Can I plug a contact mic into a guitar pedal?

Introduction

Over the years, several people have asked me if they can plug a Metal Marshmallow contact microphone directly into a guitar pedal. The issue is that Metal Marshmallow outputs a line-level signal, whereas guitar pedals are designed to accept input from a guitar, which is an instrument-level signal. In general, line-level is higher than instrument-level, so will this overload the pedal?

Microphone Comparison

To start with, the Metal Marshmallow Pro contact mic cannot be plugged directly into a guitar pedal for a different reason. Metal Marshmallow Pro requires +48V Phantom Power in order to operate, and it is not possible to supply this power backwards through guitar pedals. So even with an adapter that converts the XLR output connector of Metal Marshmallow Pro to the 1/4 inch jack input of the guitar pedal, this would not work because the mic would not be powered.

On the other hand, Metal Marshmallow II has an internal battery, so it does not require Phantom Power. Additionally, Metal Marshmallow II has a 1/4-inch output, so it is physically possible to plug it directly into a guitar pedal using a standard instrument cable and no adapter. But will this this work, or will it overload the pedal?

Reamp and DI boxes

Often when I am asked about this, I am also asked whether either a DI box or a reamp box should be placed between the mic and the pedal. The answer regarding the DI box is no. A DI box converts an instrument-level signal into a line-level signal, which is the opposite of what would be necessary in this case, so a DI box should not be used. By contrast, a reamp box does the opposite, it converts a line-level signal into an instrument-level signal. If any conversion is necessary, this would be the requisite one. But is it necessary, or can the mic be plugged directly into the guitar pedal with no intervening reamp box?

Experiment

If a pedal cannot handle the higher line-level input, you would expect it to distort the sound whenever its input is above some relatively low threshold. So in order to test this, one would need to send a relatively high level signal into the pedal and check whether the sound distorts.

I did exactly this expiriment. I started with the setup shown in Figure 1.

In this setup, I stuck a Metal Marshmallow II (Black—Higher Gain) to the surface of a cajon using Poster Putty. I plugged that directly into a USB recording interface so that I could record the sound. I also connected a multimeter directly to the output of the mic so I could monitor its output level. Finally, I placed a wine glass on the surface of the cajon near the mic. I partially filled the glass with water and ran my finger around the edge to produce sound. The closer the wine glass was to the mic, the higher the mic's output was. So I moved the wine glass around until rubbing its rim caused the mic to output around 1V RMS as measured by the multimeter. This is around maximum for a line-level signal. I recorded the output of the mic and the result is in Figure 2 below.

Note that this initial setup does not include a guitar pedal. This serves as a sanity check that shows that no other part of the setup is causing the sound to become distorted. This way, once we add a pedal into the setup, we will know that any distortion is caused by the pedal and nothing else. So to test this I inserted a guitar pedal into the setup as shown in Figure 3.

This setup is also shown in the photograph at the top of this post. It is the same as before, but I plugged the mic into the pedal and the pedal into the recording interface. The pedal I used was an old-school DOD DFX9 digital delay pedal, with the 'level' knob turned all the way up. The user manual does not specify the maximum input level. So I repated the experiment. The resulting audio is in Figure 4.

In this audio, I do not observe any distortion. The peaks are not clipped nor do I hear any harmonic distortion. This demonstrates that this particular pedal accepts line-level input from Metal Marshmallow II just fine, without a reamp box.

My Apologies

Note that the mic used in the experiment is partially disassembled (and has the wrong color heat shrink). Due to this, the above recordings are a bit hummy. Normally Metal Marshmallow II is hum-free.

Inst vs Line Input

I did the above experiments using the instrument-level inputs on my recording interface. I also tried with the line-level inputs. It honestly dosen't make that much difference. The sound going into my laptop is a few decibels higher when using the instrument inputs, but other than that I don't observe any difference.

Mic Selection

Does it matter which model / gain-variant Metal Marshmallow II contact mic I use for this? Simply put, no, the pedal does not care which one you use. They all have the same maximum output level, and the same output impedance. The higher gain models reach that level more quickly in response to quieter sounds. It is therefore recommended to select the mic that is best suited to the type of sound you plan on recording. However, no model is more or less compatible with guitar pedals than any other.

Conclusion

This experiment has demonstrated that at least some guitar pedals can accept input directly from Metal Marshmallow II contact mic with no intervening reamp box or other hardware. A quick internet search makes it seem like many pedals are designed to accept input in this range. On the other hand, there is no gaurentee that this will work for all pedals. At the same time, I don't think you are likely to damage a pedal by trying. So my receommendation would be to try it with no reamp box. If it works, you are good to go. Otherwise, you are on your own.