DIY Project #03: Buffered Multiples

First of all, definitely not a silly question!!! A very good one even. Input & output impedance took me a while to 'get it', but it is very important in audio chains (and any analog electronics chain actually).

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Well, actually there is not yet any attenuation if you just put a resistor. For attenuation, you need a resistor divider that goes to ground. If you would input for example 4V to your MCP, you would measure exactly 4V at the output of your 220 ohm resistor. 

However, when you patch this buffer output to another module, that other module will probably have a path to ground, so at that moment your signal does get attenuated. But the thing is, modules are build so that they have a low output impedance (= resistance, = 220 ohm in this case) and a high input impedance (lets say at least 100k ohm or something). So this means that the path from your input patch point to ground has high resistance. So when you connect the output of one module (eg this one, 220 ohm) to the input of another one (lets say it has 100k ohm), you get a voltage divider of 220 ohm & 100k ohm. Which means you will let your voltage drop with a factor of 220/(100k + 220) = 0.22% which is basically negligible. So as long as your output impedance is low and your input impedance is high, this 220 ohm resistor will barely atenuate your signal.

For example for this buffer module, your input goes straight into the MCP602 + input. Opamp input generaly have a huuuuuuuuuge resistance at this inputs, meaning they 'sense' the voltage, but barely use any current. The input resistance/impedance is in the range of Giga to Terra ohms (thats 1000000000000 ohms!!!!!!).

But I agree, electronics, its a very very strange discipline, and i'm only still scratching the surface haha.

All voltages in AE modular are 0-5V, so there are no negatives Involved. The oscillators oscillate around 2.5V instead of around 0V in Eurorack. So as long as you work within AE Modular, nothing will get clipped. Of course, if you want to get external signals in (eg from Eurorack) than clipping can happen, and then it's best to have a circuit for attenuation and offsetting to get everything to 0-5V.

Another way to see this: you can see the system as -2.5V - 2.5V instead of 0-5V. It's exactly the same, you just have to 'shift your perspective'. So you can see the op-amps as working from -2.5V to 2.5V.

nxn

Aha, I see now! Indeed, it should be possible with opamps and some clever circuit design. If you plan on importing voltages that might be 'unpredictable', its important to add some more protection. I think it would be best to use diodes to GND & +5V to make sure any voltage under -0.6V and above 5.6V gets clamped by the diodes. Otherwise the MCP6002/4 could be damaged by the voltages above & below its supply voltage.

You could tinker a circuit depending on the input. An input that can range from -15V to +15V needs different handling than one that will be eg 0-10V (like an envelope for example). An option can be to calculate your input attenuator depending on the MAXIMUM input range you expect (like in the case of -15V to +15V = 30V range, you can use a 30V->5V attenuator, so basically 5/30 = 1/6 attenuation). Then, after the attenuation and offsetting, you might want to add a gain stage with a setable gain with a pot (can also be done with just one opamp and some resistors). That way you have some control over the voltage without needing to make like 10 different input circuits. 

Another option would be to use some switches to switch from the buffer circuit, to a 0-10V input circuit, to a +-15V input circuit. 

Very good video on opamp summing, which you can use to introduce the offset voltage: