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PCB for Switchable balanced audio summing network

This project was awarded to ExpressElectro for €449 EUR.

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Project Description

* Design a PCB for Switchable balanced audio summing network controllable by SPI in Altium containing good OPAMP circuitry.
* Low THD
* Low Noisefloor
* use of ATMEL Microcontroller microcode
* Show simulation measurements (i.e. SPICE)
* Provide a REV0 board for evaluation and acutal measurements (i.e. matlab)

To give an idea, i have drawn something up that as an example contains 3x25 pins connectors (pinning accoording to TASCAM format). In this example there are two NJW1112 chips that would would work in unison because the audio signals are balanced into hot / cold / ground.

The NJW contains both opamp and switching from 8 stereo inputs to 4 stereo outputs.
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This simple schematic basically shows a bus aux send architecture as often found in Audio mixers. The return send is left out, but returns should ofcourse be summed as wel to a Master output.

It is difficult to get the NJW in smaller quantities and possibly the OPA series from Texas Instruments might be a better fit in regards to opamp quality. It is important to have a low noise floor. We have seen -136db noise floor chips and low THD. Asside from opamp, I have not found an audio switch that has the same quality to that of the NJW1112, but maybe you can find one.

The opamp level would be determined by the number of input channels switched to the output that needs makeup gain (each input will have get [url removed, login to view] resistance added on the source signal path). I am not proficient in opamps but I guess the voltage will have to be regulated based on the number of channels switched to the specific opamps. (more channels needs more makeup gain). i suppose you can simulate that in SPICE/Matlab to see if everything is working.

One output bus will need 4 opamps as the bus is balanced audio stereo.

Here is a typical diagram example of a typical summing network with opamp as well as its calculations.

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If you have any more questions let me know

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