The Grump has five, count 'em, five knobs:
- Input Turning this knob clockwise will attenuate the input signal, which can be useful for basses with active pickups or if the signal strength has already been increased by, say, a boost or a compressor.
- Gain Sets the amount of gain, which in turn sets the amount of fuzziness, somewhere between "old combo amp that's been sitting in the garage for fifteen years " and "grotesque."
- Bright Sets the high-frequency response, independently from Tone. With the Gain knob turned down, the effect of this knob will be less noticeable.
- Tone Clockwise: more highs, less lows. Counterclockwise: more lows, less highs.
- Vol Controls, you guessed it, output volume. The Grump is shipped with this knob turned all the way down, because the output can be rather loud compared to the original signal.
The clipping is symmetrical, which means that the positive and negative duty cycles of the square wave are roughly equal. Traditional fuzz is asymmetrical, but I call this "fuzz" because it has a cloudy, squishy quality rather than the sharp, focused tone of traditional distortion.
The Grump doesn't use op amps - the clipping and distortion come from medium-gain JFETs and a pair of diodes.
If you're curious, the little blue trimpot on the circuit board biases the transistor right next to it. Before a Grump unit ships, its little blue trimpot is set so that the voltage at the two measuring points is about 5.1 Volts. In most cases, that's the "sweet spot" for the best tonal response and smoothest clipping. You can change the value of this trimpot if you want to (by turning the white center part with a screwdriver) but the sound gets pretty thin and at extreme values. To set it back to "factory default", turn the pot until you measure somewhere around 5.1 Volts at the measuring points. Use your ears, too - you might like a slightly different bias.
The Grump, like everything from TH Audio, features:
- True bypass switching
- Metal film resistors, 1% tolerance
- Switchcraft or Neutrik jacks (depending on unit and availability)
- High-quality powdercoating over an aluminum enclosure
- Heat-treated silkscreened graphics