A recent question to inSync that could open a can of worms:

"When connecting the outputs of a Mackie board to the inputs of a Roland VS-1680 should I worry about matching impedance? I think the Roland accepts a 30-kohm impedance (not sure) but, at any rate, it is different from the Mackie board's sub outs. Is there any no-no rule in this respect?"

Interfacing different types of equipment has historically been fraught with problems, but these days it is deceptively simple. So simple, in fact, that most people aren't even aware of the concept of matching impedances. Impdance matching comes from the (old) days when equipment was all tube based and transformers were often used on inputs and outputs. Back then more power (watts) were transferred between units in a given setup. The voltage and current (amperage) were higher and the impedance was usually fairly low. This is just the way the equipment was designed (the why of this is beyond the scope of inSync). A certain amount of power (not just voltage) was required to drive each device in an audio chain. In order to have the optimal power transfer between two pieces of gear it was important to match their input and output impedances, which was often done with transformers (known as Impedance Matching Transformers). Otherwise there could be significant level losses or other anomalies between them. This was even a more convoluted problem if the cables between the devices were long, because then the impedance of the cable had to be factored into the equation. Plus, since impedance is by definition not the same at all frequencies, anomalies between two devices could sometimes show up as non-linear frequency response between them.

Nowadays with our modern solid state and integrated circuits, input impdances are very, very high. Consequently there is very little current flow between devices (meaning there is very little actual power transfer). As long as the input impedance of the receiving device is relatively high, impedance matching need not be a concern. This eliminates the primary need for transformers on most input and output stages (further reducing the cost of building the equipment). Level matching, on the other hand, is still a major concern, but that's another entire topic.