Posts in 'Tracing Journal'

Tracing Journal: Xotic AC Booster, RC Booster and Bass RC Booster

Xotic is a notorious company among the DIY pedal scene. Their pedals are massively popular despite being somewhat unoriginal, little more than Tube Screamers and Bluesbreakers with a Baxandall tone stack, and they really do sound great. But they also—up until recently—partially gooped their PCBs and sanded the markings on their ICs to make reverse-engineering difficult. On top of that, they often quietly make changes to the circuits, whether due to constant tone-tweaking or just parts availability during manufacturing.

The result is that the DIY community has traced several Xotic pedals, but almost every schematic is slightly different, and it’s very hard to sort out which of the differences are due to circuit changes by Xotic or just due to inaccurate traces.

When I first developed the Malacandra project in 2018, I gathered as many of the schematics as I could find and compared them to try to come up with a “consensus” schematic, the most accurate combination of all of them. At the time, I didn’t realize accuracy was a moving target with Xotic due to the frequency of revisions. But, when we got the opportunity to trace a few Xotic pedals directly, we decided to clear it up once and for all.

We traced three pedals that share the same PCB in Xotic’s production line: the AC Booster, RC Booster and Bass RC Booster. The AC Booster and Bass RC Booster were purchased new in 2019, while the standard RC Booster was used but only a couple of years old.

The schematics were indeed different from the community’s prior traces. None of our traces matched any of the known schematics exactly. The topography was all the same, no huge revelations there, but each of them had some new part variations that hadn’t been seen before.

Here’s the thing, though: this doesn’t mean that earlier traces by the community are inaccurate. I just felt like we needed some sort of definitive trace to satisfy my own desire for accuracy so there were no lingering questions, and so I could say that at least these schematics were accurate to these units.

So: let’s get to it!

Note: Not shown in these schematics are the bypass switching arrangement (which is standard 3PDT true bypass) or the bypass indicator LED. Xotic does use some extra circuitry around the LED, possibly to prevent it from popping when the effect is turned on and off, but it’s unimportant to the functioning of the effect. Only the main circuit and power sections are shown.

Xotic AC Booster

Xotic AC Booster Trace Schematic

The biggest thing to note about the AC Booster is the diode arrangement with D5 bridging the two pairs. Other traces show two diodes in one direction and one in the other for an asymmetric clipping arrangement similar to the Boss OD-1. Based on photos of earlier AC Booster pedals, it appears that the diode configuration was changed. Earlier traces were probably correct and Xotic intentionally changed the diode configuration along the way.

Also note the 1-meg drive pot. Earlier versions used 500kB which means they’d have less maximum gain available.

The last change is C6, which is 100n in earlier traces but has been changed to 47n. This will raise the low-cut frequency of the clipping stage for slightly less bass.

Xotic RC Booster

Xotic RC Booster Trace Schematic

This one is closer to the known traces than the AC Booster, but note the 1uF value for the input capacitor where other schematics show 47n. It likely won’t make a huge difference since 47n is plenty big enough for an input capacitor.

Xotic Bass RC Booster

Xotic Bass RC Booster Trace Schematic

I’m not aware of anyone who’s taken apart a Bass RC Booster, so this is a brand new trace, although as you can see there’s not much that’s changed from the standard RC Booster. All clipping diodes have been removed, the bass tone capacitors C3 and C4 have been doubled from 4n7 to 10n, and the minimum-drive resistor R1 has been dropped from 47k to 22k. Also, for some reason, the input capacitor C7 has been lowered from 1uF to 100n, although either of these values will still provide full-frequency operation for either bass or standard guitar.

Note that this was version 1 of the Bass RC, not the newer version 2 with a second gain-boost footswitch. The footswitch is likely just switching between two gain potentiometers, similar to the Fulldrive, but we haven’t looked inside one to know for sure.

Malacandra V2

Version 2 of the Malacandra Boost / Overdrive is releasing today, fully replacing the first version. The main change to the PCB layout is the way the diodes are configured, adding a seventh diode as a “bridge” between the two sets, used in the AC Booster. The bias supplies were also changed slightly, although this likely wouldn’t affect the sound of the unit. The documentation has been updated with part variants for all three versions.

Filed under Tracing Journal

Tracing Journal: Cornish OC-1 Optical Compressor

The OC-1 Optical Compressor is the the first of two new Cornish pedals I traced that are releasing today, alongside the SS-3 Soft Sustain.

I originally teased it on Facebook over the summer, and it attracted some attention not just because it was a Cornish that hadn’t been dissected before, but also because of the X-ray photo I included along with it:

Cornish OC-1 X-Ray

Why the X-ray, since there’s no way a complete circuit could be traced from it?

Well, the main reason was just because I had the opportunity! A friend suggested it, and said he knew a guy who could do it. I’d never seen an X-ray of a pedal before so I figured it’d be interesting.

The second reason was that we could get a really good idea of the overall topology without having to de-goop anything. We were trusting the marketing copy from Cornish that it was an original circuit, but when people cover things with goop, sometimes it’s just to hide the fact that it’s not actually as original as they want people to think. So we didn’t want to risk ruining a $500+ pedal just to find out that it was a variation of the Flatline or something similar. If I could tell that it was going to be boring, I’d have sold the pedal untouched and tried something else.

The good news is, the X-rays were enough to tell us that the OC-1 was in fact an original design, not quite like anything else we’d seen before. So we went ahead with it.

Tracing Photos

Here are a few highlights. There are 50+ photos in total that you can check out in an Imgur gallery if you’d like to see more detailed pictures.

Removing the goop

The degooping process was pretty time-intensive. Many Cornish circuits (including the SS-3, as you can see in the other tracing journal) only have a thin layer of conformal coating that is easily scratched off, but others like this one have really thick epoxy that is hard as a rock. It came off with some coaxing, but as you can see, it wasn’t pretty afterward.

From there, we took off every single resistor and capacitor and measured the values rather than relying on partially obscured color codes. We went slowly and took our time. After the initial trace was complete, three different people checked the tracing work for errors. So I can say with a high degree of certainty that this circuit is cracked.

Since people may be wondering: I’m not going to reassemble this unit and try to sell it as though it was untouched. I may keep it for reference, or I may sell or trade it privately with full disclosure that it was degooped for a reverse engineering project. But either way, it’s not going to end up on the used market.

Schematic

Cornish OC-1 Schematic

Download the PDF version.

Analysis

As mentioned, it doesn’t bear much resemblance to anything else out there. The audio path is pretty straightforward, with the classic Cornish buffer, and then a 3-way split into the compressed path, clean path for blending, and sidechain (peak detector).

The compression itself happens by means of a simple voltage-dividing attenuator, essentially a dynamically-operated volume knob, formed by R13 and the LDR portion of the vactrol. This method of volume reduction is the same type used all the way back in the first optical compressors from the 1950s such as the LA2A.

The sidechain is designed to turn the audio signal into a control current to illuminate the LED in the vactrol. I’ve never seen this topology before, but if someone recognizes it from an application note or textbook someplace, please let me know so I can add the reference and better explain what is going on.

The clean blend path gradually combines the clean signal with the compressed signal as you turn it up. It is not a balance control like some other blend circuits where one end of the knob is full clean and the other end is full effect.

The volume control is also set up so that the minimum position is unity, as opposed to zero volume. It’s an op-amp gain stage rather than a traditional volume knob at the end of the circuit. The volume control and blend control are somewhat interactive with each other and the volume may need to be backed off when the blended signal is added in to maintain a constant signal level.

Oceanid Optical Compressor

The Oceanid Optical Compressor is a new Aion project releasing today based on the OC-1. It’s a direct adaptation of the original OC-1 except that the buffered bypass is switchable to true-bypass mode.

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

Filed under Tracing Journal

Tracing Journal: Cornish SS-3 Soft Sustain

The second new Cornish pedal today is the SS-3 Soft Sustain. The SS-3 is an update of his earlier SS-2 circuit, an 1970s-style overdrive with roots in the MXR Distortion+.

The SS-2 had already been traced… sort of. There’s a pedal called the Skrydstrup ODR-2 that was rumored to be based on the SS-2, and that’s the one that was traced on FSB a few years ago. So the Cornish SS-2 schematic that’s floating around out there is really just the ODR-2 that has the Cornish buffer added.

When I found an SS-3 for a decent price, I thought it’d be a good opportunity to get to the bottom of it. The SS-3 claimed to be identical to the original except for an added low-cut control, so not only could we finally confirm whether the ODR-2 was truly based on the SS-2, but we could get the SS-3 circuit updates as well.

Tracing Photos

Here are a few of them. The rest are available in the Imgur gallery.

Schematic

Cornish SS-3 Soft Sustain Schematic

Download the PDF version.

Analysis

As mentioned earlier, we’re looking at a Cornish-flavored Distortion+, getting a similar treatment as he gave the Big Muff for his G-2 pedal. His signature input buffer is added to the beginning and a passive tone control is added at the end, along with a dual low cut control to cut the bass both before and after the clipping diodes.

With this schematic, we can confirm that the Skrydstrup ODR-2 is a fairly accurate clone of the SS-2, although there are a couple of discrepancies that could either be due to mistakes in the FSB trace of the ODR-2 or Skrydstrup’s mistakes in tracing the SS-2. However, the SS-2 schematic is very close, which is good news for everyone who has tried to build one!

Of interest is the IC2 gain recovery stage after the tone control, with R19 going to the junction of R20 and C15 rather than to ground as you’d expect. It looks for all the world like a mistake in either the community’s trace of the ODR-2 or in Skrydstrup’s trace of the SS-2. But this finally confirms that it’s not wrong. I am not sure what advantage it gives to hook up the resistor like that, but it works.

Soma Vintage Overdrive

The Soma Vintage Overdrive is a new Aion project releasing today based on the Cornish SS-3. It’s identical to the original except for the inclusion of a slide switch to select between buffered bypass and true bypass, as with other projects based on Cornish pedals.

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

Filed under 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:

 

If you’re interested, you can check out an Imgur gallery with another ~30 photos showing more detail of the PCB.

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.)

 Schematic

Xotic Soul Driven Schematic

Download the PDF version.

Analysis

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

Interestingly, the back half of the circuit (everything after #1 above) is lifted almost verbatim from the BOSS BD-2 Blues Driver. Here’s the Blues Driver factory schematic to compare:

BOSS BD-2 Factory Schematic

Only four minor changes were made:

  • Tone pot is changed from 10kB in the BD-2 to 250kB (meaning it can cut more treble than the BD-2)
  • Tone control capacitor (C101 in the BD-2, C5 in my schematic) is changed from 1n to 1n5
  • Bass resistor (R8 in the BD-2) is changed from 6k8 to four available settings: 18k, 28k, 33k, 43k (meaning the lowest setting in the Soul Driven has more bass boost than the BD-2’s fixed value)
  • Output resistor (R1 in the BD-2, R12 in my schematic) is changed from 1k to 470R. No real impact from this change.

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 lower than 100%. But hey – the Blues Driver does it, and it’s stood the test of time for almost 25 years, so I won’t criticize the engineering too much.

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 four-knob 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 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.

Filed under Tracing Journal

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.

Schematic

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)

Analysis

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.

Filed under Tracing Journal

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.

Schematic

SoloDallas Storm Schematic - Version 2 (2019)

PDF version can be downloaded here.

Analysis

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 the 1k value is only needed in this one position, it’s more cost-effective to just use two 2k resistors than to have 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!

Filed under Tracing Journal

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.

otc201-box-wide

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