Tracing Journal: Xotic BB Preamp (five variants)
Some companies like to keep their product line as small as possible so customers don’t get choice paralysis. Xotic is not one of those companies.
Think you might want a BB Preamp? Well, a few minutes of research turns into a few hours, and if you managed to gather the right info, you’re now left with a decision tree of six different variants:
- Standard (now called the “version 1.5”), post-2007 circuit
- “AT” (Andy Timmons signature), based on the pre-2007 circuit
- Bass BB Preamp
- Custom Shop BB Preamp MB (mid boost)
- Custom Shop BB Preamp Comp
- BB Plus (2-channel dual pedal)
The AT, Plus and the two Custom Shop models are no longer in current production, but still—a search for “BB Preamp” on eBay or Reverb can be dizzying.
Add to this the fact that the circuit has changed a bit throughout the years even among the same SKU, so a DIYer looking for a schematic would find several different ones. Some of the differences are based on inaccurate traces, others are likely just due to production changes by Xotic along the way.
After tracing the Soul Driven two years ago, we went on a bit of an Xotic kick and decided it was time to get to the bottom of the different BB Preamp versions as well as the AC Booster and RC Booster. We borrowed or bought three of them—the original (1.5), the AT model, and the Bass version—so we could trace them directly. Then, since the two Custom Shop models are built on the same PCB layout as the bass version, once we’d traced that one we were able to infer the Custom Shop schematics from photos and Xotic’s product descriptions.
The BB Plus is very different from the others, but we’ve got this one one of these waiting to be traced as well, so it won’t be a mystery forever.
BB Preamp AT (BB-AT)
The Andy Timmons Signature BB Preamp was released in 2009. Xotic has this to say about it:
The original version of the Xotic Effects BB-PREAMP, featured warm, compressed overdrive tone. However, for some guitarist there was too much compression that perhaps restricted the pedal from “breathing”. A new version (serial number 3643 and higher) was released with some of the compression reduced in April 2007. The result was a more aggressive BB-PREAMP that today remains one of our best selling pedals.
A few years ago, Xotic Effects and Andy Timmons partnered to recreated the original BB to what is now know as the Andy Timmons signature model BB-AT.
Based on this, it seems like it’s just a reissue of the pre-2007 BB Preamp which Andy preferred to the updated one. The only change from pre-2007 to post-2007 is the addition of one more set of diodes. But, we also didn’t have an accurate schematic of the post-2007 version, so we decided to take apart an AT and get a definitive schematic. If we did one detailed trace, the others should come easily.
So based on this, you can see that the dual-pot schematic is in fact the correct one. One half acts as a traditional volume control before the output buffer, and the second one reduces the gain of the pre-EQ opamp stage, ostensibly to avoid overloading the EQ section at lower volumes.
You can see Xotic’s design process too, if you’re interested. From the input through C7 and R13, it’s 100% identical to a Tube Screamer with the tone control hard-wired at 9:00, except that the drive pot value has been doubled to 1M. In the Tube Screamer, R13 is followed by the output volume control and output buffer. In the BB Preamp, Xotic essentially cut the connection between R13 and the output volume and inserted two additional op-amp stages: a variable gain stage and a 2-band Baxandall EQ. Then from the EQ stage output, it returns to the standard Tube Screamer circuit for the volume control and output buffer.
However, the cut-and-paste job was a bit clumsy. The variable-gain inverting opamp (IC2A) was added as a standalone circuit block without taking into account what came before it, and the end result is that it has a capacitor, resistor, variable resistor, resistor, and capacitor all in series between the two op-amps. Series resistors and capacitors can always collapse down to a single value; there is no difference between a single 11k resistor and a 1k and 10k resistor in series. So the 1uF capacitor and one of the resistors should have been eliminated.
That said… we can critique the seemingly amateur design process all day long, but we’ve never invented a pedal as legendary as the BB Preamp, so it only goes so far. Redundant parts or not, the BB Preamp is a good design, and there was probably a reluctance to change what works in exchange for a few pennies’ worth of parts.
BB Preamp v1.5
This is the post-2007 circuit which remains in production today, and for the past couple of years since they updated the cosmetics, Xotic has formally named it the “version 1.5”. The vanilla BB Preamp has been traced a few times, but there have always been a few major discrepancies in the DIY community, notably surrounding the volume control and whether it is a dual or single pot.
The original version of the Perelandra from 2018 uses the single-pot version since it seemed like a more refined design while the dual-pot version had a few clumsy redundancies. If there were really two versions of the BB, we opted to adapt the more streamlined one.
But, there was still a lingering uncertainty since we’d never seen the full inside of a real BB Preamp. So we grabbed a relatively recent current-production version and opened it up.
We don’t have photos of the unit we traced since it was closed back up very shortly after it was opened. Here’s why—check out this photo of the PCB from Xotic’s website:
The Andy Timmons model was a limited run, but after it was done they repurposed the PCB for the standard BB Preamp. The only difference on the v1.5 is that the diodes have been doubled up and “tented” so that two of them can occupy the footprint of one. We took it out of the enclosure and verified that, other than the diodes, everything else is 100% identical—same PCB layout, same components.
It’s almost redundant to include it here, but for the sake of completion:
BB Preamp Bass
The bass version came out in 2006. It appears to be discontinued as of this writing, although it was still available new as of last year (2020) so it’s possible there are just supply shortages. In any case, this one does use a different PCB layout as the BB and BB-AT we traced above, but it is the same layout used by older BBs before they moved to the hybrid SMD format.
There is one added component, a capacitor soldered to lugs 1 and 3 of IC1 on the bottom side of the PCB, which acts as a sort of treble bypass. We’ve never seen a capacitor that bypasses an entire opamp stage, noninverting input to output, but it’s definitely how it’s done. Here’s a close-up:
Custom Shop BB Preamp Comp
The two Custom Shop models of the BB Preamp came out in 2009. We’ll look at the “Comp” (compression) model first. We didn’t have a physical unit to take apart, but it’s pretty easy to tell what’s going on when you read this excerpt from the product description:
Users will have the ability to choose standard compression (think current BB-PREAMP), more compression (Andy Timmons BB-AT) and no compression. The new BBP-COMP rolls two iconic pedals into one while taking its sound to a new level with a no compression option.
The only difference between the BB-AT and current BB Preamp (v1.5) is an extra diode in each direction for version 1.5. So when they say that you can choose between standard and AT modes, with a third position “no compression” mode (i.e. diode lift), there’s not a lot of room for ambiguity.
Note that when they say “compression”, they’re not talking about true compression, like if they stuffed one of their SP Compressors into the enclosure. They’re just talking about diode clipping, and in their terminology, lower clipping threshold equals more compression. Clipping and compression are very different things, so this is a somewhat obnoxious way to talk about it. They both reduce signal dynamics, but one is extremely simple while the other usually requires an entire circuit as large or larger than the BB Preamp itself.
Here’s the BB Preamp Comp schematic:
Custom Shop BB Preamp MB (Mid Boost)
This BB Preamp MB came out at the same time as the Comp version above. Here’s a clue from Xotic’s product description:
The MB version is capable of covering all the tones that can be obtained with the standard BB Preamp by setting the mid control at 9 o’clock.
Again, familiarity with the BB Preamp circuit is crucial for translating this single sentence into a schematic. The BB has a Tube Screamer tone section in between the clipping and bass/treble tone stack, with 1k and 18k resistors essentially hard-wiring the control at approximately the 9:00 position. (1k/18k is just a little over 5% total resistance, but since it’s a W-taper control and not linear, that accounts for it being 9:00 and not 7:30.)
So with this in mind, check out this (regrettably low-resolution) photo of the BBP-MB PCB, with special attention to the two areas marked in red in the upper left:
Two missing resistors. And since it uses the same PCB as the Bass BB model from earlier, we know that these happen to be the two resistors that hard-wire the Tube Screamer tone control at 9:00. If you pulled out the PCB, you’d undoubtedly find wires going from those pads to the “mids” pot which is going to be 20kW (as on the original TS). This custom shop mod just restores the hard-wired Tube Screamer tone control.
One last thing to clarify. The tone knob in the original Tube Screamer is hardly a mid-boost control. The TS has a pretty constant 720 Hz midrange hump, and the tone control fades between a bass emphasis on the low end and a treble emphasis on the high end without much impact to the mid frequencies.
However, with the BB Preamp, since it has a Baxandall shelving tone stack afterward that shapes the lows and highs, and since the whole circuit is voiced differently than the Tube Screamer, the net effect of the TS-style tone control is that it does have more apparent impact on the mids. It’s still probably not the best name for the control, but we don’t have any better suggestions either.
Version 2 of the Perelandra is a complete overhaul of the original project, which was based on an inaccurate schematic that has been widely distributed in the DIY community. From tracing the BB Preamp variants, it’s apparent that the dual-pot version is the correct one and the BB Preamp has never had a single-pot volume control. PCBs from PedalPCB, Madbean and PCB Guitar Mania and kits from Puzzlesounds and guitar-electronics.eu all use the single-pot version of the schematic, which seems to have originated as a simplified work-alike of the BB Preamp in 2006 by a forum user named KHE and was not intended as an exact clone.
This is not to disparage: many have A/B’d the DIY single-pot version with a real BB Preamp and found that it sounded the same, and it was clearly inexperience on the part of the Xotic designers that they kept two resistors and two capacitors in between the second and third opamp stages instead of combining them into series equivalents. But we’re meticulous about accuracy, and so the original Perelandra project has been discontinued in favor of this new version that preserves the audio path of the BB Preamp exactly as it appears in the production units (redundant parts and all).
The Perelandra Deluxe is an all-new project that incorporates the two Custom Shop modifications for the “Comp” and “MB” versions. Each of these added features can be set to a position that mimics the stock circuit, so while you now have additional tone-shaping tools available, the original tone of the BB Preamp is unaffected. But since it’s a more complex build that differs significantly from the original BB Preamp, we opted to split the custom shop mods into a “Deluxe” version while keeping the standard Perelandra for the four-knob variants.