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Anyone can build this high performance four-channel audio mixer. . . Want to mix two or more audio signals together? Maybe it’s an MP3 player and a microphone so you can “play” Karaoke. Or perhaps you’ve formed the next earth-shattering band and need to mix a couple of guitars and a mic or two together. Or you’ve built a PA amplifier and want to be able to drive it from a variety of signal sources. Here’s the answer: this 4-channel mixer might be simple and cheap to build – but its performance lacks for nothing! By Nicholas Vinen Mix-It! T his mixer is something of a puts which can be configured for a controls, individual channel level reprise of two very popular wide variety of signal sources, from controls along with a master volume control and an on-board power supply. 4-Channel Guitar Mixers fea- very low level (eg, microphone or tured in SILICON CHIP – the first in our guitar) right through to quite high (eg You can build it as a stand-alone unit or incorporate it into a PA or guitar January 1992 issue and a more recent iPODs/MP3 players, CD/cassette decks [Gad, what are they?]) and the like. amplifier. version in June 2007. It has bass, midrange and treble In fact, it doesn’t even need to be a While this one has several similar PA/guitar amplifier: features, (it is an auwith almost 800mV dio mixer, after all!) output, this mixer it also has a number could be used with of improvements – • Four unbalanced inputs with 1MΩ || 100pF input impedance (see text) virtually any amplifor example, perfor• Gain of 0-36dB per channel (depending on feedback components) fier with a “line in” or mance, cost, easy to • Bass, mid and treble controls (±10dB) similar input. build – and as a bonus, • Master volume control Other features inthe PCB is actually • Input radio signal filtering clude a variety of smaller than either • Flat frequency response power supplies – it so you can fit it into a • Low distortion and noise could use a low voltsmaller case. • Four supply options: 15VAC, 12-30V DC, ±15V or unregulated split supply age AC supply – say It features four in- Features 58  Silicon Chip siliconchip.com.au An early prototype of the Mix-It! 4-channel mixer – some components have been moved or changed since this photo was taken. PCBs purchased from SILICON CHIP will also be double-sided, eliminating the need for the wire links shown on this board. around 15V – or it could use a split DC supply such as that commonly found in amplifiers (eg, ±15V). We’ll have more to say on the supply shortly. How it works pacitors with 1MΩ biasing resistors. This high value is necessary if the mixer is used with electric guitars, as their frequency response changes when driving lower impedances due to loading effects on the inductive pick-up(s). The relatively low value RF filtering capacitors (100pF) were chosen for the same reason. While most of the coupling capacitors in the circuit have been increased compared to the original designs, here we have used a lower value since the input coupling capacitors need to be non-polarised. This is because the signal source could potentially have a high DC bias or the input might be accidentally shorted to a power rail. We also wanted to use an “MKT” (polyester) capacitor as they are more reliable and linear than non-polarised electrolytics, which also vary greatly in size. Before each op amp is a 100Ω resistor, which acts as an additional RF stopper. IC1a-IC2b are TL072 low-noise JFET input op amps. Due to the high value bias resistors, the LM833s used in the original design are not suitable. They would have an excessive output DC offset due to their relatively high input bias currents. JFET input op amps have a much lower input bias current with only a small increase in noise. The gain for these op amps is set by the two resistors at their outputs. In the circuit we have used “middle of the road” values of 1.8kΩ and 220Ω, resulting in a gain of about 9.2x (18dB). Gain is calculated using the formula Each of the four identical inputs, CON1-CON4, can be fitted with either a terminal block or preferably, a PCB-mounting shorting-type RCA socket. We say preferably because unconnected inputs are then shorted to 1.8kΩ + 220Ω ground and therefore don’t introduce 220Ω any noise or hum into the circuit. Each input has an RF filter, consistThis is about half that of the original ing of a ferrite bead and 100Ω resistor design, which could not handle linein series with the signal and a 100pF level input signals without clipping. capacitor to ground. These act as lowThis one can – up to 900mV RMS or pass filters with a cutoff frequency of more with reduced gain. 16MHz while the ferrite beads greatly These values can be changed to suit improve the rejection of signals above various input devices, as we shall see a couple of hundred kilohertz. shortly. We mentioned “ground” a moment The feedback capacitors (nomiago. In this circuit, it’s important nated as 220pF) roll off the op amp to note that there are two different closed-loop gain at high frequencies “grounds”. The first is the “power” to improve stability, reduce noise ground and uses the conventional and provide a further degree of RF ground symbol ( ). The second is rejection. the “signal” ground and The op amp outputs are uses a different symbol AC-coupled via 10µF electro( ). We’ll explain lytic capacitors to 10kΩ log these a bit more when • Input range for line level output: 18-900mV volume pots (VR1-VR4). These we look at power sup- • Frequency response: 20Hz-20kHz, +0,-1.2dB (see Fig.3) capacitors are polarised, to plies shortly. • Signal-to-noise ratio: -75dB <at> 32dB gain; -92dB <at> 0dB gain minimise size and cost. We can The audio signals are • THD+N (for 20Hz-20kHz 0.015% <at> 32dB gain; get away with it because the op then AC-coupled to op bandwidth): 0.003% <at> 18dB gain; amp input bias currents (small amps IC1b, IC1a, IC2b 0.002% <at> 0dB gain) though they may be with JFET and IC2a via 470nF cainputs) cause the op amp out- Specifications siliconchip.com.au June 2012  59 +15V CON1 1 L1 BEAD 100 470nF 100 8 5 2 6 INPUT 1 CON1a IC1b 1M 100pF IC1: TL072 100 470nF 100 100F 25V 6.8k 470F 16V SUPPLY RAIL SPLITTER 220 47pF 39k IC1a 1M 10F 1 C2 1.8k 220pF VR2 10k LOG 9 47F CHANNEL 2 GAIN 100 1 470nF –15V 6 CON4 1 2 INPUT 4 CON4a 220pF VR3 10k LOG = SIGNAL GROUND 470nF 100 1M = POWER SUPPLY GROUND Adjustments to input R & C for various devices 100nF –15V 3 4 IC2a R1-R4 C1-C4 Stage Gain Overall Gain Suits 120  100pF 16x (24dB) 62x (36dB) Low-sensitivity mics 150  150pF 13x (22dB) 50x (34dB) Medium-sensitivity mics 220  220pF 9x (18dB) 38x (31dB) 390  330pF 5.5x (15dB) 22x (27dB) 910  470pF 3x (10dB) 12x (21dB) 1.8k 560pF 2x (6dB) 8x (18dB) Line level sources Omit 1nF 1x (0dB) 4x (12dB) CD/DVD/Blu-ray players 10F 1 CHANNEL 4 GAIN C4 1.8k 220pF R4 220 SC 10k R3 VR4 10k LOG 2012 R5,R6 INSTALLED FOR USE WITH CONDENSER MICROPHONES ON INPUT 4 ONLY 220 2 100pF CHANNEL 3 GAIN C3 1.8k IC2: TL072 100 7 +15V R1-4, C1-4 CAN BE ALTERED TO CHANGE GAIN OF EACH CHANNEL AND THEREFORE SUIT DIFFERENT INPUTS – SEE TABLE 10F R5 100F L4 BEAD IC2b 1M 100pF 470 PHANTOM R6 POWER 1.8k 8 5 2 INPUT 3 CON3a MIXER/AMPLIFIER STAGE +15V 100 10F 8 11 10k R2 L3 BEAD IC3c 10 220 CON3 33* –15V R1 4 3 2 100pF VR1 10k LOG CHANNEL 1 GAIN 10k 2 1 –15V 2 INPUT 2 CON2a 220pF –15V IC3a 100nF L2 BEAD 1 C1 1.8k 4 3 10F 7 +15V CON2 100nF 6.8k 10k Mics/guitars Guitars iPods, Mp3 players etc MIX-IT! FOUR CHANNEL MIXER Fig.1: the circuit diagram consists of four near-identical input stages, the outputs of which are mixed and amplified before being fed into a tone control stage and output buffer. Any of the four inputs may be altered from that shown to account for different audio devices – anything from a microphone to a Blu-ray player can be accommodated (see table above). puts to have a slightly positive DC bias. The pot wipers then connect to four 10kΩ mixing resistors which are joined together at the other end. This is the “virtual earth” point and is held at signal ground potential by op amp IC3c. Its non-inverting input (pin 10) is at signal ground potential and it is configured as an inverting amplifier with a gain of -3.9, as set by the ratio of the 39kΩ feedback resistor to the 10kΩ mixer resistors. The overall maximum 60  Silicon Chip gain of the unit is therefore 3.9 x 9.2 = 36 or 31dB. The resulting output signal is the sum of the four input signals (from the wipers of the pots). A 47pF feedback capacitor limits the bandwidth again and the output is AC-coupled to the active tone control stage with a 10µF capacitor, orientated so that it will have the correct DC bias. The tone control stage is a traditional Baxandall-style arrangement (named after Peter Baxandall, the man who first described this circuit) with three bands – bass, mid and treble. We have copied this unchanged from the original design as there is nothing wrong with it. Three 100kΩ linear potentiometers, VR5-VR7, adjust the feedback around op amp IC3d which is in an inverting configuration. The combination of capacitors across VR5 and VR6 with the capacitors at the wipers of VR6 and VR7 mean that each pot controls the feedback over a different audio “band” siliconchip.com.au K REPLACE THIS CAPACITOR WITH A WIRE LINK WHEN USING A SPLIT DC OR AN AC SUPPLY A 3 K 100F 25V A 10k A 0V DC INPUT –22V DC INPUT POWER LED1  CON6, D1 AND D2 ARE NOT FITTED WHEN HIGHER SPLIT DC SUPPLY VOLTAGES ARE FED IN THIS WAY K VR5 BASS 10k D2 1N4004 100F 50V REG2 79L15 100k LIN 15V AC IN K IN OUT CON6 A GND A –15V D1 1N4004 1.8k D4 1N4004 22nF 10k 2 100F 50V ® 1 *RESISTOR FITTED ONLY WHEN USING A SINGLE DC SUPPLY K GND 100F 25V D3 1N4004 ® CON7 +22V DC INPUT IN ® OUT ® REG1 78L15 +15V 2.2nF 10k VR6 MIDRANGE 10nF 6.8k 10F 10k 100k LIN 100k LIN VR8 10k LOG 6.8k OUTPUT LEVEL 470nF IC3: TL074 5 6 100k 7 IC3b 100 CON5 10F 1 2 100k VR7 TREBLE 1.5nF OUTPUT CON5a OUTPUT BUFFER 47pF 13 12 14 IC3d LM79L15Z LM78L15Z D1–D4: 1N4004 TONE CONTROL (EQUALISER) STAGE A –Vin COM IN K LED OUT –Vout K A COM WIRE LINK REPLACING REG1 WIRE LINK REPLACING REG1 +15V K CON7 1 D3 1N4004 100F 50V 1.8k A 2 LED1  POWER 0V IN –15V IN –15V SINGLE DC POWER SUPPLY CONFIGURATION 1 100F 25V D3 1N4004 2 A 3 K D4 1N4004 CON6 NC K CON7 +15V IN 30V DC IN A K –15V D1 1N4004 A 3 WIRE LINK REPLACING D4 +15V K 1.8k POWER 100F 25V A LED1 A  K NC +/–15V DC POWER SUPPLY CONFIGURATION (REG1, REG2, D2, D4, THE LOWER 100F/50V CAPACITOR & NEITHER 100F/25V CAPACITOR FITTED) (REG1, REG2, D1, D2 AND BOTH 100F/50V CAPACITORS OMITTED, ALSO CON6) Inset at the bottom of the main circuit are two variations for powering the mixer – two are shown on the main circuit diagram above (15V AC and ±22V DC). Each of these is further illustrated on the component overlays on page 63. R5, R6 and the 100µF capacitor on the main circuit are only needed if your microphone requires phantom power (see text). . Thus they each boost or cut a different range of frequencies. Refer to Fig.9 to see the effect of these pots; this shows the frequency response of the mixer with the controls set at their maximum extents as well as centred (blue trace). Having been inverted twice, once by the mixer and once by the tone controls, the signal at output pin 14 of IC3d is in-phase with the inputs. This is coupled to the master volume control pot, VR8. The output is taken from the wiper and then coupled with siliconchip.com.au a 470nF MKT capacitor to the noninverting input of op amp IC3b, with a 100kΩ DC bias resistor. This op amp simply buffers the signal to provide a low-impedance output. The 100Ω resistor at the output of this op amp isolates it from any cable capacitance which could otherwise cause oscillation. As with the inputs, output connector CON5 is either a terminal block or RCA socket. A final 10µF AC-coupling capacitor is used so that the output DC level is at 0V re- gardless of the signal ground potential, with a 100kΩ DC bias resistor setting this DC level. Power supply Like the original design, this unit can be powered from a ±15V regulated DC supply, via CON7. If the mixer is installed in a case with a preamplifier, there is a good chance that such rails will already be present. But if not, or in cases where the mixer is used as a stand-alone unit, June 2012  61 THD+N vs Frequency, 80kHz BW 03/22/12 11:21:15 0.1 +1 Mixer Frequency Response (1kHz) 03/22/12 10:57:01 0.1 Total Harmonic Distortion Plus Noise (THD+N) % Total Harmonic Distortion Plus Noise (THD+N) % -1 Amplitude Deviation (dBr) -2 -3 -4 -5 -6 -7 -8 0.05 0.02 0.02 0.01 0.01 0.005 0.005 0.002 0.002 0.001 20 -9 20 50 100 200 500 1k 2k Frequency (Hz) 5k 10k 20k 50k 100k 0.001 20 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) Fig.2: frequency response of the mixer with the tone controls set to their mid positions and gain at maximum. Roll-off is only 1.2dB at 20Hz and -0.75dB at 20kHz while the -3dB points are at 10Hz and 45kHz. the mixer can be run off low voltage AC or DC. An unregulated split supply can also drive the unit in some cases, as will be explained later. For low voltage AC, 15-16V RMS is supplied to CON6. Diodes D1 and D2 act as two half-wave rectifiers, charging the 100µF 50V capacitors alternately as the AC signal swings positive and negative to provide unregulated rails of approximately ±22V DC. ((16 x 2 ) – 0.6V). This is then regulated to ±15V by REG1 (78L15, +15V) and REG2 (79L15, -15V). The output voltages are filtered with 100µF capacitors. Diodes D3 and D4 prevent them from being reversebiased during operation, which could cause REG1 or REG2 to “latch up” when power is first applied. This can happen because one rail starts to 03/22/12 11:21:15 Gain = 24dB Gain Gain==32dB 18dB Gain Gain==24dB 0dB Gain = 18dB Gain = 0dB 0.05 -0 -10 10 THD+N vs Frequency,Gain 80kHz BW = 32dB Fig.3: performance with a 15VAC supply. At high gain settings, noise and 50Hz hum field pick-up dominate the distortion graph; the dip at 50Hz is when the test signal cancels some of the mains hum. charge up before the other due to the half-wave rectification. If the unit is to be run from a regulated split supply then this is connected to CON7, bypassing the regulators and powering the circuit directly. If an unregulated split supply is to be used then it can be connected via the pads for D1 and D2, bypassing the rectifier and feeding the regulators directly. The situation for a single DC supply is a little more complicated. In this case, the supply voltage is usually well below 30V. So to maximise the available headroom (the amount by which the signal can be amplified before clipping), the regulators are bypassed (linked out) so that the full voltage, minus D1’s forward voltage, is available to the op amps. D2 is also linked out and power is applied via CON7. In this case, since there is no negative supply, the signal ground potential must be positive. This bias is generated by op amp IC3a. The two resistors connected to its non-inverting input (pin Another view of the completed mixer, once again with input terminal blocks. PCB mounting RCA connectors could also be used. As noted earlier, this is an early prototype, with several component changes made to the final version (including a double-sided board). The PCB component overlay on P63 shows the final version – use that when constructing rather than this photograph. 62  Silicon Chip siliconchip.com.au 100nF IC3 TL074 47pF 39k 1.8k 33* 100k VR4 10k LOG 6.8k 6.8k 10k 10k 10k 10k 10k VR5 100k 1.5nF 100F 10F 6.8k 6.8k D1 4004 D2 4004 D3 4004 D4 4004 BEAD 470nF 470 1M 220 100 100 1.8k BEAD 100pF 100 100 IC2 TL072 470nF 1M 220 100pF BEAD 1M 220 100 100 IC1 TL072 470nF 100 100pF BEAD 100 1M 220 COMPONENTS IN RED MAY BE CHANGED TO ADJUST GAIN – SEE TABLE 47pF 22nF + POT CASE EARTHING WIRE VR3 10k LOG 100nF 10F 47F 10k 100 + VR2 10k LOG K 100k + 100nF 100F 50V LED1 POWER 10nF 1.8k 10F 10k 100F A + VR1 10k LOG 10k REG2 (25V) + 10k 79L15 R4 100F 50V CON5 470F* C4 220pF + C3 10F 10F 10F R5 + 1.8k 220pF 100F + 220pF 100F + R3 1. 8k 100pF CON7 + R2 + + + C1 C2 470nF R6 CON6 –15V 0V +15V78L15 REG1 + 1.8k CON4 + R1 220pF CON3 CON2 CON1 10F 470nF 2.2nF POT CASE EARTHING WIRE COMPONENTS IN BLUE REQUIRED ONLY FOR MICS NEEDING PHANTOM POWER VR6 100k VR7 100k PCBS FROM SILICON CHIP WILL BE DOUBLESIDED SO ORANGE LINKS WILL NOT BE NEEDED. Fig.4: the complete component overlay for the Mix-It! mixer. In this case, we have shown 220Ω resistors and 220pF capacitors in the R1/C1...R4/C4 positions which would make it suitable for guitars and many microphones. However, you can change these resistors to suit other input devices (see the table on the circuit diagram) or even add switching to one or more channels to allow the input(s) to be switched at will (see Fig.8). R5, R6 and the associated 100µF capacitor on input 4 are provided for microphones requiring “phantom power”. If you don’t need this, you can leave these components out. 3) form a divider across the supply rails, producing a voltage of roughly half the DC supply. For example, if the DC supply is 12V, this point is at about 6V. It is filtered using a 100µF capacitor, to remove supply noise. IC3a buffers this voltage, providing a low output impedance and this is filtered further using a 33Ω resistor and 470µF capacitor. The 33Ω resistor prevents op amp instability due to the large capacitive load. The RC low-pass filter formed by the 33Ω resistor and 470µF capacitor is important to achieve good performance as even a tiny amount of supply ripple coupling into the signal earth will be greatly amplified and coupled into the output, dramatically reducing the signal-to-noise ratio and increasing the distortion. We would normally use a 100Ω resistor at the op amp output, to isolate it from a capacitive load but experimentation shows that 33Ω provides better hum rejection, presumably due to the fact that higher values increase the output impedance of the buffer stage too much. To quantify the loss of headroom when running from a single supply, 12V DC can be considered equivalent to a ±6V split supply. Considering limited op amp voltage swing, this gives a maximum signal handling of about (6V - 1V) / 2 ) = 3.5V RMS. With a fixed gain of 10 at each input, the maximum input level is then 350mV RMS. siliconchip.com.au That’s plenty for most microphones and musical instruments but line level sources are generally at least 500mV and will clip unless they are attenuated somehow (or the input stage gain is reduced; more on that later). The foregoing explains why separate signal grounds and power supply grounds are required with a single rail DC supply is used. But when an AC or split supply is used, the signal ground is connected directly to power supply ground to ensure the polarised coupling capacitors are correctly biased. This is achieved by omitting the 33Ω resistor and replacing the 470µF capacitor with a wire link. All these options may seem confusing but we have provided diagrams later showing which components to install in each case. Construction The mixer is built on a PCB coded 01106121, 198 x 60mm. Refer to the overlay diagram (Fig.4). If you are not using an AC supply, refer also to one of Figs. 5, 6, 7 or 8 to see the changes required to suit your particular situation. The PCB will normally be doublesided with plated-through holes, so there will be no need for links. However, we know that some schools like to have students build their projects “from scratch”, including making PCBs where possible. Because it is unlikely students (and some readers!) will make a double- GND VR8 10k LOG sided board, six tinned copper wire links will be needed for single-sided boards (they’re shown on the PCB overlay). Follow with the resistors. It’s best to check the value of each with a digital multimeter before fitting it - you can also use the resistor colour code table as a guide but it’s easy to make mistakes (brown for orange for red, for example) so check them twice! The 1N4004 diodes go in next, with the striped (cathode) ends towards the top of the PCB. If you’re using IC sockets, mount them now, with the notches orientated towards the bottom of the PCB, as shown. Otherwise, just solder the ICs into place, taking care that they are orientated with pin 1 towards the bottom of the board. IC sockets do make it easy to place and remove ICs but we prefer to solder them in permanently, as long as there is no mistake! If installing the regulator(s), bend the leads to fit the pad spacings on the board and solder them in place. Don’t get them mixed up and ensure that the flat side faces as shown on the overlay diagram. The LED can be installed next, flat side also facing down, followed by the ceramic and MKT capacitors, from smallest to largest. Solder 3-way terminal block CON7 in place, with the wire entry holes facing the top edge of the PCB. If you are using terminal blocks for the inputs and outputs, fit them now too. Follow June 2012  63 100F K LED1 POWER IC3 TL074 47pF 1.8k A 39k D3 4004 D4 4004 470 100 IC3 TL074 47pF 1.8k 10F 100F + –22V (25V) CON7 100F LINK LIN K + 100F K + 79L15 100F 50V LED1 POWER 39k D3 4004 D4 4004 A + + 470 REG2 LINK –15V 0V +15V + 0V IC3 TL074 DC INPUTS LINK 100F 50V 33* 100k SINGLE DC SUPPLY +22V + + 100 –15V 0V +15V78L15 REG1 47pF 10F 100k DC INPUTS 100F LED1 POWER + 10F AC SUPPLY 100F 1.8k K + (25V) CON7 D1 A 39k D3 LINK 100F 470F* 4004 100F 50V CON7 4004 100 470 K LED1 POWER IC3 TL074 1.8k 47pF D1 4004 D2 A 4004 100F 79L15 100F 50V 39k D3 4004 D4 100 4004 + 470 REG2 LINK + 100F 50V CON6 –15V 0V +15V + 100F + 100F + + + CON7 LINK + CON6 –15V 0V +15V78L15 REG1 (25V) 10F 100k 100k SPLIT DC SUPPLY, +/–15V SPLIT DC SUPPLY, +/–22V Fig.5: four variations on a theme . . . the mixer is quite versatile as far as power supply goes – simply wire yours according to the power supply you are going to use. with the DC socket and then the electrolytic capacitors, all of which have the longer positive leads inserted in the hole closest to the top edge of the PCB (stripes towards the bottom edge). Ensure the correct type of capacitor, as shown on the overlay diagrams, is placed in each location. If you are using RCA sockets for the inputs and outputs, mount them now, checking that they are pushed down all the way onto the PCB and that the sockets are parallel to the board and +20 perpendicular to the edge. To minimise noise, all of the pot bodies are connected together and thence to the PCB with a 250mm length of tinned copper wire. To prepare them for soldering, hold gently in a vice and file away a patch of the passivation layer on the top of each pot (otherwise the solder won’t take). If your pots have long shafts, now is also a good time to cut them to the length you require (don’t forget to take into account any case or cabinet width). 03/21/12 13:09:04 Mixer Tone Control Extents +17.5 +15 +12.5 Amplitude Deviation (dBr) +10 +7.5 +5 +2.5 +0 -2.5 -5 -7.5 -10 Flat Max. Bass/Treble Min. Bass/Treble Max. Midrange Min. Midrange -12.5 -15 -17.5 -20 20 50 100 200 500 1k Frequency (Hz) 64  Silicon Chip 2k 5k 10k 20k Fig.6: the operation of the tone controls. The blue trace is the same as Fig.2 but with a different scale. The tone controls allow a boost or cut of around 10dB for each band with the centre frequencies around 30Hz for bass, 1kHz for mid-range and above 20kHz for treble. Solder the pots in place, ensuring that you note the difference between the three 100kΩ linear types and the 10kΩ log types. While you have the soldering iron in your hand, run a thin layer of solder over the surface of the pot where you just removed the passivation. Now solder one end of the tinned copper wire to the pad marked “GND” to the right of VR8, bend it over the top of VR8 and then solder it to the top of VR1, so that the wire passes across the top of each pot. Once it is held tightly in place, solder it to the top of the remaining pots and trim the excess. If you are using them, fit the nylon spacers to the four mounting holes and then, if you are using sockets, insert the ICs. They must be orientated with their pin 1 dots at the same end as the notches on the sockets, ie, towards the bottom of the board. If not using sockets, carefully solder in the ICs, again noting orientation. Housing it The mixer should ideally be housed in an earthed steel case, although it can be used inside an amplifier or guitar amplifier/speaker case. If putting it in a case, the pots are all 25.4mm (1 inch) apart so you will need to drill a horizontal row of eight siliconchip.com.au 8mm diameter holes in the front panel. The board can then be “hung” behind the front panel via the potentiometers. You may need to snap off the small locating spigots on each pot with small pliers (or, preferably, drill small pilot holes to accommodate them. The spigots stop heavy-handed users trying to twist the pots on the panel). While not really necessary, you can also attach the PCB to the bottom of the case using the tapped spacers – although this method of mounting might be preferable if poking the pot shafts through a thick (eg, guitar speaker box) panel. The most common input connectors for guitars, microphones and so on will usually be 6.35mm jack sockets and/or XLR sockets. The PCB is designed to accommodate RCA sockets“on board” but this may not be the most convenient to use. The altenative is to mount the sockets on a case panel – often they are mounted on the front panel or adjacent vertical panel next to their respective controls. If so, you will need to run shielded cable from the sockets to the input connectors (CON1-CON4). The output can then go to an RCA socket on the rear panel or to an internal power amplifier. Either way, use shielded cable for this connection too. When using chassis-mount jack sockets, use switched sockets and wire them to short out the input signal when nothing is plugged in, to minimise noise and hum. See Fig.7 for details on how to do this. The power supply wiring can then be run. Wire split supplies (+15V,0V,15V) up to CON7. Single DC supplies or low voltage AC go to CON6. The overlay diagrams show how the wires are connected. If you want a front-panel power indicator, it is possible to mount LED1 off-board and connect it up with flying leads and optionally, a pin header. Testing Turn all the volume knobs, including master volume to their minimum (ie, fully anti-clockwise) and set the tone controls to their centre positions. Switch on the power supply and check that LED1 lights. Plug the output of the mixer into a suitable amplifier and turn that on – with level controls at a minimum you should hear nothing! It’s then just a matter of applying a signal to siliconchip.com.au Parts list – Mix-It! Four Channel Mixer 1 PCB, code 01106121, 198 x 60mm (available from SILICON CHIP for $20 + P&P) 5 2-way mini terminal blocks (CON1a-CON5a) OR 5 PCB-mount switched RCA sockets (CON1-CON5) 1 PCB-mount DC socket (CON6) 1 3-way mini terminal block (CON7) 8 small knobs, to suit VR1-VR8 4 small ferrite beads 1 plugpack or other power supply 1 250mm length tinned copper wire (or 400mm if wire links are used) 4 M3 nylon tapped spacers 4 M3 x 6mm machine screws 2 8-pin DIL sockets (optional) 1 14-pin DIL socket (optional) Semiconductors 2 TL072 dual low noise JFET-input op amps (IC1, IC2) 1 TL074 quad low noise JFET-input op amp (IC3) 1 78L15 +15V 100mA linear regulator (REG1) 1 79L15 -15V 100mA linear regulator (REG2) 1 green 5mm LED (LED1) 4 1N4004 diodes (D1-D4) Capacitors 1 470µF 16V electrolytic 2 100µF 50V electrolytic 4 100µF 25V electrolytic 1 47µF 50V electrolytic 7 10µF 50V electrolytic 5 470nF MKT 3 100nF MKT 1 22nF MKT 1 2.2nF MKT 1 1.5nF MKT 4 220pF ceramic 4 100pF ceramic 2 47pF ceramic Resistors (all 1%, 0.25W) 4 1MΩ 2 100kΩ 1 39kΩ 9 10kΩ 6 1.8kΩ 4 220Ω 9 100Ω 1 33Ω 5 10kΩ logarithmic 16mm potentiometers (VR1-VR4, VR8) 3 100kΩ linear 16mm potentiometers (VR5-VR7) one input, then slowly turning up corresponding input and master volume controls, to check that the output sound is undistorted. Note that since there is a fair bit of gain available, if you use a line level source, you won’t have to turn the volume knobs up very far. Check each of the four inputs in turn and also check that the tone controls have the appropriate effect on the signal. If you hear a lot of hum or noise, it’s probable that it’s being induced into the sensitive input stages from whatever amplifier you’ve teamed the mixer with – in which case, you might need to house the unit in an earthed 4 6.8kΩ metal box inside the amplifier case. Alternately, hum may be caused by a hum loop, either from the power supply or the input cabling. You might need to experiment a little with earthing arrangements for best results. Making changes for MP3s etc Some constructors may wish to experiment with some component values. By doing so, you can adapt it to your particular requirements. For example, the feedback resistors for IC1 and IC2 can be changed to give different maximum gain settings for each input. You could, for example, reduce the gain of inputs 1 & 2 so that they can accept signals up to 1-2V June 2012  65 RMS, suitable for use with a CD or DVD player while leaving inputs 3 & 4 with a high gain to suit microphones or a guitar. Or you could increase the gain of one channel above the nominal 31dB to suit a microphone with a very small output signal. The easiest way to change the gain of each input is to change the values of R1 and C1 for channel 1, R2 and C2 for channel 2 and so on. Smaller values for these resistors increase the gain and larger values decrease them. The associated capacitor is changed at the same time, to keep the frequency response constant. The table on the circuit diagram shows various options for these components but other combinations are possible. You can also alter the gain for all inputs by changing the 39kΩ resistor between pins 8 and 9 of IC3c. A higher value resistor will give you more overall gain but will also increase the noise and distortion. So for example, if you change the 39kΩ resistor to 82kΩ you will double the overall gain while changing it to 22kΩ will halve it. It may be possible to gain a slight improvement in performance by replacing the TL072 and TL074 op amps with OPA2132/2134 or similar. However, the benefits will be marginal as other factors already limit the performance. It is possible that some devices such as iPods and MP3 players may not work with the mixer as published as there is no DC path for the input signals to flow to ground. This can easily be solved with the addition of a resistor (eg, 100Ω) connected across the input for that channel. Probably the easiest Improvements to a popular design Since the original 4-channel mixer was presented in SILICON CHIP in January 1992, audio design has come a long way and it was possible to make quite a few improvements in performance without adding much to the overall component count. So we have made significant improvements to the original circuit and the PCB, as follows: 1) Added RF filtering, consisting of 100Ω resistors and ferrite beads in series with each input and a 100pF capacitor to ground. These compents greatly reduce RF break-through. Testing with the prototype showed no suggestion of radio signal break-through. 2) Increased the input impedance from 10kΩ to 1MΩ, so that musical instruments with inductive pickups suffer less high frequency loss. 3) Increased the size of many of the inter-stage AC-coupling capacitors from 2.2µF to 10µF, to reduce low-frequency distortion and give a more extended bass response. At the same time, we opted to use 470nF MKT capacitors at the input instead of polarised 2.2µF electrolytic types, again to obtain lower distortion. 4) Added full AC-coupling for the input volume pots, to reduce crackle when they are turned (especially as the pots age). 5) Lowered feedback resistor values throughout, to reduce noise and hum pick-up. The feedback resistors around the initial amplifier stages have been greatly reduced, from 22kΩ/1.2kΩ to 1.8kΩ/220Ω. This results in a 70% reduction in Johnson noise, one of the predominant sources of noise in the circuit. The mixer resistors are also reduced from 47kΩ to 10kΩ. 6) Split the signal gain between the input amplifier and mixer stages. This allows line level signals of up to 900mV RMS to be fed in before clipping occurs with a ±15V supply, compared to 500mV with the original design. The maximum gain is also increased from 26dB to 31dB, to suit a wider range of microphones. 7) Slightly extended the upper frequency response, for a -3dB point at 45kHz. 8) Changed mixer to a virtual earth configuration. This eliminates interactions between channel volume settings, allows for increased gain and reduces inter-channel crosstalk for those which are turned to minimum volume. It also has the advantage of inverting the signal, which is then re-inverted by the tone control circuit, avoiding the need for a final inversion to keep the inputs and output in-phase. 9) Added provision for either PCB-mount RCA sockets or terminal blocks for inputs and output. The original design used PC stakes. 10) Added an on-board power supply. The original design required a regulated split rail power supply. This one can run from 15V AC (plugpack or small mains transformer) or from single-rail or split rail DC. The op amp stage freed up by changing to a virtual earth mixer is used as a rail-splitter (ie, virtual earth generator) for single-supply operation. 11) Added an on-power power indicator LED (which may also be mounted off-board, eg, on the front panel of the unit). 12) Reduced the op amp package count to three by replacing two of the LM833s with a TL074. 13) Reduced the size of the PCB to 198 x 60mm (compared to the original at 249 x 113mm). 66  Silicon Chip siliconchip.com.au PANEL 6.5mm MONO JACK SOCKET SHORT LENGTH OF SHIELDED CABLE 2 1 (PC BOARD) CON1 (OR CON2/3/4) Fig.7: how to wire a standard switched phono jack as a shorting jack and connect it to the PCB. This is highly recommended as otherwise, unconnected inputs may contribute noise and hum to the output of the mixer. way to do this is between the terminals of CON1a, CON2a, etc – even if there other cables going in there! However, an input modified in this manner will no longer work with some microphones, guitars and other devices with a high output impedance (normal 600 ohm “dynamic” microphones will not be too badly affected). Phantom power for condensor microphones It would arguably be fairly unusual for condensor microphones to be used with a mixer such as this but it is possible. The difficulty is that condensor microphones require a DC supply on their output (known as “phantom” power), normally around 16-48V at 1-2mA and uses the microphone cable itself to feed the microphone. Because the inputs to the op amps are AC-coupled, feeding DC “up the line” will have no effect on the mixer. Phantom power can therefore easily be achieved by connecting a bypassed DC supply between the positive supply and the “hot” side of the microphone Making inputs truly versatile We designed this mixer to be as simple as possible to build with everything “on board”. This assumed that constructors would nominate the input device required for each channel and fit appropriate resistors and capacitors for R1, C1, and so on (as per the table on the circuit). But what if you needed to regularly swap inputs with devices that had different signal levels? It happens often in, for example, a band – or where various microphones are required to suit vocals or instruments. It would be quite simple to fit a multi-pole switch to any or all of the input op amps and so switch various values of R&C. For most applications, the input bias resistors will be satisfactory. However, you could bring these all down to 100kΩ if you really want to. Small double pole (or “changeover”) slider switches are available with up to four positions (eg, Altronics S-2040), so you could in theory fit four different values of R&C on the switch (again, as per the table on the circuit) and then be able to select the input level required according to the device being connected and, of course, its signal level. (See fig.8). Alternatively, small rotary switches Resistor Colour Codes o o o o o o o o o o input. We have made provision for this on one channel only, channel 4, with R5, R6 and a 100µF bypass capacitor. If you do not require phantom power, you can simply leave out these three components. In fact, you should not connect phantom power to a microphone that doesn’t need it. Putting a DC bias on a dynamic microphone’s voice coil, for example, will usually result in a lower (or no) output and may even permanently damage the microphone. No. 4 2 1 9 4 5 4 5 1 Value 1MΩ 100kΩ 39kΩ 10kΩ 6.8kΩ 1.8kΩ 220Ω 100Ω 33Ω siliconchip.com.au 4-Band Code (1%) brown black green brown brown black yellow brown orange white orange brown brown black orange brown blue grey red brown brown grey red brown red red brown brown brown black brown brown orange orange black brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown orange white black red brown brown black black red brown blue grey black brown brown brown gey black brown brown red red black black brown brown black black black brown orange orange black gold brown 150 Ω 390Ω TO PIN6 IC1b 1.8k Ω 150pF 330pF 560pF 1 2 (SIGNAL GROUND) 3 1 2 3 TO PIN7 IC1b Fig.8: adding input switching to one or more channels is really easy and makes the mixer much more versatile (but does complicate construction a little). Here we’ve shown a 2-pole, 3-position switch capable of selecting a microphone (1), guitar (2) or line-level (3) source. 2-pole rotary switches with up to six positions are also available if you want more switchable inputs. can be configured to have two poles and six positions so most of the variations shown on the circuit diagram could be accommodated. The resistors and capacitors could be wired directly to the switch and three wires (eg, rainbow cable) run to the appropriate positions on the PCB (ie, the positions which would have been occupied by R1, C1 etc). Want more than four channels? Getting greedy, aren’t we! Seriously, adding additional channels to a design of this type is easy – you simply build additional input circuits – up to and including the 10kΩ resistor after the individual channel “gain” pots (VR1-4). The “mixed” output of the four new channels is simply connected to the negative side of the 47µF capacitor before the existing IC3c, just as happens now. Power (ie ±15VDC), can be taken from a suitable point on the existing mixer – the supply will handle it – and signal and supply grounds also conSC nected to a suitable point. Capacitor Codes Value µF Value IEC Code EIA Code 470nF 0.47µF   470n   474 100nF 0.1µF   100n   104 22nF 0.022µF   22n  223 2.2nF .0022µF   2n2  222 1.5nF .0015µF 1n5   152 100pF    NA   100p   101 47pF   NA    47p   47 June 2012  67