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BASS BLAZER 2-1/2 Octave Bass Frequency and Level Display By RICK WALTERS Do you have a subwoofer in your home theatre setup or in your car? Want to know the level of the bass signals you are hearing? This miniature 2-1/2 octave bass frequency display gives you the info. It has four vertical LED bar-graph displays to show the bass amplitude in four separate frequency bands. 32  Silicon Chip I f you're a bass fiend, you'll love this little display. It tells you the amplitude of those thumping bass signals you are listening to. Because it is powered from 12V DC it is just as happy in a car as it is in home theatre systems. It you are going to add a subwoofer to your home theatre system, you may find that one of the hardest tasks is to set the balance between the existing speakers and the sub. It's even harder if you have a variable frequency active crossover, as the combination of adjustments between this and the level control becomes huge. You can listen to some music but when you hear a low note, such as a bass drum, how do you know whether it is too soft, too loud or just spot-on? This little display gives you an indication of the relative level of the bottom octave from 32Hz to 64Hz, the next octave which is split in halves, from 64 to 96Hz and 96 to 128Hz, and lastly the range from 128 to 160Hz. The level is displayed on four vertical LED bargraphs, each covering 15dB in five 3dB steps. It is housed in a compact plastic case with the four bargraphs mounted at one end. There is quite a lot of circuit for such a small box but we've sandwiched it all onto three PC boards with rainbow cables linking them together. Theory of operation If you are going to monitor bass frequency signals in a circuit, it stands to reason that Fig.2: two of these "Multiple Feedback you need some filters so that Bandpass Filters" (MFBF) are used in each you can "hone in" on the freof the four bandpass filter stages. quencies of interest and ignore all the others. The response of the filters which drive the displays is shown in Fig.1. What we have done is to combine two Fig.1: the theoretical responses of the four filters. Fig.3: two multiple feedback bandpass filters are cascaded together and their responses combined to give and overall bandpass with an almost flat top and much steeper skirts. If you're a bass fiend you'll love this little bass frequency display with its four bargraph displays. Build it and install it in your car sound or home theatre system. It is shown here close to life-size. FEBRUARY 2001  33 filters for each band, "stagger-tuned" so that the resulting "bandpass" response has a reasonably flat top and steep "skirts". The filters we have used are known as "Multiple Feedback Band-pass Filters" (MFBF) each of which consist of an opamp with two capacitors and three resistors between input and output. The basic filter circuit is shown in Fig.2 and the values are selected to generate the response you require. R1 and R2 act as a voltage divider to control the overall gain. At high frequencies the reactance (impedance) of C1 becomes less thus rolling off the high frequency response. At low frequencies the reactance Fig.4: the circuit consists of four bandpass filter stages to monitor the bass frequency signals. The filter output signals are rectified and the DC level is fed to comparator stages to drive the bargraph LEDs. 34  Silicon Chip of C2 increases, thus rolling off the low frequency response. This is an over-simplified explanation but enough for you to get the idea. Fig.3 shows how cascading two multiple feedback bandpass filters gives an almost flat top and much steeper skirts as the response of the higher frequency filter is helping to at- tenuate the lower frequencies and vice versa. By having a small (1dB) dip at the centre frequency we get a steeper roll-off than if we had a flat top. We regard this as a good compromise. Circuit description Well, that's enough theory, let's get down to the nitty-gritty of the full cir- cuit which is shown in Fig.4. It looks pretty large but it essentially consists of the same circuit duplicated four times to give the four bands. The input circuit monitors both channels in a stereo system and mixes them together to form a mono signal which is fed through to the filter stages. Op amps IC6a and IC6b are connected as unity gain buffers to monitor the left and right channels, respectively. The buffer stages are used to avoid loading effects on the program source (CD, DVD, tape deck etc) and the outputs of the buffers are added together in op amp IC6d. FEBRUARY 2001  35 M N O Q T S GND IC4 LM339 IC5 LM339 1.6k 10F R 1 1 P Fig.5: this combined wiring diagram shows all three PC boards and most of the wires linking them together. The wiring from the comparator board to the display board (right) must be linked from point A to point A, point B to point B and so on, for “A” to “T” and ground. 1.2k F E D G H I J IC2 LM339 IC1 LM339 L K IC3 LM339 1 1 C 100F 25V 1 B 1N 4148 4.7k 68 68 68 A D5 D6 Q3 Q4 Q2 1N 4148 Q1 X 820 68 X 600 390 1k 10F 220k 10F 10F 10F .022F .047F .033F .033F .047F 4.7k 82k 91k IC8 LM324 1 820k 620k 820k .033F .033F 100k 100k .01F D8 *16V 0.1F 1N 4148 D7 47k 100k * F 100 50k 10k IC6 LM324 1 REG1 7808 10k VR1 4.7k 1M 0.1F 91k 110k 13k 6.2k 3k 4.7k 1k 0.47F D9 0.47F 1M + LEFT IN 10k 10k 10k .033F .047F .047F * F 100 110k 82k 680k .033F 1N 4148 .022F _ DC SOCKET 9.1k 1 IC7 LM324 .022F 1N 4148 .033F 2.4k 3.6k 62k .033F 56k .022F 430k 620k 680k 470k Trimpot VR1 is a preset level adjustment, to enable you to calibrate the indicators to display the correct maximum level. The summed left and right channels from VR1 are fed to the four op amp filters IC8b & IC8c, IC8a & IC8d, IC7b & IC7c and IC7a & IC7d. The bass frequencies from the output of each filter are rectified by a diode (D1, D2, D3 & D4) to a 10µF capacitor. The resistor across each capacitor discharges it and ensures that all the displays will turn off in the absence of a signal in that particular band. The resulting DC level across the respective 10uF capacitors is proportional to the bass signal level from the four filters and this DC signal is used to drive the bargraph displays. Bargraph displays 220k D4 1N 4148 D3 1N 4148 1N 4148 D1 D2 220k 220k Level set RIGHT IN RCA SOCKETS Each bargraph display uses a stack of five comparators, one for each 3dB step in signal level. The inverting inputs of all 20 comparators are individually biased to particular DC reference levels with a resistive divider fed from REG1, a 7808 8V fixed regulator. The reference voltages are set so that each successive comparator in the stack switches its output from low to high as the input level increases by 3dB. Let's now have a closer look at how each stack of five comparators works. Note that the DC signal level from each diode (D1, D2, D3 etc) is connected to the non-inverting input of all five comparators in each stack. Looking first at comparator IC1a, with no (or low) DC input level from diode D1, the non-inverting input (pin 5) will be lower than its inverting input, pin 4, which is set to +1.426V. Thus the open collector output transistor at pin 2 will be turned on and the constant current supplied by Q1 will all be diverted to ground (0V); hence no LED will be lit. Once the input voltage on pin 5 exceeds that on pin 4 the output transistor will be turned off and LED1 will light. This happens because current will pass from Q1, through LED1 and then through IC2a's output transistor which will still be turned on (as will the outputs of IC3a, IC4a & IC5a). Next, consider the situation as the DC level from D1 rises. Pin 5 of IC2a will now rise above pin 4 at +1.98V and its pin 2 transistor will now turn off allowing LED1 and LED2 to light. The output current now passes from Q1 through LED1 & LED2 and through IC3a's output transistor. So you can see how the sequence goes as the DC input voltage rises; each comparator turns off allowing the current to pass through its associated LED to the comparator which is the next in the stack. Ultimately all comparators in the stack will be turned off and so all five LEDs will be lit. The same system of operation applies to all four comparator stacks. Constant current source A constant current source is needed for each bargraph display as we can have from none to five LEDs turned on. Using a voltage feed, the LEDs would get dimmer and dimmer as more were turned on. By feeding them from a constant current source the LED intensity remains constant regardless of the number lit. 36  Silicon Chip PNP transistors Q1, Q2, Q3 & Q4 are the current sources for their respective LED bargraph. Their bases are all held at a reference voltage below the nominal 12V supply voltage by series diodes D5 and D6. Taking into account the base-emitter voltage of 0.7V there must be a voltage of 0.7V across each 68Ω emitter resistor for the four transistors and this sets the constant current to 10.2mA. This applies whether the first comparator in the stack is turned on or all five LEDs are turned on. Negative supply generator The only part of the circuit remaining to be described is the negative supply generator formed by op amp IC6c. While the quad op amp IC6 and all the LM339 quad comparators (IC1IC5) run from the nominal 12V DC supply, the filter stages involving quad op amps IC7 and IC8 need to run from plus and minus supply rails in order to get enough signal output swing for the rectifier diodes (D1-D4). This is where IC6c comes into the picture. IC6c is configured as a Schmitt trigger oscillator. Its output is used to supply a 2kHz square wave to the voltage doubler (or diode pump) formed by the two 100µF capacitors and diodes D7 & D8. The voltage doubler's output is around -8V which is used as the negative supply for IC7 and IC8. Putting it together There are three PC boards to assemble: the filter board (01102011), the comparator board (011020120) and the LED board (01102013). The diagram showing the component layouts for all three PC boards is shown in Fig.5. The first step, as always, is to inspect the PC boards for any undrilled holes, broken or shorted copper tracks. You can do this by comparing your boards to the PC patterns shown in Fig.6. It is much easier to fix any defects now, before you begin installing components on the boards. It is probably easier to assemble the comparator PC board first as it only has a few resistors and ICs. Start by inserting and soldering the four main links, followed by the six diodes and 15 resistors. If you use IC sockets fit them next, otherwise insert and solder the five LM339 comparators making sure that pin 1 on each device points towards the wider edge of the PC These photographs of all three PC boards are shown close to same size to help in construction. The boards must be connected to each other with short lengths of ribbon cable as shown opposite and then assembled in the case. The two larger boards fit one on top of the other (the board at the top of the page goes in the bottom of the case) while the small display board at left fits in vertically at the end of the case. When assembling, ensure nothing shorts out! board. I always identify pin 1 of every IC by using a rectangular pad (instead of a rounded rectangle) so use this feature to check, if you are unsure. Next, fit the four transistors and the six electrolytic capacitors. Now comes the time-consuming part: installing the on-board wiring links. These could have been avoided by designing a double-sided PC board but we like to keep the board cost as low as possible. First, pin 5 of each IC has to be connected together and linked to D1's cathode. These connections are shown as cyan (blue) on Fig.5. Similarly, FEBRUARY 2001  37 The opposite end to the bargraph display reveals the RCA stereo input sockets and (almost hidden) the 12V DC input jack. Parts List: Bass Blazer 1 plastic case, Jaycar HB-6013 or equivalent 1 filter PC board, code 01102011 1 comparator PC board, code 01102012 1 display PC board, code 01102013 2 RCA chassis-mounting sockets 1 chassis-mounting DC socket to suit your plugpack Semiconductors 3 LM324 quad op amps (IC1-3) 5 LM339 quad comparators (IC4-8) 1 LM7808 8V positive regulator (REG1) 4 BC557 transistors (Q1-Q4) 8 1N914 small signal diodes (D1-D8) 1 1N4004 silicon power diode (D9) 4 5-segment LED bargraph displays (Altronics Cat Z-0972) Capacitors 1 100µF 25VW PC electrolytic 1 100µF 16VW PC electrolytic 5 10µF 16VW PC electrolytic 2 0.47µF MKT polyester 2 0.1µF MKT polyester 4 .047µF MKT polyester 8 .033µF MKT polyester 4 .022µF MKT polyester 1 .01µF MKT polyester Resistors (0.25W, 1%) 2 1MΩ 2 820kΩ 2 680kΩ 2 620kΩ 1 470kΩ 1 430kΩ 4 220kΩ 2 110kΩ 3 100kΩ 2 91kΩ 2 82kΩ 1 62kΩ 1 56kΩ 1 47kΩ 1 13kΩ 5 10kΩ 1 9.1kΩ 1 6.2kΩ 3 4.7kΩ 1 3.6kΩ 1 3kΩ 1 2.4kΩ 1 1.6kΩ 1 1.2kΩ 2 1kΩ 1 820Ω 1 600Ω 1 390Ω 4 68Ω 1 50kΩ trimpot (VR1) Fig.6: use these actual size artworks to check or make your PC boards. 38  Silicon Chip all the pin 7s have to be joined and connected to D2's cathode (shown in purple); don't forget the link between pin 7 of IC2 to pin 7 of IC5, shown as "X". All pin 9s are joined and linked to D3's cathode (shown in magenta) and finally, all pin 11s joined and linked to D4's cathode (shown in green). Keep all the linking wires as short as possible and lay them flat on the PC board to keep them neat. Now its time to tackle the filter PC board. Fit the one link, then the resistors, diodes and trimpot, followed by the IC sockets (or ICs), the MKT capacitors and lastly the electrolytics and the regulator. Fold the regulator (REG1) over the top of the capacitor to keep its height down as the compar- ator PC board has to fit in the plastic case above the filter board. Finally, the display board can be assembled. The LED displays must be inserted with the short lead (cathode) at the end with the earth strip running the width of the PC board (marked E). They should then be pushed hard in until the wider part of the pin hits the PC board. Just tack solder the cathode pin and the anode pin at the other end of each display keeping the displays at rightangles to the PC board. Now, using the panel artwork of Fig.7 as a template, mark and cut out the four 35mm long slots for the displays. If you keep the cutouts tight you may be able to push the display assembly into position and have it stay there. Otherwise, drill a hole through the plastic case and the display PC board and use a small countersunk bolt and nut to hold it in place (or use some Blu-Tak or other adhesive to hold it in place). If the displays do not align properly, unsolder the tacked leads and adjust them until they do. Once you are satisfied, solder all the LED leads. Then cut three lengths of brown to green (brown, red, orange, yellow, green) rainbow cable 120mm long, one black to green 120mm. From the rear of the display board, with the earth track at the top, the cable with the black lead terminates the lefthand display (the highest frequency band). The wire sequence is black to (E) earth, then green, yellow, orange, red and finally brown to the next pad. The other three cables are terminat- ed in a similar manner (without the black lead). All these leads terminate on the comparator PC board. We did not use PC stakes but inserted the wires directly in the holes and soldered them. You may use PC stakes if you prefer. The high frequency display connects to pin 13 on all comparators. The brown wires all go to IC1, the red to IC2, the orange to IC3 the Fig.7: use this panel artwork as a yellow to IC4 and the green to IC5. template when cutting the slots in the All these wire links are indicated case for the bargraph displays. with the letter A to T on both the Mount these components and link comparator board and display board the two solder lugs of the RCA conon Fig.5. You will also need to connect the nectors with a piece of resistor lead offcut. Connect them to earth and the filter outputs to the diodes (D1 to D4) centre lugs to the left and right input on the comparator PC board. These on the filter PC board. wires are also shown on Fig.5. Finally, connect the DC power If you have not already done so, you connector. The positive lead from will need to drill the two holes for the RCA connectors and a hole to suit the the connector goes to the anode of diode (D9) on the filter board while power connector you plan to use in the negative lead goes to E (adjacent the plastic case. to REG1). This diode has been included othResistor Colour Codes: Bass Blazer erwise you could do damage to the circuit if you connect a DC plugpack No. Value 4-Band Code (1%) 5-Band Code (1%) with a different polarity to the DC  2 1MΩ brown black green brown brown black black yellow brown connector.  2 820kΩ grey red yellow brown grey red black orange brown Apply power and check the current consumption with a multimeter.  2 680kΩ blue grey yellow brown blue grey black orange brown It should be around 60mA. If it is a  2 620kΩ blue red yellow brown blue red black orange brown lot higher than that, turn the power  1 470kΩ yellow purple yellow brown yellow purple black orange brown off and check for bridged tracks etc.  1 430kΩ yellow orange yellow brown yellow orange black orange brown The voltage at pin 11 of IC7 and IC8  4 220kΩ red red yellow brown red red black orange brown should be around -8V. If this voltage  2 110kΩ brown brown yellow brown brown brown black orange brown is zero, it means the oscillator is not  3 100kΩ brown black yellow brown brown black black orange brown oscillating, so check your soldering  2 91kΩ white brown orange brown white brown black red brown and components around IC6c.  2 82kΩ grey red orange brown grey red black red brown If you have an audio oscillator, you can sweep through the frequency  1 62kΩ blue red orange brown blue red black red brown ranges of the filters and check that  1 56kΩ green blue orange brown green blue black red brown they operate over the correct band  1 47kΩ yellow purple orange brown yellow purple black red brown and thus all your capacitors are in the  1 13kΩ brown orange orange brown brown orange black red brown correct position.  5 10kΩ brown black orange brown brown black black red brown Once you set VR1 to let the LEDs  1 9.1kΩ white brown red brown white brownblack brown brown hit 0dB on the peaks, you may be  1 6.2kΩ blue red red brown blue red black brown brown amazed just how high the frequen 3 4.7kΩ yellow purple red brown yellow purple black brown brown cies are that sound like really low  1 3.6kΩ orange blue red brown orange blue black brown brown SC bass.  1 3kΩ orange black red brown orange black black brown brown CapacitorCODES Codes  1 2.4kΩ red yellow red brown red yellow black brown brown CAPACITOR  1 1.6kΩ brown blue red brown brown blue black brown brown Value IEC Code EIA Code  1 1.2kΩ brown red red brown brown red black brown brown  0.47µF   470n   474  2 1kΩ brown black red brown brown black black brown brown  0.1µF  100n  104  1 820Ω grey red brown brown grey red black black brown  .047µF   47n   473  1 600Ω blue black brown brown blue black black black brown  .033µF   33n   333  1 390Ω orange white brown brown orange white black black brown  .022µF   22n   223  4 68Ω blue grey black brown blue grey black gold brown  .01µF   10n   103 FEBRUARY 2001  39