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Analog Audio Workshop Project 2016 – IEEE Concordia Page |1 Analog Audio Workshop [Winter 2016] IEEE Concordia – designs by Marc-Alexandre Chan <[email protected]> Table of Contents Project Overview: Mixer ......................................................................................................................... 2 Block Diagram ......................................................................................................................................... 3 Bill of Materials ....................................................................................................................................... 4 Component Pinouts ................................................................................................................................ 5 Circuit Diagrams ...................................................................................................................................... 7 IMPORTANT: Opamp power supplies ................................................................................................. 7 Power Input (each breadboard).......................................................................................................... 8 Input amplifier .................................................................................................................................... 9 Tone Filter/Equaliser ......................................................................................................................... 10 Option A ........................................................................................................................................ 10 Option B ........................................................................................................................................ 11 Option C ........................................................................................................................................ 13 Envelope (Volume) Detector............................................................................................................. 15 LED Driver.......................................................................................................................................... 16 Option A: Linear LED driver ........................................................................................................... 16 Option B: Comparator-based LED driver ...................................................................................... 17 Mixer ................................................................................................................................................. 18 Speaker driver ................................................................................................................................... 19 Analog Audio Workshop Project 2016 – IEEE Concordia Page |2 Project Overview: Mixer An audio mixer is a device that takes two or more line-level input audio channels from external devices (mic pre-amp, computer/phone/MP3 player, effects modules) and combines the different audio signals together, effectively the electronic equivalent of what you hear when you simply play the two sounds on different speakers. Professional mixers can have dozens of input channels which can be mixed onto dozens of output buses, as well as many features like volume/gain, tone/equalisation controls, various cue/monitoring options, and level/overload detectors. In the Analog Audio Workshop, you will build a very simple two-channel mono audio mixer based on inexpensive low-noise operational amplifier (opamp) chips. This mixer illustrates the concepts of a complex analog circuit based on a cascade (chain) of simple circuit blocks, as well as how to design these circuit blocks for practical real-world opamps. While this mixer should be usable and follow general good practices, it may not produce a high-fidelity, professional-quality design—partly because there may be better opamps for that purpose, partly because the circuits are meant to be kept simple and minimise the parts count, and partly because the physical layout of the components on a circuit board or breadboard will affect quality a lot despite being the same circuit design. Our mixer will have the following features: 1. 2 mono input channels (easily extendable to more) a. Input faders, up to +12dB gain b. Activity detection and/or overload LED (after-fader) c. Tone adjustment (bass shelf, parametric mid EQ, treble shelf) 2. 1 mono output channel a. Volume fader, up to +0dB gain 3. Speaker driver with a 0.25 or 0.5W speaker (for the purpose of testing in the workshop) Some features present in commercial mixer consoles, like mute, cue/monitoring options, sends/receives, and multiple output buses can easily be extended from this design and the use of some switches. Analog Audio Workshop Project 2016 – IEEE Concordia Page |3 Block Diagram Channel 1 CH1 IN INPUT AMPLIFIER TONE/EQ FILTER [optional] Activity detector [optional] OUT ENVELOPE (VOLUME) DETECTOR LED DRIVER Channel 2 MIXER SPEAKER DRIVER CH2 IN INPUT AMPLIFIER TONE/EQ FILTER [optional] Activity light [optional] ENVELOPE (VOLUME) DETECTOR LED DRIVER The block diagram above shows the various circuit blocks that comprise the overall mixer and their interconnections. This is a medium-level view of the mixer, as it is somewhat detailed as to the circuit blocks, rather than using more abstract blocks. Depending on your skill/comfort level with the components and breadboard, you can choose to make a simpler mixer very easily. The required blocks are: 2x Input Amplifiers 1x Mixer 1x Speaker Driver (already provided) It’s that simple! Things you can add if you’re comfortable with the breadboard: Tone Filter/Equaliser (each option builds on the previous: you can start with A and build up) o Option A (easiest): Bass shelf o Option B: Bass + Treble shelves o Option C: Bass + Treble shelves + parametric mid EQ Activity light o Option A: linear (proportional to volume) o Option B: comparator (lights up above a certain level, e.g. overload detector) o Option C: Both! Analog Audio Workshop Project 2016 – IEEE Concordia Page |4 Bill of Materials A summary of all parts needed to build a complete project is listed below (including both activity detector types). Specifications are suggested (e.g. type of capacitors), but you can make certain decisions on your own in this regard (e.g. poly film capacitors can be much better for precise filters). Resistors, in this case, can all be 1/8W, 1/4W or better. Tolerance recommended is 1%; for precise gain or filters, use 0.5% for the resistors involved. For a quick test or cheap project, 5% is OK. Several external parts are not included, like the breadboard or protoboard, the power supply and voltage regulators, wires or tools, etc. Component Audio cables Audio jacks, 3.25mm TRS C 0.1uF, X7R ceramic C 0.33uF, X7R ceramic C 10uF, aluminium electrolytic C 1uF, X7R ceramic C 2.2nF, C0G ceramic C 22nF, C0G ceramic C 470uF, aluminium electrolytic C 680pF, C0G ceramic D 1N4148 (or any signal diode) D LED 5mm indicator, colour of choice MCP6004 1MHz low-power opamp, DIP8 IC MOSFET N-channel Vth=1.5 TN0702N3-G NE5532 low-noise opamp, DIP8 IC POT 100k, rotary, lin taper POT 10k fader - linear action, log taper POT 10k, rotary, lin taper POT dual-gang 100k, rotary, lin taper R 100 R 10k R 150 R 22k R 330 R 330k R 33k R 3k3 R 470k R 47k R 68k R 82k Speaker SPST switch (breadboard-friendly) or ½ SPDT switch TDA7266M 7W mono bridge amplifier Qt 3 3 19 2 1 3 4 4 5 2 2 8 1.5 4 3 4 3 4 2 2 11 2 10 6 2 2 1 2 2 2 2 1 2 1 Analog Audio Workshop Project 2016 – IEEE Concordia Page |5 Component Pinouts NE5532 (2 opamps per chip) MCP6004 (4 opamps per chip) Audio jack TN0702N3 TDA7266M Analog Audio Workshop Project 2016 – IEEE Concordia Page |6 Fader pot (PTA4543-2015DPA103) Dual-gang 100k pot (P120KGP-F20BR100K) Analog Audio Workshop Project 2016 – IEEE Concordia Page |7 Circuit Diagrams The following sections show the circuit diagrams for each of the circuit blocks. They also provide a bill of materials (list of parts needed), brief explanation/design notes and major parameters of the circuit block. IMPORTANT: Opamp power supplies The power supply pins (Vcc+ and Vcc-, or similar) are not shown in these circuit diagrams. You need to remember to always connect them and put the appropriate decoupling capacitors, connected as close to the power pins as possible (avoid connecting them directly to the rail!). The power supply pins are omitted for clarity of the schematics. This is not something you’d do in a professional circuit diagram. You may do this in documentation that explains how a circuit works. Be careful to make the correct connections depending on the chip you use. In particular, incorrectly connecting the MCP6004 could damage it/blow it up. (Don’t release the magic blue smoke!) Analog Audio Workshop Project 2016 – IEEE Concordia Page |8 Power Input (each breadboard) RefDes CF01 CF02 D01 D02 R01 R02 SW01 SW02 Part name C 470uF C 470uF D LED 5mm indicator, colour of choice D LED 5mm indicator, colour of choice R 330 R 330 SPST switch (breadboard-friendly) (half as many SPDT better if available; DPST/DPDT acceptable) SPST switch (breadboard-friendly) (half as many SPDT better if available; DPST/DPDT acceptable) This circuit is already provided, fully built, in the workshop. We have used two separate SPST switches SW01 and SW02 because we did not have any SPDT switches that fit nicely on a breadboard. It would be better to use a single SPDT switch instead. CF01 and CF02 are bulk decoupling capacitors that should be placed near to where the power comes into the board. We use two LEDs so you can tell whether either power rail is turned on, to avoid accidents and to quickly identify power problems. In a real product, you would want to use a single LED connected across +5 and -5V, with 680 ohms in series. Analog Audio Workshop Project 2016 – IEEE Concordia Page |9 Input amplifier CF11 CF12 POT11 R11 R12 R13 U11 C 0.1uF C 0.1uF POT 10k fader - linear action, log taper R 82k R 330k R 68k NE5532 low-noise opamp, DIP8 IC POT11 is not symmetrical because it is logarithmic. Connect pin 1 to the IN and pin 3 to ground. When POT11 is at maximum volume, the overall gain is: 𝐴𝑉,𝑚𝑎𝑥 = 330𝑘 ≈4 82𝑘 → 20 log10 4 = +12dB Analog Audio Workshop Project 2016 – IEEE Concordia P a g e | 10 Tone Filter/Equaliser You have three options for the tone filter, in increasing order of complexity. They each build upon each other: you can start with option A and then continue on with option B and C without undoing your previous work. If you want to do option C, I strongly suggest you build up like this and test one part of the filter at a time. Option A C21 C22 CF21 CF22 POT21 R21 R22 R23 U21 C 22nF C 22nF C 0.1uF C 0.1uF POT 100k, rotary, lin taper R 22k R 22k R 22k NE5532 low-noise opamp, DIP8 IC This circuit is a bass shelving filter. It can do either boost or cut. At high frequencies the gain is 1 = 0dB (i.e. no change in signal). At low frequencies, the maximum boost or cut is: 𝐴𝑉,𝑚𝑎𝑥 = 𝐴𝑉,𝑚𝑖𝑛 = 122𝑘 = 5.55 22𝑘 22𝑘 = 0.180 122𝑘 → 20 log10 5.55 = 14.9dB → 20 log10 0.180 = −14.9dB Analog Audio Workshop Project 2016 – IEEE Concordia P a g e | 11 This is a fairly high boost/cut and should suffice for most applications. When the potentiometer is at its centre, the response is flat (gain of 1, or 0dB). The transition frequencies (from “low” to “high” frequency gains) changes a little bit depending on the pot setting, but here are some guidelines: When the pot is at the centre, there is a theoretical boost/cut that cancel out (to give a flat response). The transition would happen between 144Hz and 474Hz. When the pot is at maximum boost or cut (fully turned to either end), the transition happens between 72Hz and 400Hz. This circuit is designed so you can hear the effect on a small speaker. For a realistic bass circuit, I suggest using C21 = C22 = 0.47nF instead, which will give you a theoretical transition of 67.7Hz to 222Hz at pot centre, and a transition of 33.9Hz to 187Hz at full boost/cut. Option B Analog Audio Workshop Project 2016 – IEEE Concordia C31 C32 C33 CF31 CF32 POT31 POT32 R31 R32 R33 R34 R35 R36 U31 C 22nF C 22nF C 680pF C 0.1uF C 0.1uF POT 100k, rotary, lin taper POT 100k, rotary, lin taper R 22k R 22k R 22k R 10k R 10k R 47k NE5532 low-noise opamp, DIP8 IC P a g e | 12 This circuit adds a treble shelving filter to Option B’s bass shelving filter. Easy to miss: Compared to Option A, R33 has been added. Don’t forget it, or the filter will not work! For treble, the maximum cut/boost is 𝐴𝑉,𝑚𝑎𝑥 22𝑘 + 2 × 22𝑘 10𝐾 = = 4.75 22𝑘 + 2 × 22𝑘 1 + 10𝐾 + 100𝐾 → 20 log10 4.75 = 13.5dB 1+ 𝐴𝑉,𝑚𝑖𝑛 = 0.211 → 20 log10 0.180 = −13.5dB When the potentiometer is at its centre, the high-frequency response is flat (gain of 1, or 0dB). The transition frequencies (from “low” to “high” frequency gains) changes a little bit depending on the pot setting, but here are some guidelines: When the pot is at the centre, there is a theoretical boost/cut that cancel out (to give a flat response). The cancelling happens at 3.72kHz. When the pot is at maximum boost or cut (fully turned to either end), the transition happens between 3.36kHz and 16.0kHz. References: Original paper: http://www.learnabout-electronics.org/Downloads/NegativeFeedbackTone.pdf A variation of this circuit: http://sound.westhost.com/dwopa2.htm Analog Audio Workshop Project 2016 – IEEE Concordia Option C P a g e | 13 Analog Audio Workshop Project 2016 – IEEE Concordia This circuit adds a parametric EQ band for mid-band control. The maximum cut/boost is ±4.4dB adjustable with POT43. There are ways of improving this cut/boost easily when it’s only a single-band EQ (see reference below: adds a resistor at the – input), but the fact that we have combined the bass/treble into this same circuit makes it less obvious (I would have to play around and see if I can make it work without affecting the bass/treble). If you wanted more gain, it might be easiest to make Option B + a wien bridge filter as two cascaded circuit blocks with two separate opamps! Frequency can be adjusted with POT44 from 593Hz to 3.29kHz. C41 C42 C43 C44 C45 CF41 CF42 POT41 POT42 POT43 Reference: Wien bridge filter (single-band EQ): http://sound.westhost.com/project150.htm POT44 R41 R42 R43 R44 R45 R46 R47 U41 P a g e | 14 C 22nF C 22nF C 680pF C 2.2nF C 2.2nF C 0.1uF C 0.1uF POT 100k, rotary, lin taper POT 100k, rotary, lin taper POT 10k, rotary, lin taper POT dual-gang 100k, rotary, lin taper R 22k R 22k R 22k R 10k R 10k R 22k R 22k NE5532 low-noise opamp, DIP8 IC Analog Audio Workshop Project 2016 – IEEE Concordia P a g e | 15 Envelope (Volume) Detector C51 C52 CF51 D51 R51 R52 R53 R54 U51 C 0.33uF C 1uF C 0.1uF D 1N4148 (or any signal diode) R 33k R 10k R 470k R 10k MCP6004 1MHz low-power opamp, DIP8 IC This circuit takes an audio input, and outputs the envelope (a.k.a. the volume, or the amplitude over time). It is basically an AM demodulator operating at very low frequency. This circuit’s maximum output will be ≈4.4V (assuming +5V power supply and a diode drop voltage of 0.6V). It is best to keep the output voltage reasonably far below this value (say 3.5V to 4V max). Furthermore, the MCP6004 uses only +5V (not ±5V – this will damage the chip!), and so it needs to be protected from a too big input coming from an NE5532. For both these reasons, R51 and R52 form a voltage divider that reduces the voltage by a factor of 4.3 (-12.7dB). You can adjust how fast it is (and thus how quickly the volume output is allowed to change) by changing the RC constant formed by R53 and C52 (higher values = slower, lower values = faster). If the circuit is too fast, you may notice unacceptable fluctuations in the envelope (e.g. if you then display it on an LED, it will visibly be pulsing). This circuit may have some oscillation issues that can also cause pulsing. I have not done deep analysis as to whether oscillations or high speed were the cause of the problem in a previous circuit; the circuit as shown above was found to provide a reasonably slow signal for a channel activity light. This circuit’s output filter (R53 and C52) has a cutoff frequency of about 1/3 Hz. Analog Audio Workshop Project 2016 – IEEE Concordia P a g e | 16 LED Driver There are two options for the LED driver. You could even use both of them connected to the output of the Envelope Detector: one could be an activity light, the other one could be an overload light. Option A: Linear LED driver CF61 D61 M61 R61 R62 U61 C 0.1uF D LED 5mm indicator, any colour MOSFET NFET Vth=1.5 TN0702N3-G R 150 R 100 MCP6004 1MHz lowpower opamp, DIP8 IC This circuit will control the current through D61 to be proportional to the input voltage 𝑉𝐶𝑇𝐿 (IN in the schematic; in our case, the input is the volume envelope). The equation is 𝐼𝑜 = 𝑉𝐶𝑇𝐿 150Ω R61 controls the relationship between voltage and current. R61 and R62 in combination control what the maximum amount of current is (for visual purposes and to protect D61 and M61 from currents beyond their limits). M61 is used as a variable resistor, in a closed-loop feedback loop (via U61), in order to control the current. The minimum is 𝑉𝐶𝑇𝐿 = 0𝑉, for 𝐼𝑜 = 0𝐴 (i.e. LED is off). The maximum is designed to be approximately 𝑉𝐶𝑇𝐿 = 1.5𝑉, 𝐼𝑜 = 10𝑚𝐴, due to R61 and R62. There is another maximum due to the opamp’s voltage output limits (in this case 0 to 5V). If we assume that M61 has a threshold 𝑉𝑇𝐻 = 1.5𝑉 (for the TN0702N3), then we find that 𝐼𝑜,𝑚𝑎𝑥 = 23𝑚𝐴 due to the opamp (see equation in slides). This means that the 10mA dominates, and the opamp isn’t a limitation for this situation. It is a good idea to limit current below the usual maximum current of 30mA. This is because the LED is often painfully bright at high currents, and appears to “max out” in brightness anyway, so there will not be much of a difference between 20mA and 30mA. In general, 5mA to 10mA is good enough to be visible, and blue LEDs need even less current. Analog Audio Workshop Project 2016 – IEEE Concordia P a g e | 17 Option B: Comparator-based LED driver C71 CF71 D71 M71 POT71 R71 U71 C 0.1uF C 0.1uF D LED 5mm indicator, colour of choice MOSFET N-channel Vth=1.5 TN0702N3-G POT 10k, rotary, lin taper R 330 MCP6004 1MHz low-power opamp, DIP8 IC This LED driver does not try to change the brightness of the LED. When the input (in our case, the volume of the audio) goes above a threshold value set by POT71, the LED will light up. You can use this as a more on-off kind of activity detector, or you can use it specifically to detect It is generally better to use a real comparator chip instead of an opamp (U71). However, since this is a slow-speed circuit, and the workshop is all about opamps, we used an opamp here. Here’s a bonus: how can you make a circuit that lets the LED turn on and off gradually (e.g. in 100ms), instead of suddenly, so that it looks better? How could you make the circuit light the LED up for at least 2s, even if the input drops back below the threshold? This second question would probably be important for an overload detector. Analog Audio Workshop Project 2016 – IEEE Concordia P a g e | 18 Mixer CF81 CF82 POT81 R81 R82 R83 R84 U81 C 0.1uF C 0.1uF POT 10k fader - linear action, log taper R 10k R 10k R 10k R 3k3 NE5532 low-noise opamp, DIP8 IC The mixer block is, as the name suggests, the core part of the audio mixer. This block allows you to add two or more signals together. This design, with R81 = R82 = R83, allows for a gain of 1 (0dB) for each signal, i.e. it adds them all at the same volume as the inputs. You could increase R83 to add some gain, or decrease R81 or R82 if you want the mixing to be uneven between different channels. The output bus has a volume control POT81. Again, POT81 is a logarithmic pot, so it’s not symmetrical and you should connect pin 1 to the opamp output and pin 3 to ground. Because of POT81, the OUT signal has a high output impedance. If you wanted to use this as an output, connected to a jack or to a lower-input-impedance circuit (<50kΩ in this case), you should use a voltage follower or an inverting buffer with a gain of -1 to buffer the output (depending on if you want to invert the phase or not). In our case, it’s going to a reasonably high-impedance speaker driver input, so the high output impedance doesn’t matter as much. Analog Audio Workshop Project 2016 – IEEE Concordia P a g e | 19 Speaker driver C91 C92 CF91 CF92 R61 R62 SPKR91 U61 C 1uF C 10uF C 0.1uF C 470uF R 47k R 47k Speaker TDA7266M 7W mono bridge amplifier This circuit is provided fully built. The speaker driver is a specially-made amplifier, in this case with a gain of about 20dB. Opamps we have dealt with in this workshop can amplify voltage signals, but they cannot provide high current from their output pins—or equivalently, they cannot provide much real power. If you were to connect a speaker to it and try to crank up the volume, the speaker would distort horribly (and your opamp might overheat after a while). This amplifier is specially designed to be able to provide up to 7W of power to one speaker. However, we are giving it only 5V of supply voltage. Remember that 𝑃 = 𝑉𝑟𝑚𝑠 𝐼𝑟𝑚𝑠 (for a resistive load) or equivalent 𝑃 = 5 √2 2 𝑉𝑟𝑚𝑠 : 𝑅 so with an 8 ohm speaker and 5V power supply, you can get a sine wave 𝑉𝑟𝑚𝑠 = and so a max power of 1.5625W. We would need around 10V power supply to get the full 7W. Why did we use 5V? First, because we were using +5V rails everywhere else, so it made it easier for a workshop setting (otherwise, you need to design for a +10V rail available too, which would be a reasonable option in a project). Secondly, because the speakers we’re using max out at 0.25W or 0.5W, so we can’t drive them with 7W anyway—they would distort horribly and start smoking!