Modular audio mixer for 8 and 4 input channels

I uploaded two new PDF documents about my audio mixer projects (1 - 2). The circuit called "audio mixer" is really simple, the required preamplifiers are little harder, if you need poweramp and EQ the device will more harder. But already I have tested and passed preamplifier project with headphone amplifier. And I have two modular equalizers ready to build. So I need audio mixer to collect these circuits to the simplest solution.

As I wrote already I have two of audio mixer project, because first time I designed very simple, with 4 or 8 input channels only, with master volume and line two outputs only. This project contains 2 very simple mainboards for 4 and 8 channel inputs, two output panels, and preamplifiers as module. The preamplifiers for mixer is same as my first designs, but can be connected with 12 pin male connectors to selected mixer mainboard.

For example, here is the microphone preamplifier with INA217:


The "original" PCB for this device is here:


But for modular mixer I have to modify the board design, the input and power connectors are removed, and changed to P4 connector on the bottom of board:


These preamplifier modules are smaller than the original versions, and contains "power filter" circuits as module (P5 and P6 connector at upper board). The preamplifier modules can be connected to the mixer mainboards, any preamplifier can be connected to the any connector of the mixer. The distance between preamplifiers is 25mm. This is the reason, why I have to design new versions of PCB for circuit called "power filter". This module must be smaller than 25mm. I have 5 versions of this module, with several sizes. All my preamplifiers (mic preamp with INA217, stereo dual jFET preamp, Dual OpAmp, 2 versions of single OpAmp) re-designed for modular mixer system.

The picasa gallery:


For the simplest version of my mixer project I have two mainboards with connectors for preamps, outputs, power supply, and balanced inputs for microphones:


4 channel mainboard for 4 inputs and 2 outputs:

For this board required 2 "power filter" modules, 4 preamplifiers, 1 volume board, and 1 line output board.

For the board can be connected two output PCBs. The one is for the master volume:

This PCB must be soldered after the 4th preamplifier via P5 male connector. This board working as 2 separated volume potentiometer for left and right channel, and possible to use only one stereo potentiometer for both channels on the pins of P1.

The another output module made for 2 line level outputs:

This board must be soldered to the last connector of mixer board via P1 header. The line level circuit designed to the mainboard, so this PCB contains the connectors only. The board useful for:

• 2 6.3mm jack
• 1 6.3mm jack and 1 stereo RCA
• 1 6.3mm jack and 1 3.2mm jack
• 1 3.2mm jack and 1 stereo RCA

The main output of full mixer is connected to the mainboard. This is the output for equalizers, poweramps, headphone amps, etc....

I designed 8 channel mainboard:


This circuit is same as than the upper 4 channel design. But here is 8 connectors for input channels can be connected to preamplifiers. The two of output boards is same too. This board width is 25cm, I think this is something big, very hard to manufacture, this is the reason why i designed "expandable" version of my audio mixer with connectable small mainboards for more than 8 channel inputs.

PDF manuals for audio mixers:

• Simple 4 or 8 channel audio mixer
  
• Expandable, any number of channels audio mixer
  

PCB sales of this project Module name Size (mm) Area (cm2) PDF SCH PCB image Tested Price (US$) Des. Sim.1 Full2 Sim.1 Full2 Man3 Power filter module 17x25 4 Yes Yes Yes - - No 3 11 Ask 4 channel mixer mainboard 149x84 125 Yes Yes Yes - - No 19 28 Ask 8 channel mixer mainboard 240x80 192 Yes Yes Yes - - No 27 36 Ask I/O board 37x60 22 Yes Yes Yes - - No 5 12 Ask 2 Potmeter board 31x62 19 Yes Yes Yes - - No 5 12 Ask Microphone preamp module V2 67x103 69 Yes Yes Yes Yes - No 11 19 Ask Microphone preamp module V3 67x103 69 Yes Yes Yes - - No 11 19 Ask 1 dual opamp preamp module 61x103 63 Yes Yes Yes - - No 10 18 Ask 2 single opamp preamp module 64x103 66 Yes Yes Yes Yes - No 11 19 Ask HQ 2 single opamp module 67x103 69 Yes Yes Yes - - No 11 19 Ask jFET guitar preamp module 65x102 66 Yes Yes Yes - - No 11 19 Ask Switch module 18x34 6 Yes Yes Yes Yes - No 3 11 Ask Simple Power supply 121x57 69 Yes Yes Yes Yes - No 11 20 Ask Power supply with 48V phantom 168x89 150 Yes Yes Yes - - No 22 32 Ask How to order? Please read the rules carefully!

Headphone amplifier with TPA6120

This is my first SMD circuit. The small size of SMD parts was my problem before. The opinions about TPA6120 is very good on all other related pages, the official datasheet is unbelievable. I have 5 pcs 6120 circuit, so I have to test it with my own PCBs.



The headphone circuit is very simple. With unbalanced balanced converter is much better, but in this first version this feature is missing from these PCB-s. I have design with unbalanced/balanced converter, but for modular mixer only. Now I posting about "independent" headphone amplifiers. Fortunately, this circuit is not the smallest SMD footprint. The distance between foots is 50 mill, the foot width is 20 mill. This size is no problem for homemade prototype.

The main IC is:


This chip have 20 "foots", but the half of these pins are not internally connected. On the bottom, small heatsink available, but this is not only heatsink, this is the GND as 21st pin. Footprint not available, have to be draw on the CAD software. The full circuit without dual power supply, and unbalanced/balanced converter is very simple:



These modifications possible:

• For both channels (left and right) have + and - inputs for balanced sources. This is optional,
  but maybe the result is better. On this schematic has no input for balanced sources.
• Separated dual power supplies possible for this one circuit. this is reduces the crosstalk between left and right channel. The dynamics of this circuit on the official datasheet is 120 dB, what is very good if true. Class D amplifier. I have "power filter" circuit to separate power sources of left and right channels, but I don`t want to use this for headphone amplifier because maybe the current source is not enough.
  

Therefore I have 3 versions of PCBs:

The first is the biggest one, to screw with another preamplifier circuits. The position of rear hole is the reason why this PCB so large:


If the same hole positions are not important between preamplifiers and the headphone amp, then here is the smaller PCB:


Finlay I have smaller. This PCB have separated inputs for balanced inputs of left and right channels, and inputs for left and right powers. this is the reason why volume potentiometer missing, and the power filter capacitors are too:


This is the smallest PCB, but possible to use after unbalanced/balanced converter and with separated dual power supply. But these solutions are required with another circuits.

6.3mm jack input soldered to all of these boards, the 1st and 2nd version of PCB have volume potentiometers too. This volume pot missing from the 3rd version, because the pot must be soldered before unbalanced/balanced converter. The TPA6120 circuit fitted to the bottom layer of all PCBs (this is the reason why this part mirrored), all another placed to to top.

See also:

• More unbalanced to balanced converters
• Simulation of unbalanced-balanced converter
• 2 input headphone amplifier
• 4 input headphone amplifier
  

PCB sales of this project Module name Size (mm) Area (cm2) PDF SCH PCB image Tested Price (US$) Des./3D Sim.1 Full2 Sim.1 Full2 Man3 TPA6120 headphone amp V:1 67x71 48 Yes Yes Yes - - No 8 16 Ask TPA6120 headphone amp V:2 55x71 39 Yes Yes Yes - - No 7 15 Ask TPA6120 headphone amp V:3 55x49 27 Yes Yes Yes - - No 6 13 Ask Power supply +/- 15V 212x57 69 - Yes Yes Yes - No 11 20 Ask How to order? Please read the rules carefully!

8 channel mainboard for modular audio mixer

I working on the PDF doc of the modular audio mixer project, but the PCB design is ready to try. This project is same as like the preamplifier project, but the preamplifiers connecting with 12 pins connectors to the mainboard of mixer circuit. First time, I made 4 channel mainboard only, but now I designed new PCB board for 8 channel audio mixer. This new mainboard is bigger, the width is 25cm. This is maximum size what I can finis with my homemade PCB creation method. In the future I will done new PCB design with 4 channel PCB with connectors to 8, 12 or 16 channel mixers.

The 8 channel mainboard:


The 8 connector on the board is for preamplifiers. The last 2 is one for master potentiometers, the another one is for 2 stereo line outputs.

PCB sales of this project Module name Size (mm) Area (cm2) PDF SCH PCB image Tested Price (US$) Des. Sim.1 Full2 Sim.1 Full2 Man3 Power filter module 17x25 4 Yes Yes Yes - - No 3 11 Ask 4 channel mixer mainboard 149x84 125 Yes Yes Yes - - No 19 28 Ask 8 channel mixer mainboard 240x80 192 Yes Yes Yes - - No 27 36 Ask I/O board 37x60 22 Yes Yes Yes - - No 5 12 Ask 2 Potmeter board 31x62 19 Yes Yes Yes - - No 5 12 Ask Microphone preamp module V2 67x103 69 Yes Yes Yes Yes - No 11 19 Ask Microphone preamp module V3 67x103 69 Yes Yes Yes - - No 11 19 Ask 1 dual opamp preamp module 61x103 63 Yes Yes Yes - - No 10 18 Ask 2 single opamp preamp module 64x103 66 Yes Yes Yes Yes - No 11 19 Ask HQ 2 single opamp module 67x103 69 Yes Yes Yes - - No 11 19 Ask jFET guitar preamp module 65x102 66 Yes Yes Yes - - No 11 19 Ask Switch module 18x34 6 Yes Yes Yes Yes - No 3 11 Ask Simple Power supply 121x57 69 Yes Yes Yes Yes - No 11 20 Ask Power supply with 48V phantom 168x89 150 Yes Yes Yes - - No 22 32 Ask How to order? Please read the rules carefully!

Preamplifiers

I finished two of my new projects with preamplifiers. One of them contains independent preamplifier schematics, another one is an audio mixer with modular system where the preamplifier-modules have to be connected to the mainboard.

The simplest preamplifier with very high gain contains only 1 dual opamp. The sound quality depend on the amplifier circuit. Recommended to try the PCB with TL072, NE5532, or LT1124. Maybe the maximum gain will lower with better quality circuits, but the noise and the sound quality will better with NExxxx or LTxxxx dual operational amplifiers. The board of this schematic contains potentiometers for gain and volume adjustment, and 6,3 mm stereo jack input. Stereo circuit, like all others except microphone preamp. The board contains all required potentiometers, jack input, and two switches for stereo/mono and true-bypass. The switches have to be soldered to the small PCB with 12 pins "L" connector. This switch-board have to be connected to the preamplifier's PCB if required, if not, just wire the required pins of the connector on the preamp-board to ignore these switches. This very simple schematic have very big gain, but for microphone is not enough, for guitars too much. The sound of circuit with TL072 dual opamp is good for mixed music, but not for instruments like guitar or guitar effect. This is the reason why I recommended for home music listening. The "P1" connector on board is option for LED output to indicate if something connected to the jack-input.

Schematic:


PCB:


The second version of above schematic with two single operational amplifier instead of one dual:

This PCB is bigger, but very good solution for single operational amplifiers. The ultra low noise single OpAmps are not cheaper, but easier to find than dual. LF356, OPAxxx, LT1028, LT1115 (instead of dual LT1124) are the alternatives for better solutions than the cheapest TL071. The board contains one "power filter" circuit to reduce the noise of power supply.

Finally I have version no.3 for the Opamp solutions. The stereo circuit contains 2 single Opamps instead of one dual like above, but two power filter circuits included, one-one for both operation amplifiers. This solution is better for ultra low noise circuits like LT1028 or LT11115.


The small circuit contains switches:

This circuit contains 3 switches, but SW1 and SW2 are same. Only the place of these parts are different. If required only one switch, use SW3 only.

The PCB of switches:

This really small board required for all preamplifiers, except Dual-jFET preamplifier. If the switches are not required, just wire the correct pins of the P1 connector on the amplifier's board.

The dual-jFET preamplifier.

This is my favorite schematic for guitars or guitar-effects. The circuit contains one dual jFET for stereo input. No noise, good dynamics, warm sound. No symmetrical power required, therefore the power filter is smaller and simpler. The gain and volume potentiometers, and the stereo jack input connected to the PCB. The maximum gain is lower than the Opamp preamplifier's, but enough for guitars, guitar effects, and keyboards, or another instruments, except microphones.

See the schematic:


PCB:

This PCB have no bypass and stereo/mono switches. Maybe later will be included. The originally planned part 2N3958 Dual jFET is not available now, but any low frequency dual N-channel jFET compatible with this solution. Here is the table to find another dual jFET.

The next schematic is a microphone preamplifier with very cheap and good quality operational amplifier INA217. This is the recommended application schematic of this integrated instrument preamplifier. The PCB contains two mono potentiometers for gain and volume, and one 6,3mm jack input for unbalanced inputs like guitars. The switch-module contains only one switch to select between unbalanced/balanced inputs. The unbalanced input is the 6,3 jack, the balanced have 3 pins connector, and external XLR input required. For 48V phantom power required an independent external switch to the case if you want to use condenser microphones.

Very cheap and good quality circuit. No noise, no feedback, very high gain, good dynamics. Compared to cheap Phonix mixer, this circuit was much better with Sennheiser and Behringer dynamic vocal microphones. The 48V input for phantom power included to the PCB but power supply required for this feature. Unbalanced to balanced converter circuit included after jack input to the board. This is cheap and simple converter, but with the recommendation of Jensen. Later will be changed this converter to better one. The PCB contains 3 integrated circuits, but one power filter circuit included.

The PCB:


The PCBs of these preamplifier circuits drilled with same positions on the bottom. With these holes possible to fix to the case of mixer or preamplifiers. Later, all of these preamplifiers modified to module of audio mixer. The inputs, outputs, and powers wired to the pins of connectors.

Finally here is the recommended +-15V power supply:


PCB:


The PDF about preamplifier projects.

As I posted, all preamplifiers converted to module of audio mixer with 12 pins connectors. With these connectors the preaplifiers can be connected to the mainboards of mixer. All of the power filter circuits connected as module to the new PCB of preamp-modules. The mixer-mainboard contains only mixing circuits and 2 stereo line outputs:


Gallery about preamplifiers:



Picasa gallery about modular mixer:

PCB sales of this project Module name Size (mm) Area (cm2) PDF SCH PCB image Tested Price (US$) Des. Sim.1 Full2 Sim.1 Full2 Man3 Microphone preamp V:2 130x105 137 Yes Yes Yes Yes - No 20 30 Ask 1 dual opamp preamp 95x104 99 Yes Yes Yes Yes - No 15 24 Ask 2 single opamp preamp 89x110 98 Yes Yes Yes Yes - No 15 24 Ask HQ 2 single opamp 105x112 118 Yes Yes Yes - - No 18 27 Ask jFET guitar preamp 70x108 76 Yes Yes Yes Yes - No 12 20 Ask Switch module 18x34 6 Yes Yes Yes Yes - No 3 11 Ask Simple Power supply 121x57 69 Yes Yes Yes Yes - No 11 20 Ask Power supply with 48V phantom 168x89 150 Yes Yes Yes - - No 22 32 Ask How to order? Please read the rules carefully!

UREI546 parametric EQ like URS VST EQ bundle plugin

See again the project of parametric UREI EQ project. I posted I really like URS VST audio plugins. I wanted UREI cloned circuit, to make EQ like URS VST. Unfortunately this is impossible, because the required values of parts not available, but I think the differences are not too important because the parameters are adjustable, and maybe with some tricks (parallel connections for example) possible to create something like this. Therefore I choose two parametric EQ plugin from URS bundle, and trying to simulate the values with UREI EQ circuits. I think logarithmic potentiometers required for this simulation.

Look at my new values for 4 bands equalizer:
Lower values in URS VST plugin: 30 Hz, 75 Hz, 800 Hz, 2,5 kHz


Upper values on VST plugin: 400 Hz, 1 kHz, 12,5 kHz, 20 kHz:


The another VST plugin have 5 bands. I have only 4 and 6 bands of EQ PCB, but within 6 bands UREI have to be insert 5 EQ modules only. With this example, from the 5 bands 3 bands are similar. These 3 bands can be adjusted to different settings. Logarithmic potentiometers required again. No standard values of potentiometers like in the tables, but something with similar values will be useful.

Low values: 31 Hz, 75 Hz 3x, 2 kHz.


Upper values: 400 Hz, 12,5 kHz 3x, 20 kHz.


The table with capacitors and adjustable resistors:


I hope it's useful tip.

See also:

• Simulation of UREI LP/HP filter module
• Modular, expandable parametric equalizer based on UREI546
• Simulation of 8 band parametric equalizer
• Simulation and software analysis of UREI 546
• 6 and 10 bands for UREI 546 parametric equalizer
• Math expression for UREI546 parametric EQ design
• Parametric EQ Project - Modular UREI545 and 546
• Modular equalizer with gyrator filter
• PDF manual of modular gyrator EQ
• PDF manual of UREI 545 and 546 EQ clone

6 and 10 bands for UREI 546 parametric equalizer

It is true, I cannot found really correct math expression to modify the band-frequency of UREI 546 EQ circuit, but I think I have a very useful Excel sheet. I think the not 100% exact expression is not a big problem, because the frequency parameter is adjustable. I have a plan UREI 546 with 6 bands, but I counted out values of potentiometers and capacitors for 10 bands. For 10 bands EQ I will design mainboard later.

The first table for original 4 bands, 6 bands, and 10 bands:


The result is the value of P1 stereo potentiometers for frequency adjustment, and the capacitor of filter C(a) = C(b). If somebody have better expression please send me a message.

Another table just for variations:

See also:

• Simulation of UREI LP/HP filter module
• Modular, expandable parametric equalizer based on UREI546
• Simulation of 8 band parametric equalizer
• Simulation and software analysis of UREI 546
• UREI546 parametric EQ like URS VST EQ bundle
• Math expression for UREI546 parametric EQ design
• Parametric EQ Project - Modular UREI545 and 546
• Modular equalizer with gyrator filter
• PDF manual of modular gyrator EQ
• PDF manual of UREI 545 and 546 EQ clone

PCB sales of this project Module name Size (mm) Area (cm2) PDF SCH PCB image Tested Price (US$) Des./3D Sim.1 Full2 Sim.1 Full2 Man3 Parametric EQ module 56x61 34 Yes Yes Yes - - - 7 14 Yes 2 Potmeter module 31x62 19 Yes Yes Yes - - - 5 12 Yes 3 Potmeter module 41x83 34 Yes Yes Yes - - - 6 14 Yes Power filter module V:1 51x22 11 Yes Yes Yes - - - 3 11 Yes UREI545 clone mainboard V:2 217x143 310 Yes Yes Yes - - - 43 53 Yes UREI545 clone mainboard V:1 217x198 430 Yes Yes Yes - - - 59 70 Yes 6 band UREI546 mainboard 173x143 247 Yes Yes Yes - - - 35 44 Yes Power supply 121x57 69 Yes Yes Yes - - - 11 20 Yes How to order? Please read the rules carefully!

Math expression for UREI546 parametric EQ design

While I found math expression easy for gyrator EQ project, I cannot found mathematical help to customize UREI 545 and 546 EQ circuits. For the EQ module I found nothing, the biggest problem that all parameters can be adjusted by resistors. Therefore I made expression with excel, to count out frequencies to make more than 4 band parametric EQ. The expression is not the best, but working well with several C(a) and C(b) values where C(a) = C(b). If somebody design more than 4 channel UREI 546 EQ, the potentiometes of Q adjustment must be changed too. For example, if the Q adjustment is 10kOhm for 4 bands, for 8 bands must be half (5 or 4,7 kOhm). I hope it's enough and useful for customized parametric EQ design.

Look at the expressions for UREI 546 band-frequencies:

C(a) = C(b), the adjustable resistors for frequency 55kOhm stereo, the permanent resistors are 4,7 kOhm like in the schematic. You have to change the value of C(a) and C(b) capacitors. In the last expression you can get the fpot, which is the frequency depend on the value of potentiometer. The variable Rpot can be set between 0 and 55000 like in the "real life". The result getting in Hz.

The module contains EQ circuit, most be placed to the mainboard PCB:

See also:

• Simulation of UREI LP/HP filter module
• Modular, expandable parametric equalizer based on UREI546
• Simulation of 8 band parametric equalizer
• Simulation and software analysis of UREI 546
• UREI546 parametric EQ like URS VST EQ bundle
• 6 and 10 bands for UREI 546 parametric equalizer
• Parametric EQ Project - Modular UREI545 and 546
• Modular equalizer with gyrator filter
• PDF manual of modular gyrator EQ
• PDF manual of UREI 545 and 546 EQ clone