The Meteor Distortion project is a clone of the Maxon/Ibanez Sonic Distortion, which first hit stores in around 1983. Advertised as a heavier Tube Screamer, in reality it shares nothing at all in common with the classic TS-9 circuit. It has more similarities to hard-clipping circuits such as the BOSS DS-1 or MXR Distortion+.
This project is for the original SD-9, but features one of the two mods from the SD-9M, the “Mids” switch. (The second SD-9M mod, the gain switch, is not backwards-compatible with this circuit.) The second switch on this PCB allows you to switch between different sets of clipping diodes.
The SD-9 was not reissued until 2002 when Maxon broke off from Ibanez and released the Nine Series under their own name. Aside from true-bypass switching, the circuit was identical to the version from the 1980s.
A “modded reissue” of the SD-9 was released by Ibanez in 2012 as the SD-9M. While on the outside it appears to be just an SD-9 with two switch mods, the inside is more than a little different, with a couple of additional op-amp gain stages not found in the original. This helps with the output volume issue, but it doesn’t sound the same as an original SD-9.
Input buffer → Adjustable gain boost → Clipping diodes → Tone control → Volume & output buffer
The input buffer is a simple emitter-follower transistor setup, which converts a high impedance input to low impedance while maintaining a gain of very near 1. This is the first and last stage of every electronic bypass (flip-flop) pedal such as those by Boss and Ibanez. In these pedals, the input & output buffers are always in the circuit even in bypass mode. The input buffer normalizes the input signal and makes it so that what follows is predictable. This is the only way to guarantee reliable electronic switching.
When one of these pedals is converted to true bypass, either for clones like this one or for modifications to the original units, these buffers are usually left intact since they are part of the sound. However, the bypass switch is always wired outside of the buffers so that the buffers get switched in along with the rest of the circuit. You can leave off the buffer if you’d like, and in fact many of the most popular Tube Screamer clones omit the buffers—in general, this will change the touch sensitivity of the circuit and make it more reactive to the volume control on your guitar, but it also opens up a level of unpredictability in how it will interact with other effects before and after.
Since this circuit uses diode-to-ground hard clipping, the gain stage is an adjustable clean boost. By increasing the amplitude of the signal, you change the point in the waveform where the signal is hard-clipped later on, and the severity of this clipping determines how gainy and distorted the signal is. However, for now the signal is clean.
This stage also prepares the signal by cutting the lows with a corner frequency of 1.54 KHz, meaning the lows are cut by a factor of 6dB per octave beginning at 1.54 KHz. (In comparison, a Tube Screamer cuts starting at 723 Hz.) This is a much more significant low cut than you normally see in a circuit, but any more bass and the clipping would be too muddy or fuzzy. We’ll make up for it later on.
At the minimum Drive setting, this stage feeds the clipping diodes with a gain of 71. At maximum drive, the gain is 603.
This is a pretty run-of-the-mill hard-clipping configuration, with back-to-back diodes going to ground. These are 1N914s, which means the signal is clipped to around 0.6V on both sides of the waveform. This has a direct impact on the signal’s volume, and 0.6V isn’t a lot to work with, especially considering the passive tone stack that follows, which cuts the overall signal level even further.
Many hard-clipping circuits have a gain recovery section if there’s any significant tone-shaping that happens (see the Shredmaster). The biggest complaint against the SD-9 is that it doesn’t have enough available output volume, so it’s perhaps a design flaw that there’s no gain recovery section in the original pedal. However, it’s a pretty easy fix if you’re building one from scratch, or even modding one: just use a clipping diode combination with a higher clipping threshold, and it will directly impact the available output volume. By adding a BAT46 or germanium diode (0.3V forward voltage) in series with each 1N914, you’ll increase the available volume by half. By stacking two 1N914s, you’ll double the available volume.
It’ll change the tone—the end result will be less compressed and a little more open sounding—but probably for the better. (The Meteor project has a switch that lets you use two sets of diodes, so you can keep the original mode intact while also having a modified mode available.)
The SD-9’s tone control is pretty similar to the one in a Big Muff Pi or Boss DS-1 but with tweaked values. The end result is a mid-scoop between 234 Hz and 867 Hz, which stands in stark contrast to the Tube Screamer’s midrange emphasis. A common modification changes the value of a capacitor in this tone section which changes the mid-scoop to a slight mid-boost. This modification was incorporated into Ibanez’s reissue SD9M as one of the two switches. The Meteor project incorporates this modification as well.
From there, it’s a straight shot through a standard volume control and into another buffer, identical to the first but adjusted for the input impedance, and then out.