I was talking to a friend about distortion boxes and the Box of Rock came up. Which got me thinking, I’d never heard one before, and Z Vex always makes good stuff. I found a schematic in the usual place. It looked like a pretty easy build.
The Box is basically two pedals in series, a distortion followed by a booster. The Box has two foot switches, The first switch engages the distortion and the second engages the booster. The controls for the distortion are Gain, Tone and Volume. The booster adds a fourth knob, Gain/Boost.
The distortion section is made of three BS170 MOSFet stages. The first stage is a SHO followed by a Marshall style high pass filter made of a 470p cap and a 470K resistor in parallel. Then come two more BS170s configured gains of approximately 51 and 15.
Next is a BMP style tone stack followed by an extra low pass filter. The low pass filter is exactly the same as used in the BSIAB II. The BSIAB II also uses the same Marshall style, 470p and 470K, high pass filter between the first two stages.
The B of R includes an SHO booster on the output. I had one of these built already so i decided not to build the stock B of R and instead build just the distortion section. I figure I can place my SHO or any other booster after it for different sounds.
Here’s a simple idea that you will work on almost every transistor
circuit out there. We’ll apply it to the MosFET boost here.
Remember the gain of the MosFET boost was determined by the Drain and
Source resistors. For the example circuit, as the Source resistor
decreases in value the gain increases.
A capacitor acts as a resistor when an AC signal is applied to it. The
resistance is dependent on the value of the cap and the frequency.
Lower frequencies see more resistance when the cap is smaller. This is
why you use a smaller cap to limit the amount of bass entering a
circuit. Or, use a larger cap to get more bass.
This idea can also be applied to the gain control to make a range
booster. Take a look at the attached file. Note that without C3 the
gain is 1 (assuming R3 and POT12 are equal). Add C3 to the wiper of
POT1 and you get a variable amount of gain to a frequency set by the
value of C3. Lower frequencies get attenuated.
When the wiper is at max resistance, you’re getting at least 5K before
the pot, so gain is 1. When the wiper is at minimum value the cap does
the variable resistance thing dependent on the frequency. Smaller cap
more highs get through. Higher frequencies see less resistance and
more gain, while lower frequencies see more resistance and less gain.
You can use this as an alternative to the SHO gain control by using a
large cap to boost a full range. Oops wait looks like Jack Orman did
it first. Take a look at his MosFET booster.
Here’s a typical MosFET boost. The gain is roughly determined by the ratio of Drain and Source resistors. Divide the Drain resistor (Rd) by the Source resistor (Rs) to find the gain. For example if Rd = 5k and Rs = 5k the gain is 1.
Use a Pot for Rs to set variable gain. Imagine POT1 as a 5k pot. Notice how the gain changes exponentially.
- 5k/5k = 1
- 5k/4k = 1.25
- 5k/3k = 1.67
- 5k/2k = 2.5
- 5k/1k = 5
- 5k/.5k = 10