dB at around 35Hz. Circuit details Fig.2 shows the circuit details. It uses two op amps per channel, all in a TL074 quad op amp package. We will discuss only one channel, since both channels are identical. The input signal for the left channel is fed through a 1mF capacitor and a resistive attenuator to the noninverting input (pin 5) of op amp IC1a, which is wired as a unity gain buffer. The 68kW and 39kW resistors at pin 5 result in a loss of 2.74 times (-8.76dB). To compensate for this loss, op amp IC1c provides a gain of 2.74 (+8.76dB) so that the overall circuit gain is unity; ie, zero gain. Apart from providing some gain, IC1c is configured as an equal compo- SPECIFI CATION S Frequency response................... -3dB <at> 61kHz (see graph for bass response) Signal to noise ratio....................... -70dB unweighted, -83dB A-weighted (with respect to 1V, 20Hz - 20kHz bandwidth) Total harmonic distortion........................ 0.02% at 1kHz and 20kHz (1V input) Signal handling....................... 2.5V RMS maximum input level (12V DC supply) Crosstalk................................................................................ 60dB (typical) 62  Silicon Chip nent Sallen-Key filter. How it works is quite complex but in simple terms, the resistors from the output (pin 8) to the junction of the two 100nF capacitors provide positive feedback below a certain frequency. Thus the gain increases to provide the bass boost characteristic we want. This is shown in Fig.4. Naturally, the shape of the bass boost curve will need to vary, depending on whether we are compensating for a sealed enclosure or a vented enclosure (bass reflex) and the rated cutoff (-3dB point) of the loudspeaker system. Accordingly, the values of resistors R1, R2 & R3 on the circuit are for vented enclosures. If you have sealed enclosures (bass reflex), R1 should be changed to 27kW, R2 to 47kW and R3 to 39kW. Similarly, the value of the four resistors marked RS depends on your speaker’s cutoff frequency and this is calculated using the formula: RS = RT - 33kW where RT = 3,180,000 ÷ fc and fc = speaker cutoff frequency. This formula applies to both sealed and vented enclosures. For example, if your speakers have a cutoff frequency (-3dB point) of 70Hz, RT = 3,180,000 ÷ 70 = 45.4kW. Subtracting 33kW from this figure gives a value of 12kW for RS. You will have to do the calculations for your own system before you can assemble this project. Power supply The circuit can be powered from 12-20V DC. Diode D1 provides input polarity protection. Two 10kW resistors divide the supply rail in half (VCC/2). This is used as a bias voltage for IC1, necessary to allow the op amp to work with AC signals when running from a single supply rail. Provision has been made for a power indicator (LED1) but we expect that most readers will not install this. It should not be installed if the board is to be powered from a DC plugpack, as the extra current drain will increase supply hum. Construction All parts for the Bass Extender mount on a small PC board, measuring 74 x 56mm (code 01104051). As usual, begin by checking the PC board for defects. Now is also a good time to enlarge the mounting holes for the siliconchip.com.au Par t s Lis t 1 PC board, code 01104051, 74 x 56mm 1 UB3 size plastic case (Jaycar HB-6013 or similar) (optional) 2 dual PC-mount RCA sockets 1 2.1 or 2.5mm PC-mount DC socket 2 6G x 6mm self-tapping screws for RCA sockets 1 16-pin IC socket Fig.5: use this diagram as a guide during assembly. Take care with the orientation of the diode (D1), op amp (IC1) and the 100mF & 330mF capacitors. The 1mF & 2.2mF capacitors are nonpolarised and can go in either way. Semiconductors 1 TL074 op amp (IC1) 1 3mm or 5mm red LED (optional; see text) 1 1N4004 diode (D1) Capacitors 1 330mF 25V PC electrolytic 1 100mF 16V PC electrolytic 2 1mF 16V non-polarised PC electrolytic 2 2.2mF 16V non-polarised PC electrolytic 4 100nF 50V metallised polyester (MKT) 1 100nF 50V monolithic ceramic 2 10pF 50V disc ceramic Right: this view shows the prototype PC board assembly. Note that there are some minor differences between this prototype and the final version shown in Fig.5 above. Resistors (0.25W 1%) 2 1MW 2 27kW 2 68kW 2 22kW 2 47kW 2 10kW 2 39kW 1 1.5kW 6 33kW 2 100W RCA sockets and/or power socket, if required. Next, install the single wire link, diode (D1) and all of the resistors, using the overlay diagram (Fig.5) as a guide. It’s a good idea to check resistor values with a multimeter before installation. Note that the banded (cathode) end of the diode must be oriented as shown. Follow up with the IC socket and all of the capacitors. The larger 100mF and 330mF electrolytic capacitors are polarised and must be inserted with their positive leads oriented as indicated by the “+” markings on the overlay. The two RCA sockets and power socket can be left until last. Push them all the way down on the PC board before soldering them in position. That done, plug in the TL074 (IC1), watch- Table 1: Resistor Colour Codes o o o o o o o o o o o siliconchip.com.au No.   2   2   2   2   6   2   2   2   1   2 Value 1MW 68kW 47kW 39kW 33kW 27kW 22kW 10kW 1.5kW 100W 4-Band Code (1%) brown black green brown blue grey orange brown yellow violet orange brown orange white orange brown orange orange orange brown red violet orange brown red red orange brown brown black orange brown brown green red brown brown black brown brown 5-Band Code (1%) brown black black yellow brown blue grey black red brown yellow violet black red brown orange white black red brown orange orange black red brown red violet black red brown red red black red brown brown black black red brown brown green black brown brown brown black black black brown April 2005  63 DIY Loudspeaker Measurements H ow do you measure your speaker’s resonance in its enclosure? For both types of enclosures, you will need an audio oscillator, an analog multimeter, AC millivoltmeter or oscilloscope and a 47W resistor. A frequency counter can be used to set your oscillator’s output if it lacks an accurately calibrated scale. Bass reflex (ie, with a vent): connect the oscillator’s output to the speaker terminals, running one of the connections via the 47W resistor. That done, monitor the voltage across the speaker terminals (set your meter to its lowest AC range) and slowly reduce the oscillator frequency, starting off at about 200Hz. The reading should rise to a maximum then fall then rise again. The middle of the dip is the resonant frequency of the speaker and enclosure combination. Sealed (closed box or infinite baffle): the same setup is used as for a bass reflex design but instead of a dip between two peaks, your meter should rise to a maximum then fall. The peak is the resonant frequency of the system. In most cases, the system resonance will be near your speaker’s free-air resonance but can be a little higher or lower depending on the enclosure size. If you cannot get a reasonable reading on your multimeter, perhaps due to the low output level from your oscillator, you will have to feed the oscillator into an audio amplifier. Place the resistor (preferably 5W or so) in series with the ungrounded output of the amplifier and the speaker. Connect the multimeter across the speaker terminals and set the oscillator output to give about 1V on the multimeter at 200Hz (with the amplifier turned on, naturally). Then follow the relevant procedure above. ing that you have the notched (pin 1) end around the right way. Testing Fig.6: this is the full size etching pattern for the PC board. To test the Bass Extender you will need an audio oscillator and a multimeter or oscilloscope. Start with the oscillator set to about 1kHz, with 450-500mV RMS output. Check the output of the oscillator with your multimeter (or millivoltmeter) if it doesn’t have a calibrated amplitude scale. Apply power and connect the oscillator to the left and right RCA inputs in turn. Measure the amplitude of the signals at the corresponding RCA outputs; they should be almost identical to the inputs. Now set the oscillator to your speaker’s resonant frequency; eg, 80Hz. Fig.7: if you’re installing your board into a case, a photocopy of this drilling guide will make life much easier. 64  Silicon Chip siliconchip.com.au The PC board can either be mounted inside an existing stereo amplifier or it can be mounted inside a small “UB3” size plastic instrument case as shown here. You will need to drill holes in one side of the case for the RCA sockets and to provide access to the DC power socket (see Fig.7). Measure each channel again and this time you should find that the outputs read about 40% higher (+3dB). Finally, measure each channel while tweaking the oscillator frequency to obtain the maximum possible reading. For a bass reflex (vented) enclosure, the maximum output should be around twice the input (+6dB), while for a sealed enclosure it should be about 3.5 times higher (+11dB), in line with the performance of our prototype (see Fig.4). If the results aren’t what you expect, then go back and re-check your resistor calculations. If you don’t get any bass boost, it is likely that the value calculated for RS is much too large. For those without the appropriate test gear, a listening test will quickly tell whether the Bass Extender is doing its job. Simply hook the project into one channel of your hifi system and listen to the bass with a suitable music program; the difference between channels should be noticeable. Housing The Bass Extender could be used in a variety of ways. For example, it could be installed inside a stereo amplifier and patched into a tape loop or inserted between the preamp and power amplifier stages. It could also be used in a car sound system. siliconchip.com.au Where a separate enclosure is required, the board can be installed inside a small “UB3” size plastic instrument case. Mounting details for this option are as follows: Photocopy the drilling template (Fig. 7) and place it centrally along the open edge of the plastic case, fixing it in place with adhesive tape. Mark and drill the holes, starting with small pilot holes and working up to the final size in several steps. A tapered reamer can also be used to enlarge the holes. The three ribs on the inside of the case should be removed with a sharp knife or chisel to allow the power socket to fit flush with the inside. The bottom 5mm or so of the three ribs on the other side may need to be removed if the board is reluctant to fit. Drop the PC board into the case and then slide the board backwards. The sockets will drop into their holes and the two self-tapping screws can then be fitted to hold the RCA sockets and PC board in place. References (1). A. Neville Thiele, “Loudspeakers in Vented Boxes,” Proceedings of the IRE Australia, August 1961; reprinted Journal of Audio Engineering Society, SC May & June 1971. Select your microcontroller kit and get started... From $295* Fax a copy of this ad and receive a 5% discount on your order! Feature rich, compiler, editor & debugger with royalty free TCP/IP stack RCM3400 • Prices exclude GST and delivery charges. Tel: + 61 2 9906 6988 Fax: + 61 2 9906 7145 www.dominion.net.au 4007 Ozitronics www.ozitronics.com Tel: (03) 9434 3806 Fax: (03) 9011 6220 Email: sales2005<at>ozitronics.com PC Printer Port Relay Board Put that old PC to good use. Direct connection to printer port. Eight relays with LED indicators. DOS and Windows software. 12VDC. 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