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Tiny 4 Channel Powered Stereo Mixer
A Tiny form-factor 4-channel stereo mixer for use with TV/PC/Turntable/Amplifer
Requirements
- 4 stereo inputs
- 1 stereo output
- Inputs will all use stereo RCA/Phono connectors
- volume controls for each of the 4 input channels
- volume controls should be mounted in the front of the case
- External power input shall be 9V DC
- Internal power supply shall be +/-9V DC
- Input signals shall be buffered
- Volume controls for each channel shall be independant and not subtract gain from the other channels.
- Cross-talk between channels should be minimised.
Top level Design
The opamps shall use TL072
Using opamps to buffer the input signals and mix the channels together will ensure that the volume of each channel can be mixed independently from the others.
The design will implement an inverting opamp “virtual ground” topology. The virtual ground refers to the inputs of the mixing opamp: the non-inverting input will be referenced to ground, which means the voltage at both opamp inputs will be referenced to ground because the potential difference between inputs will always try to be zero. The absence of voltage at these inputs will prevent cross-talk between the channels. There is a far superior explanation of this concept at sound-au.com
Power Stage Design
The old PSU design for Rev A/B can be found in OLDPSU.md.
The negative voltage rail is generated from the positive rail using a TPS54202 SMPS IC. This is normally a buck regulator. However, it can be configured to generate a negative voltage when pin 2 is clamped to ground, forcing pin 1 to a negative voltage.
Further details can be found in the SLVA933 app note
Soft Start
Unlike the LM2611, the TPS54202 has an internal soft start circuit. This can be seen in the Function Block Diagram of the TPS54202 datasheet.
Output Voltage
Output voltage is selected using the voltage divider between pins 2 and 1, creating feedback into pin 4. This is very similar to the normal operation of this IC.
Current Draw and Load Regulation
Output ripple is shown in the SLVA933 app note as 5mV at no load and 20mV at 0.8A (see pages 21-22).
For my own test I created a small board. The original layout example with the new configuration violated the layout guiidelines from the TPS54202 datasheet and proved to be noisy (240mV @ 0.6A). A second revision was made with a better layout that observed the guideline and was a significant improvement (80mv @ 0.6A).
Load regulation was around 5%, dropping from 12V to 11.4V @ 0.6A.
These results were acceptable so this PSU design was used.
Schematics
Layout
I used a 4 layer board so I could have a layer per voltage rail. This makes routing the traces significantly easier. I kept the board dimensions below 10cm to minimize manufacture costs.
Enclosure
I abandoned the 3D printed enclosure idea and made the PCB to fit this Hammond RM2005MTBK clam shell enclosure.