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Build an 8-cha stereo mixer; At last there’s a comprehensive mixer that’s easy to build. This unit features eight main input channels, an auxiliary input channel, LED bargraph level meters, effects send and comprehensive headphone monitoring facilities. 20  Silicon Chip annel Pt.1 By JOHN CLARKE W HEN IT COMES to mixers, every­- one has their own ideas about how the signal should be directed from the input to the output. And of course there are numerous options to be decided on in the design process. In this design, we have produced a practical arrangement which should be suitable for most mixer users. There are of course the usual level controls for each of the inputs, plus separate output pots for the left and right channels. Stereo effects are provided by separate Pan pots for each input channel. These allow tailored mixing into either the left or right channel bus, or a mixture in both. Each of the eight Main inputs includes tone control facili­ties. The monitor signal, however, is not affected by the tone controls and is a mono output only. This output can be fed to a foldback amplifier and speakers, so that musicians can hear themselves on the stage. An Effects signal output is also provided on each channel, immediately following the tone controls. It is intended for use with reverberation or other effects boxes for added sound en­ hancement. The resulting signal from the effects box can then be applied to the Auxiliary input of the mixer and subsequently level controlled and panned to the left and right outputs. If effects are not required, the Effects bus can be used as a second monitor output. Note that both the Monitor and Effects outputs for each channel have individual level controls. These are situated in two rows along the top of the front panel. All signal monitoring is provided via a headphone output which is situated near the top righthand corner of the front panel. An adjacent 12-way switch allows any of the eight main inputs to be monitored (after the tone controls), or the Monitor Bus, Effects Bus, Left Main Bus or Right Main Bus can be moni­tored. Two 10-step LED bargraph displays are used to indicate the left and right channel output levels. These cover signal levels from -24dB to +3dB and allow the operator to see what’s going on at a glance. Design considerations With all those facilities, the new 8-Channel Mixer is quite large. It fits into a 125mm deep metal case and this carries a front panel that measures 485mm wide x 310mm high. These dimen­ sions comply with a 7-unit rack sizing, which means that the unit can be mounted vertically in a rack frame. Alternatively, it can be used as a “standalone” unit which sits either horizontally or vertically. In addition, at least one retailer has indicated that they intend producing a complete kit of parts for this design and that their case will have a sloping front panel. Despite the amount of circuitry involved, the unit is easy to build since virtually all the parts go on a single large PC board. Even the input and output plugs and sockets mount on the PC board. This leaves only a small amount of wiring to be run for the power supply. By contrast, many other mixer designs use a separate PC board for each input channel plus several others for the output controls. The amount of wiring between these PC boards is consid­erable. Our new circuit is also much simpler than previous designs with similar facilities. This has been made possible by using a spe­cial purpose balanced input amplifier IC which provides a low noise signal for the following stages. Let’s now take a look at how it all works. Block diagram Fig.1 shows the general signal path arrangement of our new mixer. There are eight Main inputs plus a single Auxiliary input. Note, however, that only one Main input channel (IC1-IC3) is shown here in order to simplify the diagram. The other seven input channels are identical. Each main input can accept either an XLR plug or a 6.35mm stereo jack. For unbalanced inputs, you can ground one of the input pins (2 or 3) on the plug. This is standard practice and is done to avoid hum pickup when an unbalanced lead is connected to a balanced input. IC1 is the input amplifier and it can be switched to pro­vide either +30dB or +10dB of gain. The +30dB setting (LOW) is suitable for microphone levels and provides the mixer with an overall sensitivity of 4mV. The +10dB (HIGH) setting is suitable for higher input levels, such as from electric guitars and key­boards. In the latter mode, the overall sensitivity is reduced to 40mV November 1996  21 Fig.1: the general signal path arrangement of the new mixer. There are eight Main inputs (although only one is shown here) plus a single Auxiliary input and an Effects input. The outputs are metered using LED bargraph displays. and clipping occurs at 9V RMS. Note that the input does not provide phantom power for electret microphones. If you want to use electrets, they will either have to be battery-operated or powered from some other source. Following IC1, the signal is split two ways and drives level control pots VR1 (Main) and VR2 (Monitor). VR2 in turn feeds op amp stage IC2 (+12dB) which then drives the monitor bus via a mixing resistor. VR1, on the other hand, feeds IC2a which also provides +12dB of gain. Its output then drives a tone control stage con­ sisting of IC3 and potent­ iometers VR3 (bass) and VR4 (treble). 22  Silicon Chip The bass control provides a nominal 10dB boost or cut at 100Hz, while the treble control gives a 12dB boost or cut at 10kHz. Note that the bass control is usually only used to remove any “boominess” from instruments, while the treble control can help with sibilant (S) sounds by curtailing high frequencies in voice signals. The output of the tone control stage drives Effects level control VR5 and Pan control VR6. It also provides the Channel 1 headphone signal via an isolating resistor. The Effects control sets the signal level to be applied to the Effects bus (again via a mixing resistor), while the Pan control sets the signal levels fed to the left and right buses. If the signal is intended for the right channel only, then the Pan pot is fully rotated to shunt the left channel signal to ground. Conversely, if the signal if for the left channel only, the Pan pot is fully rotated in the opposite direction to shunt the right channel signal to ground. If the Pan pot is centred, then equal amounts of signal are applied to both the left and right buses. By contrast with the Main channels, the Auxiliary channel provides an unbalanced input only. This input is buffered by IC10a and this then drives pan control pot VR11 via level control VR10. The resulting left and right channel signals are then mixed onto the left and right main buses. From there, the left bus signal is fed to IC4a, while the right bus signal goes to IC7a. For the left channel, IC4a provides 11dB of gain and drives IC4b via output level control VR7. IC4b provides an extra 12dB of gain. Its output directly drives pin 2 of an XLR output socket and pin 3 of this same socket via inverting amplifier IC5a. As a result, two out-of-phase signals appear on pins 2 and 3 to provide the balanced output. Alternatively, an unbalanced output can be obtained by connecting between pin 2 and ground. In addition, IC4b drives a 10-LED bargraph display which indicates the signal level in 3dB steps. This display has a range from -24dB to +3dB. IC7a, IC7b, IC5b and the LED bar­ graph process the right channel bus signals in exactly the same manner. The Effects bus and Monitor bus output stages employ iden­ t ical circuitry. For the Effects bus, IC9a amplifies the mixed signal and drives IC9b via level control VR9. IC9b then drives the Effects Send socket to provide an unbalanced output signal. IC11a, VR12 and IC11b do exactly the same job for the mixed Monitor bus signal. Finally, the headphone amplifier is connected as an invert­ing stage (mixing) so that it can monitor the selected bus via switch S9. You can listen to all the input channel signals and each of the buses as shown. The amplifier is mono only, which means that the left and right bus signals are monitored separate­ly rather than in stereo. Circuit details Refer now to Fig.2 for the circuit details. To simplify matters, this shows only one of the eight input channels (channel 1) and only one of the two main output stages. IC1 is an Analog Devices SSM2017 Self-Contained Audio Preamplifier. This is a balanced input amplifier with a typical common mode rejection of 74dB at a gain of 10. Its total harmonic distortion and noise figures are also very low. IC1’s gain is determined by the resistance value between pins 1 and 8. In this case, the gain can be switched between +10dB and +30dB by S1 which selects either a 4.7kΩ resistor or a 330Ω resistor respectively. The 10kΩ resistors at the pin 2 and pin 3 inputs of IC1 ensure that it oper- Features • • • • Eight Main inputs plus Auxiliary input. • • • Bass and treble controls on eight Main inputs. • • • Special purpose low noise input amplifier. • Headphone monitoring for eight Main channels, plus Monitor, Effects, Right main and Left main buses. • Easy to build – single PC board construction eliminates all external wiring except for power supply. • Case conforms to 7-unit rack sizing; suitable for vertical or horizontal use. Stereo outputs. Effects and monitor for all eight Main inputs. Panning between left and right channels for all eight Main inputs and Auxiliary. High and low input signal selection for eight Main inputs. Balanced inputs for eight Main channels using XLR sockets or 6.35mm sockets. Balanced left and right main XLR outputs. Signal level metering for Left and Right output channels using dual LED bargraphs. Specifications Signal-to-Noise Ratio at Left and Right Main outputs 80dB unweighted <at> 1V out and 100mV input (all channel inputs unloaded and set at maximum) Bass and Treble controls ±10dB at 100Hz and ±12dB at 10kHz Sensitivity for 1-8 Channel inputs 4mV RMS for 1V output on LOW setting 40mV RMS for 1V output on HIGH setting Sensitivity for Monitor and Effects outputs 2mV RMS for 1V output on LOW setting; 20mV RMS for 1V output on HIGH setting Sensitivity for Auxiliary input 120mV RMS for 1V output Maximum input levels before clipping 2.9V RMS on LOW setting; 9V RMS on HIGH setting Frequency response -3dB at 20Hz and 32kHz (Main, Monitor and Effects) Total Harmonic Distortion 0.008% at 1kHz (100mV in and 1V out); 0.02% at 10kHz (100mV in and 1V out) ates within its correct common mode range when no DC connection is made to the input. If a balanced microphone is connected, its low 600Ω impedance will reduce the input load resistance, with a subsequent reduction in noise. The 270pF capacitor shunts any high frequency signals to improve common mode rejection at high frequencies and reduces the possibility of RF pickup. Note that the input to IC1 is not AC-coupled via a capaci­ tor. That’s because microphone and guitar signals November 1996  23 24  Silicon Chip Fig.2 (left): this circuit diagram shows only one of the eight Main input channels and only one of the two output stages. Note that all the Main channels have balanced inputs and feature tone control circuitry. The headphone monitoring circuit uses op amp IC12 to drive complementary output pair Q1 & Q2. are from a balanced or unbalanced transformer or inductive pickup and hence carry no DC voltage. What’s more, any small DC offset from say a line signal or keyboard will not cause problems since the amplifier can handle high DC offsets before any superimposed AC signal will be clipped. The output from IC1 is AC-coupled to prevent any DC flow in the following Main and Monitor pots (VR1 & VR2). This is neces­sary since any DC flow in these potentiometers will cause noise in the signal as they are adjusted. The output from VR2 is AC-coupled to the input of IC2b (pin 5) via a 2.2µF non-polarised (bipolar) capacitor. A 22kΩ resistor to ground sets the input bias, while the 10Ω resistor in series with the input reduces RF pickup. The gain is set to four by the 6.8kΩ and 2.2kΩ feedback resistors, while a 270pF feedback ca­pacitor rolls off the high frequency response from about 87kHz to prevent high-frequency instability. The amplified output from IC2b appears at pin 7 and is mixed onto the Monitor bus via a 10kΩ resistor. IC2a in the main signal path functions in exactly the same manner as IC2b. Besides providing gain and a high impedance load for level control VR1, IC2a also acts as a low impedance source for the following tone control stage based on IC3. This stage has the tone control pots (VR3 & VR4) connected in the negative feedback network. When the bass and treble controls are centred, the gain of the stage is unity, up to at least 50kHz. Winding the bass or treble controls toward the input side of IC3 increases the gain for frequencies above 2kHz for the treble control and 300Hz for the bass control. Conversely, when the tone controls are rotated in the opposite direction (to apply bass or treble cut), the gain is reduced above 2kHz and below 300Hz. This is because the negative feedback has been November 1996  25 AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 15.000 23 SEP 96 10:10:41 10.000 5.0000 0.0 -5.000 This stage operates with a gain of 4.09 and has a 100Ω resistor in series with its output to prevent instability when driving ca­pacitive loads. Following IC4b, the signal is fed to pin 2 of the XLR output socket via a 47µF capacitor. It also goes to pin 2 of inverting buffer stage IC5a, which then drives pin 3 to provide the other side of the balanced output signal. As with IC4b, IC5a has a 100Ω resistor in series with its output to prevent in­stability. Note that the outputs of IC4b and IC5a are both AC-coupled to the output XLR socket. This is to prevent any DC in the out­put. LED level indicator -10.00 -15.00 20 100 1k 10k 20k Fig.3: this graph shows the frequency response of the tone controls at their maximum boost and cut settings and at the flat setting. Note that the amount of boost and cut is set to ±12dB maximum in both instances. increased, giving a reduction in gain at these frequencies. The maximum bass boost and cut is limited to about ±12dB by the 22kΩ resistors on either side of the bass pot, VR3. Simi­larly, the amount of treble boost and cut provided by VR4 is limited to ±12dB by the 4.7kΩ resistors on either side of the treble pot, VR4. Fig.3 shows the action of the tone controls at their maximum boost and cut settings and also at the flat setting. Note that OP27GP op amps have been specified in the tone control circuitry. The reason for this is that the DC across VR3 must be as low as possible to limit noise when adjusting the bass con­trol. The input offset voltage for the OP27 is typically just 30µV while the input offset current is only 12nA and so the resulting DC in VR3 will be negligible. The output from IC3 appears at pin 6 and drives Effects pot VR5, the 10kΩ headphone signal resistor and the left and right channel Pan control circuitry. As mentioned previously, the Pan control operates by shunting signal in the unwanted channel to ground. When the wiper of VR6 is towards the left main bus side, the left channel signal is shunted to ground. Similarly, when the wiper of VR6 is towards the right bus side, the right channel signal 26  Silicon Chip is shunted. Finally, when the pot is centred the left and right signals are attenuated and so are equally mixed into the left and right channels. The seven other inputs are identical to this first input circuit but with different IC and pot numbering. Op amps IC10a and IC10b are used to process the unbalanced auxiliary input signal. IC10a is wired as a unity gain buffer stage, with its pin 3 (non-inverting) input biased to 0V by a 22kΩ resistor. The output signal appears on pin 1 and is AC-coupled to the Auxiliary level control (VR10) via a 2.2µF capaci­tor. Following VR10, the signal is fed to IC10b which operates with a gain of 4.09. IC10b in turn drives the Auxiliary pan control circuitry which mixes the signal onto the left and right main buses. Output stages The mixed left main bus signal is fed to the pin 2 input of IC4a via a 2.2kΩ resistor. This op amp (an LM833) is wired as an inverting stage and amplifies the left bus signal by a factor of 3.4. The 27pF capacitor across the 68kΩ resistor provides high frequency rolloff above about 87kHz. IC4a’s output at pin 1 feeds the Left Main level control (VR7), after which the signal is coupled to gain stage IC4b. IC4b also drives the LED level indicator circuit. This circuit is based on IC6 which is a logarithmic LED display driver wired to operate in bargraph mode. The signal from IC4b is ap­plied to pin 5 via a 100Ω de­coupling resistor. Inside the IC, the negative-going signal excursions are clamped via a diode while positive signal excursions are fed to comparator circuits which then drive the individual LEDs. The meter circuit responds instantaneously to the waveform and thus shows the peak voltage of a sinewave. Note, however, that the peak LED does not light on very short transients and so the meter can be considered to be an averaging display. The meter calibration is set by the voltage on pins 6 & 7. This voltage is determined by first applying the 1.2V internal reference that appears between pins 6 & 8 to a 330Ω resistor. The resulting 3.6mA current then flows to ground via a 68Ω resistor, which thus has 0.25V across it. As a result, pins 6 & 7 of IC6 sit at 1.45V (1.2V + 0.25V). This means that the LED bargraph reaches full-scale (equivalent to +3B) when the applied signal level reaches 1.45V, corresponding to a nominal 1V RMS sinewave. The 0dB level (LED 9) occurs at 0.7V RMS. The 270Ω resistor in series with the LED anodes limits the dissipation in IC6, while the associated 100µF capacitor decou­ples the LED supply rails. A 10µF capacitor decouples the supply rail to the IC. The right channel output stage is identical to the left channel circuitry. In this case, the devices and their Despite the amount of circuitry involved, the assembly is really very easy since virtually all the parts are on this single large PC board. Note that this photo shows an early prototype board. corre­sponding pin numbers are indicated in brackets. Monitor & effects stages IC11a is the Monitor bus summing amplifier and this stage operates with a gain of about 5.7. Its output is fed to level control VR12 and from there to IC11b which provides a gain 2. IC11b’s output is then fed via a 100Ω resistor (for stability) and a 47µF capacitor to the Monitor output socket. The Effects summing and output amplifier stages (IC9a and IC9b, respectively) operate in exactly the same manner as the Monitor stages. Headphone amplifier The headphone amplifier is based on op amp IC12 and this operates in combination with transistors Q1 and Q2 which form a fairly conventional push-pull output stage. The transistors are there to boost the output current cap­ability of the TL071 op amp. Note that they are slightly forward-biased (to minimise cross­ over distortion) by connecting diodes D1 and D2 in series between their bases. The distortion produced by the output transistors is also minimised by incorpo­rating them inside the feedback loop of the op amp. The 33Ω emitter resistors have been included to maintain the bias stability. Together with the 68Ω output series resistor, they also provide short circuit protection and protect the head­phones against damage in the unlikely event of an amplifier failure. Power supply Power for the circuit is derived from a toroidal transform­er which delivers a 30V centre-tapped AC output. The primary of the transformer is fused for safety, while a .001µF 2kV capaci­tor is connected across the power switch to minimise switch-off transients. The secondary voltage is applied to bridge rectifier D3-D6 and the resulting DC rails filtered using two 1000µF capacitors to obtain ±21VDC. These rails are then fed to 3-terminal regu­ lators REG1 and REG2 which provide stable ±15V rails for the mixer circuit. Note that the output of each regulator is decoupled with a 10µF capacitor to maintain stability. In addition, there are many other 10µF capacitors scattered around the circuit which decouple the supply rails to the ICs. These are important to ensure stability in the op amps. Power indication is provided by LED21 which is connected in series with a 4.7kΩ resistor between ground and the -15V rail. By the way, the toroidal type has been specified to mini­ m ise hum induction in the mixer circuit. Do not use a standard E-core type since the signal-to-noise ratio will suffer greatly and hum will be heard in the mixer output. That’s all we have space for this month. In Pt.2, we shall present the parts list and give the full construction SC details. November 1996  27