Kintsugi, a JFET drive with dynamic sag circuit.

Last year I built my first Smallsound/Bigsound Mini based on a PCB of my own creation. Over the course of the months that followed, I started becoming more curious about the F- Overdrive, which is a JFET based overdrive/fuzz/distortion that features a dynamically modulated starve circuit that is said to mimic the sound of a dying tube amp. To me, the effect is akin to a compressor and tremolo working in tandem. On the most subtle level, the dynamic sag circuit can make a drive pedal sound like Neil Young’s “exploding small amp” from Weld, and on the most extreme level, the pedal will struggle and strive to illicit a sound. For reference, the demo at the bottom of this page is set to about 75% of the dynamic sag’s full capabilities.

What I believed would be a simple project derived from researching a series of Google searches exploded into a full fledged R&D project. As I would later find, the information needed to construct the dynamic sag circuit was either non-functioning and/or incomplete. I solicited advice from the usual forums (FSB, DIYStompBoxes, Reddit), and ultimately rebuilt the dynamic sag circuit from scratch. While I am still not completely sure my dynamic sag circuit is 100% a part for part remake of what’s in the SSBS model, I believe I have come very close and am approximating similar sounds (effects). Furthermore, up until now, the only person I have seen publicly claim to get the dynamic sag circuit to work is Brian from SSBS. Even Andrew/The Tone God who first shared the dynamic sag circuit which later made its way into the SSBS drive has never made a claim to producing a functioning unit.

This project started back in April and it wasn’t until now that I’ve completed the product I set out to create: A JFET drive with a dynamic starve circuit. The primary circuit is the SSBS Mini, but I did make some changes along the way like implementing some snippets from the Fairfield Circuitry Barbershop and Benson Preamp. I also added an additional control to the dynamic sag circuit: “Brokenness” that acts sort of like an attack/decay knob. Just like the SSBS F- Overdrive, the dynamic sag unit is footswitch bypass-able, meaning the pedal can either be run with the normal power supply or be run with the sag unit activated. I opted against making it momentary activated because (1) it would require an additional SPDT toggle switch that I had difficulty fitting on to the PCB layout and (2) for my style of music/playing, I found myself preferring it to be a “latching” effect versus momentary.

Controls

• Volume - Controls audio level output.

• Gain - Controls two gain stages simultaneously and increases distortion levels.

• Bass - Passive EQ control for bass frequencies.

• Treble - Passive EQ control for treble frequencies.

• Threshold - Determines aggressiveness of dynamic sag.

• Sensitivity - Adjust for audio input. Hotter signals and pick-ups may require less sensitivity.

• Range - Determines how much voltage (although technically, current) gets starved but also acts like an attack/decay knob.

• If you would like to play around with the dynamic sag and see its effect on voltage, see this model: https://everycircuit.com/circuit/4638353567514624.

Demo

Updates

• August 5, 2023: Added a “full band” demo. Made changes to some of the wording regarding the controls. Updated with dynamic sag model.

A deep dive into deciphering the elusive dynamic sag circuit in the Smallsound/Bigsound Overdrive.

I originally posted two similar versions of this blog on Reddit and the PedalPCB forum. This version includes changes based on feedback from those posts.

Summary

I spent the past few months figuring out the dynamic sag portion of the Smallsound/Bigsound (SSBS) F- Overdrive, had a breakthrough, and then stuck it into a Fairfield Circuitry Barbershop + SSBS Mini-hybrid drive.

The companion effect, an envelope-controlled voltage starve circuit, mimics a dying tube amp and responds to an individual's playing: As described by Brian/SSBS, the dynamic sag circuit is “dynamically modulated (envelope controlled)…turn up the threshold and the harder you play, the more the overdrive crackles, tries to keep up… and fails miserably.” I call it a practice in kintsugi - or discovering and embracing the musicality in the “brokenness” of the pedal.

In this post I have included video demos, a schematic of the dynamic sag circuit, background information, a schematic of the dynamic sag circuit in practice, and final notes on shortcomings and future things to explore.

The post is quite lengthy, so I implemented headers in case you want to jump ahead to a specific section. Also, instead of posting ongoing updates or progress reports, I will just update this post.

(Last updated: 7-4-2023)

Video Demos

• Diagram of the signal path.

• Rhythm guitar demo.

• Lead guitar demo.

Background

A few months ago, after having built a SSBS Mini clone, I came across a video demo of the F Overdrive. The video detailed a feature within the pedal that was described as an envelope follower that starved a single gain stage. I thought the process would be as simple as a Google search, but whenever I did, I found threads suggesting people just Google the answer which leads to more threads suggesting people just Google the answer which leads to more threads…so on and so forth. In all my hours of Googling and perusing forums like FreeStompBoxes and DIYStompBoxes, I had only found one person who figured out the circuit and produced a functional dynamic sag circuit: Brian from SSBS.

Eventually I happened across a thread on Reddit in which someone used the phrase “dynamic sag,” which lead me to the Tone God Punisher circuit.

The Tone God Punisher circuit is not so much a pedal as a companion piece for a pedal. Utilizing an envelope filter, a NPN BJT, and a JFET, the circuit starves current and mimics a dying tube amp. The core concept of the circuit (envelope follower and voltage controlled resistors) can be used for a myriad of applications in which an input signal dynamically controls another part of the pedal - be it, gain, volume, delay time (like the Retroflect), clipping, etc.

The circuit itself is derived from the Nurse Quacky, which in turn is derived from the Dr. Quack, which in turn is based on the EHX Doctor Q.

The dynamic sag feature is a hallmark of the SSBS F Overdrive. More recently The Shields Blender sported a similar feature. Fender ostensibly claims the dynamic sag - or “reactive sag” is “the first of its kind,” but clearly it isn’t, lending credence to the fact that Brian/SSBS is (was?) quite ahead of his time. To clarify, I don’t know if the sag circuitry in the Shields Blender is the same as the SSBS, but I do take issue that the reactive sag is being purported as being revolutionary when another pedal had the same function years prior.

Research

To say it took me a couple of months to figure out how to get the dynamic sag circuit to work would be an understatement. The original Tone God Punisher schematic was not functional. Through a series of trial, error, soliciting advice, multiple breadboards layouts, meticulously looking at internal of the SSBS OD and unpopulated SSBS OD PCBs, and a random late night epiphany, I managed to get the circuit working.

The big changes I had to implement were (1) the biasing of the op amp and (2) the removal of the JFET. The latter was a last minute decision brought about by a FreeStompBoxes.Com post in which a user felt the JFET and BJT interaction would be too unpredictable and warrant careful (laborious) selection of components. Alas, I got the circuit working and stuck it into a Fairfield Circuitry Barbershop/SSBS Mini hybrid.

Because of the alterations I have made and lack of the heavy/light control in my circuit, I have to state that this is not 100% a replication of the dynamic sag circuit in the SSBS OD; however, it is very similar in topology and produces the same effect.

Schematic of the Dynamic Sag

Note: Since originally sharing this schematic, I have made some changes based on feedback I received. The above schematic also includes a few proposed changes that I have learned about since my initial postings on the forums.

Practical Application in a Pedal Schematic

Based on my analysis of the SSBS Overdrive PCB, my circuit is probably only 80% similar to what's in the SSBS Overdrive - but I do believe it is a fair approximation.Above is a schematic of the Fairfield Circuitry Barbershop with the implementation of the dynamic sag unit. Note this is not the same schematic as the demo videos. I just wanted to share how the schematic would look once all the pieces are put together.

Explanation

(Note that parts referred to in this section is referring to the primary Dynamic Sag Circuit schematic.)

The input of the guitar signal splits off into the actual audio circuit and into the dynamic sag circuit. The signal goes through a buffer into the Sensitivity control. Thereafter, the signal goes to an envelope follower, which is a oddly-biased op amp. The envelope follower triggers an LED. LED charges up a capacitor, which in turn discharges to the BJT and allows current/power to flow to ground. The current/power that flows to ground is current/power that’s meant for the gain stage, effectively starving the stage in the process. LEDs’ forward voltage must be matched. Vout goes to the power source of the gain stage and R10 is whatever the bias resistor is.

The two knobs, Sensitivity and Threshold, control circuit. Sensitivity should be adjusted based on playing style - eg. Picking/strumming strength and Threshold determines how much starve should occur. Mark Hammer proposed that the RC network could be changed in order to increase/decrease the decay or even make the effect less aggressive overall. I postulate that the SSBS Fuck’s compression switch (heavy/low) goes between two different RC networks (R8, C4). In my own personal builds, I usually make C4 10uF. I find that values greater than 22uF drain too much power and take longer for the circuit to recover. Another proposal is to replace R8 with a potentiometer in series with a 50-100 ohm resistor in order to make an Attack control. The two foot switches are Bypass and Dynamic Sag. Bypass activates the drive and Dynamic Sag activates the Dynamic Sag.

The schematic for the dynamic sag footswitch bypass wiring is shared in the schematic under "Pedal Schematic or Practical Application." This will enable you to toggle the dynamic sag on or off. You can also opt for a monetary DPDT switch so that pressing/holding activates the sag and releasing immediately turns it off.

Shortcomings and Proposed Changes

Based on my analysis of the SSBS Overdrive PCB, my circuit is probably only 80% similar to what's in the SSBS Overdrive - but I do believe it is a fair approximation. In the future I would like to find a better way to control the rate of voltage change. I think a slew rate limiter is a plausible solution.

Updates

July 4, 2023

When my Skeptical Buffer schematic came under criticism - which I invite for the sake of correcting my work, I decided to proceed with using Simulation Program with Integrated Circuit Emphasis (SPICE). The program I chose was EveryCircuit. I plugged in the Dynamic Sag circuit and experimented with the different suggestions I posted in my original schematic. To my surprise, none of what I proposed did much. The one change that produced the most change - a more subdued voltage starve - was increasing R9, the resistor from Threshold to the base of the BJT.

Furthermore, as a result of this change, I am inclined to believe that the Compression switch on the SSBS overdrive toggles between two different resistors. The larger the value of R9, the higher the minimum voltage will be when the gain stage starves, and the smaller the value of R9, the lower the minimum voltage will be. Also in future builds, I have considered either omitting the Sensitivity control or making it an internal trimmer because I find that if you can fine tune R9, the circuit may not need a sensitivity knob and the Threshold control will offer more usability.

September 22, 2023

Dino from DEAD END FX has stated that their company plans to release a SSBS F*ck PCB. He has offered the following insight as to my research/development:

Theoretically, there is nothing wrong with the TG (Tone God) schemo. It really depends on the application, as well as the gain structure of what you want to influence with it. Where the F*ck is concerned, there is no Vref scheme in the power structure at all, meaning that the opamp input is not biased, and it works fine. Even with a guitar signal. The sag structure is more or less verbatim to the TG schemo, EXCEPT... there is one resistor to ground added at the back end of the envelope. You're not influencing the main rail voltage here, but rather, the bias voltage of one JFET within the audio gain structure. As such, minor (and I mean MINOR) tweaking might be required to get it to work with the destined application. From my breadboarding, I find that the resistor acts as a bleed control of the sag function (biasing the base of the NPN transistor). My experimenting led me to replace the resistor with a pot, which enhances the sag depending on the gain structure of the circuit being influenced, which is important. I found the F*ck that I had to lack sufficient "fade, crackle, pop" when the gain was turned up high, which using a pot instead of a fixed resistor value (as previously mentioned), helps compensate for.

As for the F*ck, I can tell you from experimentation that even the type of JFET's used as the VCR is important. The 2N5457's gave the smoothest voltage transitions, while others (ex.PF5102) were much more abrupt. At the initial attack, the bias voltage of the JFET used in the audio gain side of the circuit can drop from it's normal 4.5v to less than 1v when being "sagged", adequately providing the "edge of tube disaster" effect that is sought after.

Progress Report: Envelope Follower, Voltage Controlled FETS, and the Elusive "Dynamic Power Supply Sag"

“...the highlight of this pedal is a unique blown amp feature which provides crackles, compression, gating and eq changes similar to that of a tube amp pushed past it’s (sic) limits...”

— Smallsound/Bigsound F- Ovedrive Manual

The Smallsound/Bigsound F- Overdrive has been on the market for some time, and alongside its amazing overdrive pedal, is a highly unique feature: A Crackle/Burst/Threshold system that is an enveloper follower circuit based on the Dr. Quack schematic. The Dr. Quack is an off-shoot of the Dr. Q by Electro-Harmonix. The Dr. Quack was used by The Tone God to create The Punisher, and the Punisher is widely said to be the schematic behind the F- Overdrive’s Crackle/Burst/Threshold (CBT) feature.

Go to 3:35 for demonstration.

“The Punisher is a dynamic power supply sag simulator. The Punisher is not an audio effect but a companion effect that is to be used to control another effect to provide new sounds. You plug your audio signal and power supply into Punisher in series and connect the output audio signal and power supply into the effect to be controlled.”

— The Tone God, "Punisher"

The envelope follower feeds a control voltage to a JFET, which in turn alters its resistance, producing a voltage-controlled resistor in the process. The JFET is run in series with the power supply to a JFET stage, thus mis-biasing the JFET stage in the process. The resulting sounds is said to emulate that of an amp on the verge of explosion whose tubes wax and wane.

For further reading, I suggest visiting this page and scrolling down to “Driving Effects Circuits with the LFO Waveforms.”

Having built the Smallsound/Bigsound (SSBS) Mini, I started to set my eyes on the SSBS F- Oveddrive. However, I had little clue as to the underworkings of the CBT. Initially I thought the circuit was some sort of phaser/tremolo circuit that altered the volume/output of a specific stage of the SSBS Mini. But I wasn't too sure. Also, pictures of the F- Overdrive PCB did not look anything like a tremolo/phaser circuit. Although, oddly enough, the concepts of JFETs as voltage-controlled resistors (VCRs) is closely associated with phasers.

I began researching the most readily available information out there: It's a voltage-controlled envelope follower circuit. I pulled up schematics for the Glowfly Retroflect and Frantone Vibutron, which features envelope followers affecting FETs. I shot off a few Google searches, which immediately turned up the Dr. Q schematic. Unfortunately, there was very little information about how that particular circuit fit in the CBT. I wish I could say that the subsequent research was streamlined once I aligned the CBT circuit with the Dr. Q schematic, but in reality, I spent quite a few days perusing various forums to no avail. A culmination of 2-3 days of digging deep into forums helped me learn that the envelope follower controlled a "resistor," the resistor was associated with the second stage of the SSBS Mini, and the overall result was the JFET being mis-biased.

Simple enough, right?

A few more hours of Google searching lead me to Reddit where someone used the keywords, "dynamic sag." (Coincidently, I had just completed a LM317-based sag unit.) A few more Google searches lead me to the Pusnisher circuit. Furthermore, if you Google the Punisher circuit, the CBT pops-up alongside it!

I breadboarded the circuit, but I could not get it to function. Furthermore, I found several posts of individuals identifying the Punisher as being a key part of the CBT, but only a handful of people who successfully implemented the circuit: The Tone God, author of the circuit; some random guy on DIYStompboxes; and the mind behind SSBS. After a day of troubleshooting the circuit, I crowdsourced Reddit and DIYStompboxes for assistance.

The key piece missing from the original schematic was just a pair of resistors: The op-amp was not biased! The addition of the transistors allowed the circuit to work. I also experimented/ with and implemented some other changes from forum suggestions and the Dr. Q, which allowed the envelope signal to be much stronger. However, one change that helped was found on accident: I was trying out different biasing resistors and left one resistor in. This made the circuit work much better for reasons I am unsure of.

Schematic with many changes.

And like that, I figured it all out...sort of.

See, I call this "Progress Report" because there are still a few of issues to solve:

• How to calculate the resistance that is bestowed upon by the VCR/JFET.

• How to make the circuit smoother as the attack/release is really fast.

• How to elicit more crackling. I actually did manage to do this when I was using two 1M ohm resistors for biasing, but the envelope signal was not as strong.

• What the “Heavy/Light” function changes. I think it either changes the electrolytic capacitor attached to red LED or it may be the Attack function seen in the Nurse Quacky.

As for now, this is it.

Left side is the Crackle/Burst/Threshold function. On the right is a dummy unit to test the circuit. The dummy unit based on the Fairfield Circuitry Barbershop, which is like a minimalist SSBS Mini.

Updates:

• May 8, 2023: Grammar and spelling errors corrected.

Variable Voltage Regulator Pedal, or the Dying Battery Simulator Pedal

A dying battery simulator allows the voltage of an analog pedal to be reduced to a suboptimal level in order to mimic a dying battery effect. At lower voltages, distortion/overdrive/fuzz pedals are said to produce different sonic textures. For example, fuzz pedals may become gated, and high gain distortion pedals may take on a more subtle overdriven sound.

While the design is presented in pedal form, the schematic can be minimized to fit into another pedal in order to allow a user the ability to vary voltage within a pedal. Furthermore, pedal builders can also place the design into their test boxes in order to provide a wide range of voltages. In the lattermost case, building a 35V charge pump with a LM317 would allow a builder to have a wide breadth of voltages from ~32V to 1.25V - although I personally believe, 25V, 18V, 9V, and 4.5V would be satisfactory for many pedal designs.

The LM317 is a 3-terminal adjustable regulator that is capable of outputting a voltage range of 1.25 V to 37 V depending on input voltage. The regulator has three terminals: Input, Output, and Adjust.

Source: Wikipedia

When the JHS Volture first came out, I immediately knew it wasn't simply a potentiometer in series with a power supply. As soon as gut shots for the Volture became available, it was pretty clear the circuit was based on RG Keen's original concept, which in turn is probably from the actual datasheet for the LM317.

For my version, I utilized a video from Volos Projects to corroborate/refine the design. I then added in a LED voltmeter from Amazon and an internal Age control. New batteries differ from old batteries in that, “while new batteries can put out a lot of current, old ones can't - they develop a high internal resistance.” The Age knobs acts a rheostat. Putting “a resistor on the output of our regulator…will fake the internal resistance of the battery" (source). Voltmeter addition was inspired by MAS Effects’ UV Meter Drive and Silktone Fuzz. I did attempt to have a rotary switch for a Battery Simulator (carbon zinc, alkaline, “wall wort”) to play off the JHS April Fool’s joke, but the idea was too ambitious and overly complicated to implement. I also would've loved the Age control to be an actual potentiometer, but I could not find a reasonably priced and appropriately sized 100 ohm potentiometer.

This circuit allows you to go from ~7.5VDC to 1.25VDC which is similar to the Voluture specs on the JHS website ("...adjustable voltage from 7.5VDC down to 1.25VDC..."). The basic circuit is 5-6 different components and takes a matter of minutes to piece together.

Schematic.

Two changes I would've liked to implement were:

• Reverse polarity protection.

• 100uF to 1000uF electrolytic output capacitor for stability.

Demonstration. Note that the LED I used was very cheap and requires calibration. The read-out is not as smooth or accurate as I would like, but I am sure if a better, well-calibrated LED was used, the result would be better.

Files:

• (Pending)