Box of MOSFET

Rock Box!

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.

Blaxendall/James Tone Stack

I also decided to change up the tone control for a little more variety and to make this into something a little different. I had heard a few good words about the James Tone control a.k.a. Baxandall tone stack. This is a two knob type with a Bass and Treble control. A good description of this tone stack can be found here. Here’s a shorter less technical description.

Here’s an image of the James/Blaxandall tone stack. RT and RB are the Treble and Bass control. I had run into this tone control before at Freestompboxes.org in a project by forum member Mictester. It was included as part of a project called Bigmuff Plus. This was sort of a BMP on steroids. My drawing includes values for the Orange Amp tone controls and the values used in the Big Muff Plus.

Note that the Blaxandall uses the Audio taper pots for the Bass and Treble controls. These are not required but, without them the usable adjust range is bunched up at one end of the pot rotation.

Initial Schematic

I drew everything in my notebook. At this point I had the following (note this omits the extra low pass filter and volume pot):

I built everything on a breadboard to test out the idea. I tested each stage as I built it. One thing that impressed was how bad the distorted sound was without a tone stack. I shouldn’t really say “bad” as the sound wasn’t terrible. Heck, it’s distortion right, so it might sound good to somebody. What is “bad” when it comes to distortion? The sound did lack the refinement and had some extra high end hash that wasn’t helping in my opinion. Through headphone the sound was unbearable. The headphone, I’m guessing, were reproducing more high end then would come out of a guitar speaker. After adding the tone control the sound was much smoother and had a lot to recommend it.

Later I added the extra low pass filter following the tone control. This really moved the sound into the Marshall territory. This kind of extra fixed filter stage added to the end made a noticeable difference in the sound. Seems like it might be a good addition to a lot of boxes.

Building

Originally I had planned on using the Orange tone stack. Turns out I could only find a single A1M pot. Looking over Mictester’s take on the Blaxandall, he used different resistor and cap values alone with a A470K pot for the treble control. I did happen to have an A500K pot (with detents, it clicks at each of sixteen steps). So I Decided to go with those values. Some times you have to just work with what’s available.

I drilled a box fit all the parts and wired up the standard box connections. I drew up a perf board layout which placed all of the transistors in a row. I noticed at this point that the BS170 is DGS while the 2n7000 (another MOSFET) is SGD, seems like it would possible to swap these.

First test

I’m not the greatest at making flowery descriptions, but here goes. The sound is tight and crunchy. You can dial in a surprising amount of low end with the bass control. The bass is tight and doesn’t get muddy. It’s got a sound you would associate with Marshall amps. I’d say it does AC/DC to Van Halen. It doesn’t quite get to metal.

The added low pass along with the higher impedance tone stack cut the output noticeably. The volume needs to about 3 o’clock for unity gain. Might be good to add another transistor on the end to boost the volume. Then again maybe tinkering with the volume pot might be enough.