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Build this low-noise 4-channel guitar mixer Fancy building a guitar mixer but don't like the units that are currently available? Well, why not take a look at this 4-channel mixer? It has separate bass, midrange & treble controls and has very low noise and distortion. By DARREN YATES These days, most musical instruments have higher quality audio outputs than the average cassette deck. This particularly applies to keyboards that use digital voice synthesis but electric guitars have picked up their game as well and now provide high quality outputs. An audio mixer, therefore, must meet certain minimum standards if it is going to faithfully reproduce the music and not add unwanted coloration of its own. Although there are a couple of mixer kits still currently available, these 16 STI,ICO N CHIP were designed many years ago and don't take advantage of the very lownoise, low-cost op amps now available. This new design features National Semiconductor's LM833 dual low-noise op amp to achieve excellent performance, plus board-mounted potentiometers to ensure easy con. struction. In addition to the tone control facilities already mentioned, the unit features separate input level controls plus an output level control. It is ideal for use with most musical instruments, from keyboards to guitars to tape decks. In fact, you can feed it with just about any audio signal - it's not just limited to guitar outputs. The accompanying specifications shows the performance of the new 4Channel Mixer. Two specifications are quoted for the signal noise ratio: (1) with all inputs closed, and (2) with all inputs open. In practice, since the four input level controls will neither be all fully open or fully closed but at some intermediate setting, the signal to noise ratio will be somewhere around 90dB. This is a very good figure. Block diagram To see how the 4-channel mixer works, refer now to the block diagram of Fig.1 . Each input has its own preamplifier stage which provides a fixed 26dB of gain for the input signal. The four outputs are then resistively mixed together and connected to the mixer preamplifier. This stage has 12dB of gain to overcome some of the inherent loss in the resistive mixer. Fig.2 shows how this loss comes about. Each output is effectively connected to a 47kQ resistor and the opposite ends of these resistors are all joined together. Thus, as Fig.2 shows, each output is effectively connected to a potential divider consisting of one 47kQ resistor (the one connected to its output) and three other 47kQ resistors in parallel (these correspond to the other input stages). The audio is taken from the tap and fed to the mixer preamplifier stage. Because the bottom three resistors are in parallel, the signal level applied to the mixer preamplifier is only a quarter of that at the output of the input preamplifier stage. This loss is overcome by giving the mixer preamplifier a gain of 12dB (ie, we amplify the mixed signal by four). The output of the mixer preamplifier is thus approximately equal to the sum of the four input preamplifier outputs. This mixed signal is then passed through a 3-band graphic equaliser which provides separate bass, midrange and treble controls. These allow you to tailor the sound to suit your own tastes, whether you like beefy bass or squeaky-clean treble. Each frequency band is individually tailored by a single control which operates around a common inverting op amp configuration. The bass control provides about ±14dB of gain at lOOHz, while the midrange and treble controls provide ±1 ldB of gain at lkHz and lOkHz respectively. These figures mean that the tone controls should provide more than adequate boost or cut in most situations. PREAMP 1 PREAMP 2 BASS MIXER PREAMP INVERTING BUFFER TREBLE PREAMP 4 .,. INVERTING STAGE Fig.1: block diagram of the 4-Channel Mixer. Each input is first preamplified, then resistively mixed and fed to a mixer preamplifier stage.-The output from this stage then drives the tone control section which in turn drives an output buffer stage. MIXER PREAMP gain of-1 and, in addition to buffering the tone control output, ensures that the output signal is in phase with the signal inputs. Circuit details 47k 47k .,. Fig.2: each input preamplifier effectively drives a potential divider consisting of a 47kQ resistor & three other 47kQ resistors in parallel. Finally, the signal output from the tone control stage is fed to an inverting buffer. This stage operates with a SPECIFICATIONS Signal to noise ratio: {at 50mV input<at> 1kHz) All inputs closed : -96dB All inputs open: -82dB Distortion: (at 1kHz and 100mV input)< 0.0075% Graphic equaliser: Bass: ±14dB<at> 100Hz (±1dB interaction with midrange); ±19dB<at> 50Hz (±0.3dB interaction with midrange) Midrange: ±11dB<at> 1kHz (±0.8dB interaction with bass, ±2dB interaction with treble) Treble: ±11dB<at> 10kHz (±2dB interaction with midrange) Maximum output: approx. 23V p-p before clipping Frequency response: 18Hz-35kHz {±3dB) Fig.3 shows the complete circuit details for the 4-Channel Mixer. It uses four LM833 dual low-noise op amps which can be directly related to the various stages shown in Fig.2. Because the four input stages are identical, we'll save some time by describing the channel 1 input stage only. The input signal is coupled into the circuit via a 2.2µF electrolytic capacitor, while a lOkQ resistor to ground sets the input impedance. A lkQ stopper resistor is also included in the input stage to protect the circuit against RF radiation. What happens here is that long audio input cables can act as radio antennas. When combined with the residual circuit capacitance, this can produce a tuned circuit which couples radio signals straight into the sensitive first junction of the op amp. This junction can act as a detector and thus detected radio signals will be amplified and fed through the mixer! By adding the lkQ stopper resistor, we greatly reduce the sensitivity of the first stage to RF signals while JANUARY 1992 17 2.2 INPUT 25VW 1 er-=t1-·-.-"",Ntr-"1 .022 +15V 10k 10k 10k 6.8k TREBLE VR7 100k LIN 6.Bk 10k .0015 39pF VO}~:E~~~~ 10k LOG -15V +15VREGO ovo INPUT 4 2.2 25VW 1001 6x0.1l +15V • -15V 25VWI 100 + 25VW! o--:a1--·......-Wilr--"I --!- 3,0.1I -15V REGO 10k .,. 1.2k FOUR CHANNEL MIXER Fig.3: the main circuit can be directly related to the block diagram shown in Fig.1. ICta, ICtb, IC2a & IC2b all function with a gain of about 19. Their outputs are mixed via the level control pots and the resulting signal amplified by IC3a and fed to tone control stage IC3b. Finally, the output from IC3b is fed to unity gain buffer stage IC4a via volume control potentiometer VR8. still allowing through the wanted audio signals. Following the stopper resistor, the signal is fed into the non-inverting input (pin 3) of ICla. This stage is configured as a non-inverting amplifier with a gain ofabout 19.3, as set by the 22kQ and 1.2kQ resistors (ie, Gain = 1 + 22kQ/1.2kQ). The 100pF feed- 18 SILICON CHIP back capacitor across the 22kQ resistor set the upper 3dB frequency response to about 72kHz, while the 1.2kQ resistor and 22µF capacitor set the lower 3dB frequency response to about 6Hz, thus ensuring good wideband audio response. The output from IC1a appears at pin 1 and is coupled via a 2.2µF elec- trolytic capacitor to a 10kQ pot (VR1), which acts as the input level control for that channel. Similarly, VR2 , VR3 & VR4 act as the input level controls for the other three channels. Not.e that, because these level controls follow the input preamplifiers rather than precede them, we can reduce the amount of noise present in the final output in the event that a particular channel is not required. By simply rotating the unwanted level control(s) to minimum, we remove the associated input preamplifier .022 ~.t-2_--1:~:k~OS_T_..__W,,H,__-Wfr---, BASS SECTION noise from the rest of the circuit. Following the level controls, the signals are mixed together via four 47kQ resistors, as described previously. The signal is then fed to noninverting op amp stage IC3a which operates with a gain of about 4.3 to make up for mixing losses. Graphic equaliser Next, the signal is fed into the 3band graphic equaliser section. This is based on IC3b and potentiometers VR5, VR6 & VR7. These pots, plus their associated resistors and capacitors, form an AC negative feedback loop around the op amp. To understand how the tone controls work, let's consider the bass boost/cut circuitry based on VR5. Fig.4 shows the basic arrangement. In addition to the pot, this circuit consists of two lOkQ resistors (one on each side of the pot) and a .022µF capacitor across the pot. The capacitor and the 1ookn pot set the frequency response, while the two lOkQ resistors limit the amount of boost and cut. Because VR5 forms part of the feedback loop and part of the input network, we can change the low frequency gain ofIC3b by simply varying the pot. , For example, if the pot is adjusted so that the feedback loop has minimum resistance, the result is an amplifier with a gain of approximately 10kQ/110kn or .09 (ie, the low-frequency output is cut). Conversely, if the pot is fully wound the other way, then we have maximum resistance in the feedback loop and minimum resistance in the input stage. The gain is now 110kn/10kn or 11, which results in a boost in the low frequency output. The midrange section works in a similar manner except that there is now a .0lµF capacitor in series with the inverting input. This, along with the .0022µF capacitor across VR6, Fig.4:: basic arrangement for the tone control section, showing the bass boost/cut circuitry only. The .022µF capacitor & the pot (VR5) set the frequency response, while VR5 also allows the low frequency gain of the op amp to be varied. The midrange & high ranges work in similar fashion. gives a bandpass filter arrangement. The .0lµF capacitor sets the lower frequency limit, while the .0022µF capacitor sets the upper frequency limit. Again, the pot is connected so that it forms part of the both the input and feedback circuits, which allows the gain to be varied as before. The treble control (VR7) works in similar fashion on the high audio frequency range , as set by the .0015µF capacitor in series with the inverting input of IC3b. Finally, a 39pF capacitor is included in parallel with the feedback network across IC3b. This reduces the overall response ofIC3b to very high frequencies to ensure stability. The output from the graphic equaliser appears at pin 1 ofIC3b and is fed to VR8, which forms the master volume control. IC4a is an inverting amplifier with unity gain. It buffers the output from the volume control and, as a bonus, provides a 180° phase shift to bring the signal back into phase with the input signals. This phase-corrected signal appears at pin 1 of IC4a and is AC-coupled to the output via a 2.ZµF capacitor. PARTS LIST 1 PC board, code SC01102921, 249 x 113mm 3 100kn linear potentiometers 5 10kn log potentiometers 13 PC stakes 1 ±15VDC power supply board, DSE Cat. K-3435 or Jaycar Cat. KC-5038 (see text) Semiconductors 4 LM833 dual op amps (IC1-IC4) Capacitors 2 100µF 25VW electrolytics 4 22µF 16VW electrolytics 11 2.2µF 25VW electrolytics 9 0.1 µF 63VW 5mm-pitch polyester 1 .022µF 63VW 5mm-pitch polyester 1 .01 µF 63VW 5mm-pitch polyester 1 .0022µF 63VW 5mm-pitch polyester 1 .0015uF 63VW 5mm-pitch ceramic 5 100pF 5mm-pitch ceramic 1 39pF 5mm-pitch ceramic Resistors (0.25W, 5%) 5 47kn 2 6.8kQ 1 33kQ 4 1.2kn 8 22kn 41kQ 11 10kn 1 4700 Miscellaneous 5 audio sockets to suit, 1 piece of blank PC board measuring 149 x 113mm (see text), solder, screws, nuts, washers, hookup wire, shielded cable, tinned copper wire. Power supply Because of the mixer's very low noise characteristics, we elected to keep the power supply off the main board. This reduces the amount of mains hum and interference picked up by the circuit, since we no longer have the unregulated supply on the board. The circuit is designed to run off ±15VDC but will work with slightly reduced specifications down to ±12VDC. If you include this circuit with another piece of audio gear, you should be able to find ±15VDC somewhere. If not, you can use the universal power supply board published in the August 1988 issue of SILICON CHIP. You can buy this kit from Jaycar Electron- ics (Cat KC-5038) or from Dick Smith Electronics (Cat K-3435). Construction All components for the 4-Channel Mixer are mounted on a single PC board measuring 249 x 113mm and coded SC0l 102921. Before you start construction, check the board carefully for defects by comparing it against the published pattern. If you find any, use a small dash of solder or a small artwork knife as appropriate to repair the problem. Once the board is OK, check that the PC-mounting potentiometers (VR1-VR8) fit snugly into the holes on JANUARY 1992 19 INPUT 1 GND _ INPUT 2 _.fil!!! _ INPUT 3 _fil!!l _ INPUT 4 _fil![I _ -------------------"" F ;?!;?!~ ., ) . - -~ -:.-:.-:.,-_-_ ..._-_-ii . ,. ...L..J, _ _ 0 22uf • GAIN 2 VR2 GAIN3 VR3 • GAIN 4 VR4 BASS VR5 Fig.5: follow this parts layout diagram carefully when wiring up the 4-Channel Mixer. For best results, the unit should be equipped with a groundplane by mounting it in a metal case or on a piece of blank PC board (see text). the board. If they don't, enlarge the holes with a slightly larger drill bit until they do fit. While you're still at your rough workbench, you can hammer in the PC stakes. These go into the 13 external wiring points at the back of the board - see Fig.5. Solder them in, then install the 13 wire links. If necessary, you can straighten the link wire by clamping one end in a vice and then stretching it slightly by pulling on the other end with a pair or pliers. Next, solder in the resistors. The table below shows the resistor colour codes. If you cannot make out the colour bands clearly, use your multi- meter to check the resistor values. The 5mm fixed-pitched capacitors can now be installed, followed by the electrolytics. Make sure that you install the electrolytics with the correct polarity, as shown on Fig.5. The same goes for the four ICs which can also be installed at this stage. You can identify pin 1 of an IC by an adjacent dot or notch in the plastic body at one end of the device. All four ICs face in the same direction. Finally, install the eight potentiometers along the front of the board. Take care to ensure that you don't get any of the pots mixed up. Logarithmic pots have the letter "A" stamped into the metal casing, while linear pots are MID VR6 TREBLE VR7 VOLUME VRB stamped with the letter "B". Because of the circuit's sensitivity, we suggest that you earth the potentiometer casings. This can be done by soldering a single length of tinned copper wire to the back of the pots and then soldering one end to the earth pattern underneath the board (see photo). A groundplane will greatly reduce the circuit's sensitivity to hum. For CAPACITOR CODES Value IEC Code EIA Code 0.1µF 100pF 39pF 100n n10 39p 104 101 39 RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 20 No. Value 4-Band Code (5%) 5-Band Code (1 %) 5 1 8 8 2 4 4 1 47kQ 33kQ 22kQ 10kQ 6.8kQ 1.2kQ 1kQ 470Q yellow violet orange gold orange orange orange gold red red orange gold brown black orange gold blue grey red gold brown red red gold brown black red gold yellow violet brown gold yellow violet black red brown orange orange black red brown red red black red brown brown black black red brown blue grey black brown brown brown red black brown brown brown black black brown brown yellow violet black black brown SILICON CHIP this reason, we suggest that you install the board in a metal case. Alternatively, you can fashion a suitable groundplane from a piece of blank PC board. Don't forget to connect the circuit earth to the groundplane (or to the metal case). Testing Before you hook up your power supply, check the board assembly carefully to ensure that all components are at their correct locations and are correctly oriented. When you're sure that everything is OK, connect your power supply to the board, with your multimeter (switched to amps) in series with the positive supply rail. Now switch on and check the current flow. If it is greater than about 30mA, switch off immediately and check the board carefully for solder splashes, shorts between tracks and other possible wiring errors. Assuming everything checks out so far, use your multimeter to check the supply voltages to the ICs. Pin 8 of each IC should be at+ 15V while pin 4 of each IC should be at -15V. To test that each audio channel is working, simply feed in an audio signal of about 100mV or so (a tape deck RCA output is fine) and connect the output from the mixer to your hifi gear or to a bench amplifier. If you now wind up the level control for the relevant channel, along with the master volume control, you should hear the audio at the output. The bass, midrange and treble controls should also be checked for correct operation. To do this, set the tone controls on your amplifier to the flat position (or switch them out of circuit), then check that the mixer tone controls have the desired effect on the low, mid and high frequencies as appropriate. All that remains now is to mount the mixer board in with your existing audio gear or mount it in a case of its own. Note that, for optimum performance, you should use shielded audio cable for both the input and output connections. Finally, remember to turn the level controls for any unused channels down to minimum when you are using the mixer. This will eliminate noise from the preamplifier stages in the unused channels and give the best signal-to-noise ratio for your music or recordings. SC l----0 0 __..; ru (J\ __..; C) __..; __..; 0 C) u (/) ..... Cl) Cl) :.s cc .....00 ~ 0 > 0 > a--•---D9 lQ. JANUARY 1992 21