Audio mixer build

Audio mixer detailed construction guide

This page details how to build the Audio mixer MK2 PCB.

Equipment list
PCB (order here).
Audio connectors.
Complete kit of components (detailed here).
Soldering iron with fine tip and solder.
Fine nose pliers.
Flush cutters.
Some 7/0.2 size wire.
Optional dual gang, 10K LOG potentiometers
Schematic diagram printed out
PCB layout for reference
 

Build options
The design has a couple of selectable build options, it is easiest to decide, before commencing the build, which options you wish to configure. The Circuit description, defines the various options
The available options are:
1) Fixed or adjustable input volumes
2) Optional 'Better Paula' circuit.
3) Utilization of on-board 8V regulator to cut out ATX PSU noise or use 6-12V input supply.
4) Either a DC power jack or wires for DC power input


Detailed construction steps

Step 1
If you have chosen to implement the fixed input option, apply a blob of solder across the two pads shown in red on the underside of the PCB.

AM_Build_S_1-medium

Step 2
If you have chosen to use the fixed input option, short out the two solder pads highlighted in red.

It is easiest to fit the 33K (R19 and R23) and 100R (R5 and R6) resistors, next.
AM_Build_S_2-medium

Step 3
Next fit the numerous 10K resistors as shown here.

AM_Build_S_3-medium

Step 4
Before soldering the components, bend the leads and check the correct parts are in each location as shown here.
AM_Build_S_4-medium

Step 5
Fit the diode D1.
AM_Build_S_5-medium

Step 6
Fit the decoupling capacitors C6, C7 and C11.
AM_Build_S_6-medium

Step 7 (optional)
If you are not using the 8V regulator, fit a wire link (using a snipped component leg) across R20.
AM_Build_S_7-medium

Step 8
If preferred, fit 3 x 8 pin DIL sockets
Fit R24 and R25 if using the optional linear regulator.
Fit the gain resistors R9 and R10, nominal value is 10K but refer to circuit description for details.

AM_Build_S_8-medium

Step 9
Fit JP1 and JP2.
AM_Build_S_9-medium

Step 10 (optional)

If required, fit the LM317LZ linear regulator, IC4.
AM_Build_S_10-medium

Step 11
Fit the 1uF electrolytic capacitors C5 and C15.
AM_Build_S_11-medium

Step 12
Fit the 10uF electrolytic capacitor C12.
AM_Build_S_12-medium

Step 13
Fit the 2.2uF input AC coupling capacitors C1, C8, C9 and C10
AM_Build_S_13-medium

Step 14
Fit the output 2.2uF AC coupling capacitors, C3 and C4
AM_Build_S_14-medium

Step 15 (optional)
Fit the audio connectors to the PCB
Note: The pins do not align properly on SK2, carefully move them into alignment with the PCB holes

AM_Build_S_15-medium

Step 16
This shows both audio connectors fitted
AM_Build_S_16-medium

Step 17
Measure the power supply resistance using the left most pad of R20 and the 0V pad as shown here:
AM_Build_S_17-mediumExpected value is 3 K ohms

Step 18 (Optional)

Wire up the dual gang potentiometers for the two inputs according the the following picture and table
AM_potentiometer

PCB pad #

Potentiometer pin #

Signal

1

3

Ami_L_Audio_In

2

6

Ami_R_Audio_In

3

2

Ami_L_Audio_Out

4

5

Ami_R_Audio_Out

5

1

Ground

6

4

Ground

7

3

Ext_L_Audio_In

8

6

Ext_R_Audio_In

9

2

Ext_L_Audio_Out

10

5

Ext_R_Audio_Out

11

1

Ground

12

4

Ground

Note: a signal with a _in suffix is the input from the audio connector and connects to the 'top' of the potentiometer. Likewise
a signal with a _out suffix is the output from the potentiometer and should connect to the center pin.
The ground connection always goes to the left most pins, when viewed from the front, this is to allow clockwise rotation to increase
the volume level.
Step 19
Apply power to the PCB
Measure across pins 4 (0V) and 8 (+V of each socket. If using the linear regulator, you should measure 8.0V +/- 0.1V, if using a 6-12V supply,
 you should measure the input voltage.
If the voltage is incorrect, please re-check your soldering and if applicable, the orientation of the linear regulator.

Step 20

Insert the LM358 op-amps and using a multimeter, in series with the power lead, measure the supply current, it should be less than 10mA.