Posts in 'Tracing Journal'

Tracing Journal: Xotic Soul Driven Boost/Overdrive

Xotic is best known for their AC & RC Booster pedals as well as the BB Preamp. The Soul Driven was released in 2016, making it a relatively new addition to the Xotic family, developed in collaboration with Allen Hinds to be his signature drive pedal.

Here’s a demo/interview video with Allen:

It sounds pretty great, so we thought we’d tear one apart and see what was going on. Xotic makes some killer pedals, but they also tend to rehash a lot of their own stuff with only a couple of tweaks, so we were curious how different the Soul Driven would be from the AC/RC/BB pedals.

As it turns out: pretty different!

Note: The Soul Driven was originally released in 2016 as the limited-edition “AH” model with blue enclosure, as seen in the demo video above. The one we traced was the later non-limited chrome enclosure model, without the “AH”. It’s not known whether the “AH” version differs at all on the inside or if it’s just cosmetic.

Tracing Photos

Here are some photos of the Soul Driven guts:


Xotic really has an interesting method of PCB construction with the sandwiched boards. They do this on all their other 1590B-format pedals as well. It gives them the best of all worlds: top-side DC jack, PCB-mount pots, through-hole components, space for a battery, and high-end Switchcraft in/out jacks. The only downside is that it makes troubleshooting and repair a little bit harder. (Tracing as well, incidentally.)


Xotic Soul Driven Schematic

Download the PDF version.


Nothing super groundbreaking, but interesting nonetheless. Here’s the basic topology:

  1. A Tube Screamer style clipping stage with slightly higher clipping threshold (1N914 + BAT46 in series) and variable high-pass frequency (similar to the Bass control of the Timmy or the Voice control of the Zendrive)
  2. Passive treble cut, similar to the RAT or Zendrive
  3. Volume control
  4. Bass boost stage @ 120 Hz by means of a single-transistor gyrator in the feedback loop of an op-amp. A dual DIP switch selects between four preset resistor values to determine the amount of bass gain, from flat (no boost) up to 6dB.
  5. Transistor-based output buffer

It’s an effective circuit and sounds great. The only quibble I have from a design standpoint is putting the volume control before the EQ rather than after. As can be expected, there is a small amount of background hiss when the volume is all the way down, which could be eliminated if the volume control was after the bass stage, before the output buffer. The overall background noise level would be lower across all volume settings.

However, judging by the placement of the two diodes between the inputs of the second op-amp (which have the effect of clamping high signal levels to prevent op-amp clipping), it’s very possible that they tried it the other way first but intentionally moved it to the middle. It seems to have been done thoughtfully, so I’m not going to say it should be changed.

Regarding the bass boost: Technically speaking, it functions the same as a potentiometer, just with four presets instead of a continuous control range. The only reason not to use a real potentiometer here is if you had a design restriction that limited you to no more than four knobs. Since Xotic is committed to the 1590B form factor, it’s likely that they experimented with the Bass control as its own potentiometer during the design phase but then settled on making it an internal DIP switch to save space.

Their advertising claims the bass boost is centered at 125 Hz, but using a gyrator calculator, I found that it was actually slightly under 120. The difference wouldn’t be audible, but since 120 is also a nice round number, I’m not sure why they didn’t just call it what it is.

Thulcandra Boost / Overdrive

The Thulcandra Boost / Overdrive is an Aion project releasing today based on the Xotic Soul Driven.

The one major modification is that the internal DIP switch for bass level was changed into a potentiometer for smooth control across the whole range. At minimum rotation, it’s equivalent to the Flat setting, and at maximum rotation it’s equivalent to the highest boost setting (6dB), but instead of only having two in-between settings, you can set it however you’d like.

Disclaimer: Aion FX is in no way affiliated with Xotic. Any trademarks are property of their owners and are used for comparative purposes only.

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Tracing Journal: Seymour Duncan Pickup Booster

The Seymour Duncan Pickup Booster is a clean boost pedal with the added feature of being able to simulate the resonant peak of different pickups, letting a single-coil assume the character of either vintage or modern humbucker. It was first released in 2004, discontinued for awhile, and then resurrected in 2015 with an improved Class A design.

Here’s the intro video from Seymour Duncan themselves:

Now, here’s the deal. Seymour Duncan is a great company. The build quality and aesthetics of the Pickup Booster are good, and they’re very reasonably priced, even new. So if you build one yourself, the overall cost savings ratio isn’t killer.

But, I used to own the first version when it originally came out, and it always struck me as an interesting circuit. The 2015 update hadn’t been traced yet so I wanted to see what had changed.


I didn’t get any good photos of the inside of this one, and we had to destroy the SMD board to trace it. But here’s the schematic:

Seymour Duncan Pickup Booster Schematic

Here’s the PDF version.

Compare it to the schematic from version 1 (redrawn from Analogguru’s trace in 2007):

Seymour Duncan Pickup Booster Schematic - Version 1 (2004)


As you can see from comparing the schematics, the major difference in version 2 is the addition of a Class A push-pull output stage instead of the single PNP transistor. The Boost control has also been rearranged so that the pedal can go all the way down to unity gain. (One of the quirks of the original is that there was always some amount of signal boost even at the lowest setting.)

The new version uses uncommon and specific resistor values such as 30.1k and 4.77k. For some reason, you tend to see more of this type of thing in SMD work. There isn’t any engineering reason for this amount of precision, so the nearest common value would work just as well, such as 30k or 4.7k.

One other thing to note is the lack of power filtering. There’s a single 220nF capacitor on the +V supply, and then Vref has a 4.7uF filter capacitor. Interestingly, though, there was a mistake in the PCB layout and the 4u7 capacitor isn’t actually connected to the ground plane like it should be. It’s just a dead-end. The unit we traced was manufactured in 2018, so it’s likely an issue with all versions. It was easy to tell what they meant to do, so for the schematic I went ahead and corrected this mistake.

Photon Booster / Line Driver

The Photon Booster / Line Driver is a new Aion project based on the Pickup Booster. It’s a direct clone with no added features other than standardized (and improved) power filtering.

The high-precision resistor values used in the original have been carried over to the DIY version since they were all available in through-hole format at Mouser. However, the circuit should perform identically using more common values, so if you aren’t ordering from Mouser, we’ve provided a list of more common substitutes in the documentation.

Disclaimer: Aion FX is not affiliated with Seymour Duncan. All trademarks are property of their respective owners and used for comparative purposes only.

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Tracing Journal: SoloDallas Storm

The SoloDallas Storm is a pedal conversion of the Schaffer-Vega Diversity System (SVDS), the first wireless system for guitar that was reliable enough to be used by major touring acts. The SVDS was designed by Ken Schaffer and originally released in 1976 by the Vega Corporation. It was used by several famous guitarists including Ace Frehley, David Gilmour and Eddie Van Halen—and most importantly for this particular story, Angus Young of AC/DC.

The SVDS was eventually lost to time once better wireless systems became available in the 1980s. It wasn’t until an AC/DC fan named Filippo Olivieri (known by his forum handle SoloDallas) started an obsessive journey to recreate Angus Young’s guitar tone that he came across a 1984 interview with Angus where he mentioned using the SVDS in the studio. The SVDS had its own tonal coloration even without the wireless aspect, and it had become part of his tone.

You can read the full story on the SoloDallas website, but the short version is that he managed to reverse-engineer the SVDS and create a replica of it. Fil eventually got in touch with Ken Schaffer who joined him in the venture, and it culminated with Fil getting a chance to meet Angus Young during the recording of Rock or Bust (2014) and deliver unit #001 in person, which was used for all the guitar tracking on the album.

Since then, SoloDallas has become a full-fledged company with several products, including amps and pickups as well as pedals, all centered around recreating classic rock tones.

The Storm is an adaptation of the audio path of the SVDS that captures the essence of the full-size unit in an easy-to-use and affordable format, since the SVDS is about the size of a small desktop computer and costs over $1,000. It replaced the compander in the original SVDS (also used in the Schaffer Replica pedal, the Storm’s precursor) with an optical limiter based on a new reverse-engineer of the SVDS.

Anyway, that’s enough history for now. Here’s a video of the Storm in action:

Tracing photos

The final two photos show the PCB with all components removed and the solder mask partially sanded down to show the traces. Needless to say, as with previous SMD traces, this one did not survive the dissection.


SoloDallas Storm Schematic - Version 2 (2019)

PDF version can be downloaded here.


There are essentially three major sub-circuits that comprise the Storm, each corresponding to one of the knobs.

  1. Input pre-gain volume control to prevent clipping for high-output instruments
  2. Optical limiter (similar to something like the Flatline Compressor or Philosopher’s Tone) with variable threshold
  3. Output amplifier powered by the LM386 that delivers approximately 25dB of clean boost, with a volume control at the end

It’s a very well-designed circuit and does exactly what it says. It doesn’t produce any distortion or clipping on its own, but can drive the input of an amplifier to great effect, and the limiter can deliver plenty of squish along with it.

Omitted parts

The Storm has two capacitors on the PCB that are populated, but linked by jumpers on the bottom side so that they are disconnected unless the jumpers are in place. This is a common manufacturing trick in SMT assembly that lets them decide on the fly whether to use either of the capacitors, even on a per-unit basis.

C1 is 120pF capacitor on the input, which is commonly used to tame radio frequencies, and is marked “input RF filter” on the PCB.

C5 is a 10uF capacitor in parallel with the vactrol LED which helps control the attack time, marked “isolator threshold” on the PCB. This capacitor is very commonly used in similar optical compressor/limiter circuits.

On the unit that was traced, both jumpers were unpopulated, so both capacitors were disconnected. Even though R19 and R20 are not populated on the PCB, it can be inferred that they are zero-ohm jumpers that just activate or deactivate their respective capacitors.

There is also another quirk with R5 and R21. These are two 2k resistors in parallel, for an effective value of 1k. This is done because the rest of the board only uses 2k resistors. Since 1k is only needed in this position, it helps them save on production costs by not having to stock another resistor value and add a step to the SMT assembly process.

Gale Booster/Limiter

The Gale Booster/Limiter is a new project releasing today based on the SoloDallas Storm. It’s the same circuit shown above in the schematic, except for the following three changes:

  1. Power filtering has been revised slightly to work better with the PCB layout of the Gale.
  2. R5 and R21 have been combined into one 1k resistor.
  3. R19 and R20 are omitted because it’s more straightforward to just choose to include or omit C1 or C5 directly.

Disclaimer: Aion FX is in no way affiliated with SoloDallas. Any trademarks are property of their owners and used only for comparative purposes. SoloDallas makes a wide range of incredibly high-quality products. If you like what they do, support them!

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Tracing Journal: Dinosaural OTC-201 Opticompressor

Back in January, I had the opportunity to collaborate on a trace of the Bouteek Distorter Preamp, a super-rare drive pedal that was on the verge of extinction. The result was the Vortex, released in March.

It was a lot of fun to work on, and rewarding to bring a new circuit to the DIY community that no one had ever seen before. I’ve gotten great feedback from several people who have built it.

So… why not keep it going?

I decided to buy a few interesting pedals as an experiment to see if this idea could pay for itself. Well, it turned out to be more than a few—I’ve got about 15 brand new traces underway right now, with schematics already completed on all but a few of them. They’ll all be released as PCBs over the coming months, before the end of the year.

Some of them are very high-profile. Some are outrageously expensive. Some are, like the Bouteek, so rare as to be nearly extinct. Some of them are traces that had been done before, but not accurately, or with a lot of misinformation.

This one falls under both “expensive” and “rare”. Tracing log entry #001: the Dinosaural OTC-201 Opticompressor.


You may not have heard of it, but you likely know the designer: Dan Coggins from Lovetone, the UK company known for its super-complex analog modulation designs in the mid-90s and early 2000s. Dinosaural was Dan’s solo venture after Lovetone folded, and he released three pedals under this name: the Tube Bender (similar to a silicon Tone Bender, based on a design he did for Guitar and Bass Magazine), the Overdriven Pre-Amp OPA-101 (an updated version of the Tube Bender with a buffer), and the OTC-201 Opticompressor.

The Opticompressor is the one new design of the three, and in an engineering sense, it’s a thing of beauty. Probably only a few hundred were made in total, in limited batches between 2013 and 2016. This particular unit was from 2014 based on the serial number.

Tracing Photos


The trace was complicated owing to the on-board hardware and hybrid SMD design. Unfortunately, the pedal did not survive the trace, but it gave its life for a good cause. Here’s the schematic.

Dinosaural OTC-201 Schematic

And here’s a PDF version if that’s more to your liking.

Introducing the Convex Parallel Compressor

The Convex Parallel Compressor is a new Aion project based on the OTC-201 and is releasing today. It’s an exact clone except for one added feature: there’s an internal slide switch to go between buffered mode and true bypass, as with the Refractor. Additionally, the Direct/Blend switch has been moved to the face of the enclosure.

This build is shown using the Input/Output Module Kit which I use for all of my prototypes.

I hope you enjoy it! It was a fun project to design from start to finish, and I’m excited to be able to add a new circuit to the DIY pantheon.

Filed under Tracing Journal