The Solstice Amp Distortion is a recreation of the Marshall Shredmaster, first released in 1991 alongside the Bluesbreaker and the Guv’nor, which was previously released in 1988 as the Drivemaster.
Despite its name, the Shredmaster is not an ultra-gain metal monster like the Boss Metal Zone. Unlike the Guv’nor, it does have the ability to scoop the mids, so you can get some sounds out of it that approach the metal territory—but for the most part, the name is misleading.
The Shredmaster has a lot in common with the Guv’nor / Drivemaster, and in fact the first half of both circuits are nearly identical. However, the Shredmaster has an additional gain recovery stage, and even though they have three tone knobs, the tone stack is very different under the hood.
The Solstice is faithful to the original, but with one added modification: a switch that lets you go between different clipping diodes. The original has a pretty low clipping threshold with just one 1N914 diode in each direction, meaning its clipping character is more gainy and compressed. But some different sounds can be coaxed out of it if you stack two additional diodes (as in the Bluesbreaker) or use LEDs (as in the Guv’nor) to get a higher threshold.
After the success of the Guv’nor (released in 1988), in 1991 Marshall decided to release two more pedals—the Bluesbreaker (a low-gain drive pedal intended to mimic the classic Model 1962 amp, nicknamed the Bluesbreaker) and the Shredmaster, a higher-gain variant of the Guv’nor.
Despite its name, the Shredmaster is not an ultra-gain metal monster like the Boss Metal Zone. Unlike the Guv’nor, it does have the ability to scoop the mids, so you can get some sounds out of it that approach the metal territory—but for the most part the name is misleading.
The Shredmaster was discontinued in the mid-1990s along with the other two pedals in the series. It was resurrected in 1998 as the JH-1 Jackhammer, but this new line of pedals from Marshall is almost universally considered vastly inferior to the originals.
The Shredmaster is probably most noted for being the favored distortion pedal of Kevin Shields of My Bloody Valentine and Jonny Greenwood from Radiohead (as well as being spotted on the boards of the other two guitarists at times). 
The most notable clone of the Shredmaster is the Truetone Jekyll & Hyde—specifically, the Hyde half of the pedal is a Shredmaster with the bass control fixed at around 90% rotation.
Adjustable gain stages & filter → Hard clipping (LEDs) → 2-band tonestack & recovery → Contour & high cut → Volume
The first half of the Shredmaster is identical to the Guv’nor / Drivemaster, but with lower gain in the first stage and higher gain in the second stage. As a result, you’ll notice this section is almost word-for-word identical to the description on the Equinox page.
The first circuit block is comprised of two op-amp stages: an adjustable op-amp boost & filter, set up as a non-inverting stage which has a bass roll-off corner frequency of 1026 Hz, which means it passes slightly less bass than the Guv’nor. This is followed by an inverting stage with a very high treble roll-off corner frequency, which means that overall, the Shredmaster is starting out with a much flatter signal than the Guv’nor. That’s probably for the best because it’s also going to do a lot more shaping later on.
The Gain control is interesting—it’s the same variety as found on the Guv’nor and Bluesbreaker pedals, but I’ve not seen it elsewhere. Let’s break it apart.
First, a bit about op-amp gain: in a non-inverting op-amp configuration, the gain is a factor of the feedback resistor divided by the ground resistor, plus 1. In an inverting op-amp configuration, the total gain is a factor of the feedback resistor divided by the input resistor.
In this circuit, the Gain pot is actually set up as a voltage divider rather than a variable resistor. When resistance is added to the feedback loop of the non-inverting op-amp, it’s also subtracted from the input of the inverting op-amp—which in both stages increases the gain.
So the gain is calculated like this:
Even though the pot is linear taper, in this setup it behaves somewhat logarithmically, so it’s very usable across the range.
One interesting thing to note here is that technically the Guv’nor actually has more maximum gain in this first section than the Shredmaster does (46 x 68 = 3128 in the Guv’nor, as opposed to 22 x 83 = 1826 here). However, due to clipping diode arrangement, the Shredmaster ends up sounding gainier—we’ll talk about this next.
No frills here: just two back-to-back 1N914 diodes to the reference voltage (which is seen as ground by the circuit, so it’s still essentially diode-to-ground clipping). This clips the circuit much more heavily than the Guv’nor / Drivemaster (0.6V here, as opposed to the Guv’nor’s 1.8V) and it’s the main reason that this circuit sounds much more “gainy” even though technically the Guv’nor has more gain, at least in this first clipping section where the distortion sound is created. The signal is clipped above 0.6V, but there are two gain recovery stages later on that put the two pedals closer in available volume.
The Shredmaster has two tone control stages. The first is nearly identical to the classic Fender/Marshall tonestack, but with fixed midrange (a 1k resistor where a 10k potentiometer would normally be) and the values of the potentiometers and capacitors changed to suit the context. Following that, there’s a non-inverting op-amp gain recovery stage with a gain of about 6, a low-cut corner frequency of 15 Hz (which is inconsequential; essentially no bass is cut) and a high-cut corner frequency of 329 Hz (which is very steep).
The designers knew what they were doing when they cut this much treble at this point in the circuit, and it affects how the Contour control works in the next stage. If you do feel that this circuit is lacking in treble, which some do, wait until the next section—we can fix it there.
Immediately after the first gain recovery stage is a second tone control, a single Contour knob that pans between two capacitors. For a description of how the Contour knob functions, I can’t do any better than Mark Hammer’s description (with part numbering revised to my schematic):
[C13/C14] and [R11/R12] form a wide notch mid-scoop filter that bleeds mids to ground through the Contour pot. As the resistance between [C13] and ground increases, the scoop becomes less severe. At the same time as the resistance to ground through [the Contour pot] gets bigger at one side, the resistance between [C12] and ground gets smaller. When it is small enough, [R10] and [C12] behave like a 1-pole lowpass filter, bleeding highs to ground. So, in one direction the Contour control can act like a mid scoop, and make the treble and bass seem more pronounced. In the other direction, the scoop goes away and the circuit attenuates some of the highs while letting the mids and lows through easily.
Past that, we have a second op-amp stage, this time set up in an inverting configuration. For this part I’m not actually sure how it’s calculated, since there are two 33k resistors in series with the 100k input resistor that are part of the Contour control but may factor somehow (or more likely, they cause the gain to be slightly different across the frequency spectrum).
At any rate, if you only consider the 100k input resistor along with the 100k feedback resistor, it has a gain of 1, so it’s a tone-shaping stage with unity gain rather than being a gain recovery stage. With a 100k resistor and 1n capacitor in parallel, it has a high-cut starting at 1.6 KHz.
If you feel like this circuit cuts too much treble even at the highest treble settings, this is where to fix it. The Visual Sound Jekyll & Hyde includes a Sharp/Blunt switch that switches between a 100pF (Sharp) and 1n (Blunt) capacitor here. The Solstice does not have space for a switch, but you could socket this value and try it out to your liking. I preferred 470pF or 820pF as a static value; the stock 1n was a little too dull, but the 100pF was too bright.
Nothing of interest here, just a standard volume control and a 220n output capacitor—though note that the volume control is referenced to Vref rather than ground, since it’s directly coupled to the last op-amp stage. This is followed by a 1M output pulldown resistor, which the three Marshall pedals in this series all share.
If you’re keeping score, there are two non-inverting and two inverting stages in this circuit, meaning the output is in phase with the input. This is in contrast with the Guv’nor and Bluesbreaker which both have only one inverting stage, meaning the output is out of phase with the input. This only really affects split-signal or parallel-processing setups, but it’s worth mentioning.