Does Impedance Matter?

February 14th, 2022
experiments, mandolin, music, tech
Traditionally, you plug an electric instrument into an amp. Amps are big and heavy, however, so if you are already planning to play with a PA and monitors you might like to connect your instrument directly to the PA. There are two main downsides to this:
  • In many styles of music you use an amp that isn't faithfully reproducing the input, and these modifications are an important part of your sound.

  • Passive pickups are designed to work with high input impedance (~1MΩ), but a PA's line inputs are much lower (~10kΩ). When driving a low impedance input you don't get accurate frequency response, especially at the high end.

I'm primarily interested in the second downside, the loss of high frequencies, and I'm curious how much of a thing it is. What happens if I do just go ahead and connect my mandolin to the soundboard? What if I use a passive DI? Let's find out!

I tested with a Gold Tone GME-4, tone knob at 10 (no low pass). I tried the following inputs:

  • Line: the line input of a Soundcraft EPM-8 analog mixer. Rated 10kΩ.

  • Line II: the line input of a Mackie SRM150. Rated 10kΩ.

  • DI: a Radial ProDI passive direct box. Rated 140kΩ.

  • Pedal: a Boss BD2 in bypass mode. Rated 1MΩ.

  • Pedal II: an MXR M222 in bypass mode. Unspecified input impedance.

  • Instrument: the instrument input of a Mackie SRM150. Rated 1MΩ.

Here's how they sounded with a range of things I do on the mandolin:

Chords
(Line, mp3)

(Line II, mp3)

(DI, mp3)

(Pedal, mp3)

(Pedal II, mp3)

(Instrument, mp3)

High riff
(Line, mp3)

(Line II, mp3)

(DI, mp3)

(Pedal, mp3)

(Pedal II, mp3)

(Instrument, mp3)

Low riff
(Line, mp3)

(Line II, mp3)

(DI, mp3)

(Pedal, mp3)

(Pedal II, mp3)

(Instrument, mp3)

High melody
(Line, mp3)

(Line II, mp3)

(DI, mp3)

(Pedal, mp3)

(Pedal II, mp3)

(Instrument, mp3)

Medium melody
(Line, mp3)

(Line II, mp3)

(DI, mp3)

(Pedal, mp3)

(Pedal II, mp3)

(Instrument, mp3)

Low melody
(Line, mp3)

(Line II, mp3)

(DI, mp3)

(Pedal, mp3)

(Pedal II, mp3)

(Instrument, mp3)

Overall, I'd rate the SRM 150 on instrument mode and MXR M222 on bypass > Radial ProDI passive DI and Boss BD2 pedal >> mixer line input. That the ProDI and BD2 sound similar is a surprisingly good performance from the ProDI and surprisingly bad one for the BD2. While I do think in the general rule of using an active DI for a passive source holds up, it does seem to depend some on the specific gear involved.

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