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I've been interested in electronics for couple of months now so my knowledge is still very limited. I like microcontrollers so I'm trying to include them in my projects.

I'm trying to build a similar device as Millenium HP4 but with 8 channels. Idea is to have 8 inputs and to map them freely between 8 outputs (you can combine multiple inputs). There are a lot of great articles about splitting, mixing, combining etc. audio signals.

I am stuggling to find a component which can assign one signal input to 0..8 (or 0..16 if available, for stereo sound) outputs. It can be controlled with i2c or simple address inputs (I can control with for example ATMega328p and ATTiny85 slaves).

I'm looking for something like MC74HCT4851A demultiplexer but which can output to multiple channels simultaneously. Seems like I can't find one.. Any recommendations or other ways I can achive what I want?

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I am stuggling to find a component which can assign one signal input to 0..8 (or 0..16 if available, for stereo sound) outputs.

Because that doesn't typically exist – you'd use a system of analog switches to achieve that. So, basically, for every single connection you'd want, you'd have to have a switch that's either open or closed.

In essence, each of your 16 outputs would have a summing amplifier (to combine the signals if you connect multiple to one output).

Each of these summing amplifier would be designed by you to have 16 inputs. Each of these inputs would be connected to one of your signal inputs by a switch that's either open or closed.

That makes 16² = 256 switches.

You can realize something like that using a gigantic matrix of 256 relays; there's also analog switch and multiplexer ICs. These would use a lot less power. Your MC74HCT4851A is 16 of these switches – you could use it right after the input resistors of a 16-channel summing amplifier. However, that will require the signal being pushed through to be buffered well, so your overall system realistically would also include input amplifiers (buffers) for each individual input.

An alternative would be having 16 ADCs (e.g. sound cards), and do the summation in software (e.g. running on a PC, or on a relatively potent microcontroller) or digital hardware, e.g. FPGAs, outputting signals through 16 DACs.

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You're thinking about this backwards. What you're describing is a full-matrix mixer, in which each of the outputs can have a mix of any of the inputs. In other words, treat each output independently as an N-channel mixer, where N is the number of inputs. Then build as many mixers as you have outputs.

You might need buffer amplifiers (unity gain, low output impedance) on the input channels in order to have adequate drive for all of those mixer inputs.

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