Tracing Journal: Pigtronix Philosopher’s Tone Micro
This one’s a bit out of the ordinary. The Philosopher’s Tone was released in 2009, an original design by Howard Davis, who designed many of Electro-Harmonix’s most famous effects of the 1970s including the Deluxe Electric Mistress.
Here’s a demo video of the Micro version, which is the same core circuit minus the Grit control:
The original Philosopher was traced by the Freestompboxes community in 2011. A couple of years ago we released the Polyphemus project, which is an amalgamation of several different iterations of the circuit, including a charge pump from the Micro along with optional germanium clipping diodes from the Germanium Gold version.
But something always seemed off with the circuit—not just ours, but going back to the original trace. Many in the FSB community noticed that their builds seemed to have very low volume, not getting much above unity gain at maximum. But as is often the case, most people building the pedal did not have an original to compare it with, and it was always assumed that this was just a characteristic of the design.
Subsequent traces of the Philosopher’s Rock seemed to confirm that the core circuit had been traced correctly, because all the component values in the shared parts of the circuit were the same. There were a few community mods to increase the volume, but no independent traces of other units to confirm or disprove anything.
So, after getting feedback on our Polyphemus project from those who built it, we decided to trace the newest version of the Philosopher’s Tone (the 1590A-sized Micro) to see whether the current version of the circuit is any different.
As they say in every modern clickbait news headline… the results may surprise you.
Tracing photos
Schematic
Analysis
Overall, it’s pretty well identical to the original Philosopher’s Tone. A charge pump has been added so that it can be used with a standard 9V adapter[1], and the Grit section has been removed so it’s as though the knob is turned down all the way.
The much-discussed capacitor is left intact, still hooked up in a way that breaks the rules since a capacitor should never be connected directly to the output of a op-amp—but it may be exploiting some characteristic of the LM741 and contributing to the circuit’s unique characteristics.[2]
But, there is one key difference: R19, the 6k8 resistor from the original trace, is 68k in the Micro. This resistor sets the gain ratio of the wet signal in the op-amp stage, and higher resistance means a higher signal level. So in this case the compressed/grit section will have 10 times the gain, while the dry signal will remain at unity since it is taken directly off the input buffer.
Original 1590B Philosopher’s Tone
So now the big question: did the original Philosopher’s Tone use 6k8, as shown in the original trace as well as the later trace of the Philosopher’s Rock—or was this a mistake that was never caught?
In the original trace, there’s only one photo of the component side of the PCB on FSB, and it’s difficult to tell whether the fourth band of the resistor (labeled R12 in the original unit) is red or brown, which would be the difference between 68k and 6k8.
So we picked up an original Philosopher’s Tone (that weird horizontal 1590B one) to check. Ours is SMD, but from the same generation as the original, just slightly newer (around 2011-2012).
Here’s a photo of this same resistor, labeled R7 in this version:
As you can see, it’s marked “6802”, which is 68k.
This prompted us to revisit the Philosopher’s Rock trace. While the photos are a bit blurry, the resistor does appear to be 6802 as well. (It’s R17 on the lower left side of the PCB.)
It’s possible that it was in fact 6k8 in the very earliest Philosopher’s Tone units and the original FSB trace was accurate. If anyone has a through-hole unit and can verify the resistor value, we’ll add the info to this page. But at the very least, we know Pigtronix was using 68k within a year or two of the original release, and the Philosopher’s Rock schematic incorrectly carried over the 6k8 value from the original schematic.
Other differences
Along with this one very consequential difference, in our Micro unit the tone cap was reduced from 6n8 to 1n5, which raises the center frequency of the tone control so it leaves the mids more intact. This was not another error in the original trace—we verified the 6n8 value in our 1590B unit.
But, this also leads to further questions. The original manual doesn’t mention the tone control frequency, and the Rock/Bass both omit the tone control entirely, but the manual for the Micro lists it at 2kHz. We ran a frequency analysis on the original 1590B unit and the tone control frequency was right at 2k with the 6n8 capacitor.
Therefore, the Micro be a lot higher if it uses a 1n5 tone capacitor, and in fact based on LTSpice simulations it’s closer to 8kHz—meaning that the current manual’s listed tone control frequency is correct for older units, but not current ones. It’s possible the 1n5 value may have been an error in our unit, but it’s more likely just an outdated spec in the manual.
Two more minor change: the Sustain pot has been changed from 25kB to 100kB and the adjacent coupling capacitor was raised from 560n to to 1uF. These changes were made around the time of the Philosopher’s Rock.[3] Since the Sustain pot is wired as an adjustable resistor, it means the last quarter of the knob rotation is identical to the full range of the original unit. They kept this change for the Micro. The capacitor increase would cause it to pass more low frequencies to the compressor portion, which makes the circuit more suitable for bass guitar.
Polyphemus update
Fortunately, this is an easy update with no PCB revisions needed, and just requires swapping out one resistor: change R16 in the Polyphemus from 6k8 to 68k. We recommend everyone who has built a Polyphemus (or any other Philosopher clone from other layouts) make this change. Even if you like it fine as it is, it’s going to be a lot more versatile with more volume on tap and the tone will not otherwise change.
It’s up to you whether you want to swap out the capacitor to narrow the frequency of the tone control, but in the Polyphemus this would mean changing C3 from 6n8 to 1n5.
Notes & references
- They actually use two ICL7660 charge pumps in parallel for extra current handling, since each chip can only handle 20mA. In our estimation it’s better to use a single LT1054, which can handle up to 100mA of current and has better specs all around. ↩
- After 16 years and no reputation for failure across at least five different revisions, we should conclude that even if it’s wrong, it seems to be working. The Blues Driver does the same thing with the buffered reference voltage, and there are hundreds of thousands of them out there. ↩
- According to Ovni Labs, the pot change originated from his direct request to Pigtronix to raise the threshold (i.e. reduce the sustain) on the original unit. ↩
