Download Audio Mixer (2016) project document

Analog Audio Workshop Project 2016 – IEEE Concordia
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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
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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
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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
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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
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Component Pinouts
NE5532 (2 opamps per chip)
MCP6004 (4 opamps per chip)
Audio jack
TN0702N3
TDA7266M
Analog Audio Workshop Project 2016 – IEEE Concordia
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Fader pot (PTA4543-2015DPA103)
Dual-gang 100k pot (P120KGP-F20BR100K)
Analog Audio Workshop Project 2016 – IEEE Concordia
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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!