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I'm sorry to disagree with F1EKU's answer, but it is only partially correct and therefore misleading in general. As I posted earlier, what you are implementing this way is effectively a noise impedance transformer. In the regime where you want to transform downward, paralleling can be effective (noise current goes up, so noise impedance goes down). But if you want to transform upward, paralleling only makes things worse; what you want to do in that case is put things in series. The specific example of moving coil cartridges he cited is not generalizable. Those have extremely low impedances, so downward transformation is the right strategy in that instance. But that's a specific case. No universal truth can be inferred from it.

None of this need be based on guesswork and lore. Just model each element properly, with a noise voltage source in series with the input, and a noise current source in shunt. To simplify life, assume that these are uncorrelated. When you parallel the inputs, the noise currents add (in rss fashion). When you put the inputs in series, the noise voltages add (again, in rss fashion). The ratio of noise voltage to noise current is the impedance that yields minimum noise. So, merely series- or parallel-connect the amplifying elements until you get as close to the noise impedance of the source as you can.

That's a much-simplified presentation, but it captures the essentials needed at this level of discussion.

-- Cheers
Tom

-- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070