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Ask, July 2011

July 1, 2011
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The two combo input connectors on MOTU''s Audio Express can accommodate mics or instruments.

The two combo input connectors on MOTU''s Audio Express can accommodate mics or instruments.

I see interfaces with “high-impedance instrument inputs,” and they''re supposed to give better sound. But when I patch in my pedalboard output, I really don''t hear any difference. My P-bass sounds great, though. So when should you use, or not use, the instrument input? Can you fry it by putting in too strong a signal?
Spence Feder, Ft. Worth, Texas
via e-mail

Good question—most companies just assume you know how to use instrument inputs. They came about because standard guitars and basses with passive pickups generate too much output for mic inputs, but often not enough for line level inputs; more importantly there''s an impedance-matching issue, as guitar pickups have a high output impedance compared to line-level devices. Furthermore, pickup impedance increases with frequency, so plugging in to a low-impedance input “dulls” the sound, as it loads down high frequencies more than low frequencies.

Your pedalboard already has a low-impedance output (as would any individual effect) that''s probably line-level, or close to it. So, as it can produce sufficient level and doesn''t have impedance-matching problems, you can use it with a line input. It should also work with the instrument input, although you may not need as much gain as for standard pickups. Your P-bass, on the other hand, has passive pickups and is exactly the type of signal an instrument input wants to see. You would not “fry” an instrument input when using normal instrument or line levels.

Guitars and basses with active pickups, or onboard preamps, generally don''t need instrument inputs although they may produce relatively low-level signals in order to be more like a traditional guitar. In this case, there''s no harm in going through an instrument input, and it may actually be a more suitable match because extra gain is available compared to line-level inputs.
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