amplifiers-----confused

[geoffery francis james]

what is the difference between 
a----- a summing amp
b----- a mixing     amp
c----- an adding   amp
i've asked this of many a tech head and we seem to get the same sought of
confusion when asking what is the difference between a"partial and a
harmonic" and what is the difference between "pitch and frequency".
i am confident the answer lies with synth DIY--------prove me wrong!!
regards
geoff james