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Boost Pedal Distortion

Comparing Three Types of Boosters

I configured my test rig so that each pedal would give about a -20dB output level. With the Mini-Booster shown above, the output volume is maxed and the 100 Hz test signal was reduced so that the proper level registered on the test graph. As you can see, this mini-booster pedal is providing about 34dB of gain (-54db into the pedal and -20dB out). You will notice that the 2nd is the only even harmonic that registers above the noise floor, but there are numerous odd harmonics (3, 5, 7, 9 and more). The noise floor rises steadily until it hits the 20kHz filter in the M-Audio Quattro sound interface. While this is decent performance (-72dB is about the noise floor of a 2" pro multi-track machine), it is easily beat by the other boosters.

The mosfet boost will give better performance, though it is misleading since you can turn back the gain on this boost pedal to unity. The noise floor is significantly below that of the mini-booster with a modest 2nd harmonic and small 3rd and 5ths. The sound of this booster pedal will be clean and have little added noise.

The last booster pedal in this test is a direct box made with the AMZ multi-purpose opamp pc board. This box has an NE5532 opamp chip and provided a clean, low noise signal. The 2nd harmonic was missing and only odd harmonics are seen, but those are around 90 dB below the signal so they are insignificant. This is a very clean and high definition sound.

To summarize, the mini-booster will give more harmonics for a given input signal, which can add some grit and character. The mosfet boost is clean with low noise and little distortion, but for the purest sound, the opamp based pedal is the way to go.

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