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Fulltone - OCD (Obsessive Compulsive Drive)

Fulltone OCD V4

Fulltone, an American effects manufacturer founded by Michael Fuller, has been active since 1991 in Los Angeles, California. Although their production is not quite in mass production scale, Fulltone has delivered tens of thousands of high quality hand-built pedals and sound devices globally. More than 25'000 OCD:s have already at this point been sold, thus it is questionable if the term "boutique" can be applied to their production, although their appearance might qualify for it.

The OCD (Obsessive Compulsive Drive) has lately received a lot of attention through outstanding reviews and by winning the Guitar Player "Editors Pick"-award (Read more here.) The OCD has found its way to the pedalboard of great guitar heroes like Robin Trower (listen to an excellent sound clip on Fulltone's website) and Paul Gilbert (Guitar One -Issue 12/06). So let's have a closer look at the unit.


The Function and Sound of the OCD

The OCD is basically a booster/overdrive-type distortion box, which provides a soundscape ranging from mild clean boosting all the way to a really gritty, almost fuzz-type distortion. The pedal offer a maximum of around 30 dB of volume gain (a voltage gain of 1000), which basically means that your neighbors will receive their share as well.

The controls are simple and straightforward: Three knobs labeled Volume, Tone, and Drive, and one switch to select between LP (Low Peak) and HP (High Peak) modes. The Volume- and Drive- controls affect the signal in a familiar way controlling the output volume level and the amount of distortion, respectively. The Tone control on the other hand affects only the high frequency content, leaving the bass signals pretty much unaffected when the high range is tuned. Thus you don't lose the bottom end when dialing in more high-end.

According to the manufacturer, the LP mode is designed to affect the original sound as little as possible, which especially in a clean boost is very desirable. The HP mode on the other hand has a more pronounced high end with a sharper frequency response and boosted volume.

Up until now this pedal seems like a quite common overdrive pedal - right? But there is one feature on this pedal, which distinguishes it from most other overdrives – the dynamics. The sound of the pedal follows the intensity of the playing and the volume level in a very profound way. Even with the Drive control turned up quite a bit, the sound can be kept almost clean by just picking very gently or rolling down the volume of your instrument a bit. By increasing the volume and/or increasing the intensity of your playing (i.e. really digging your pick into the strings) the sound swells up and increases to a growling, full-bodied distortion – the playing touch thus dictates the amount of distortion of the signal, without changing the output volume level too much. This pedal works and sounds pretty much like you would expect a good tube amplifier with tubes at the verge of cooking to sound.

A clear example of the dynamic behavior of the OCD can be seen by examining the input and output signal waveforms (voltage) while playing the same sample through the OCD with different volume levels without touching the settings of the pedal. A typical plot of the input and output voltage swings of a sample played at guitar volume levels of 10, 7, 4 and 2 (on a regular volume pot) looks like this:


OCD Input/Output waveforms
(Click to enlarge)
Play
Input sample audio


Play
Output sample audio


As can be seen from the waveforms, the absolute voltage level ( volume ) stays quite constant in the output waveforms, although a dramatic decrease in volume level is noticed in the input signal level. From the audio samples you can clearly hear how the amount of overdrive radically decreases as the input volume level decreases. (The used OCD for this sample was a V2, the exact settings can be seen in the sound sample section)

The sound remains very transparent, very clear and punchy. Even broken chords played with heavy distortion remain very clear and articulate. The low frequencies cut through strongly and the sound remains strong and full, with lots of high end and great tone.


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TECHNICAL DETAILS:
 
First off, the pedal has true-bypass, meaning that the signal is unaffected when the effect is off. The blue led (red in v4) is bright enough to lighten up half the stage (well... at least half your pedalboard) in dark surroundings.
During its short history the OCD has already had some revisions and modifications, and the currently sold units are already version 4.

It was slightly harder than usual to explore the functionality of the OCD circuit - the internet did not offer a single schematic, so the circuit topology had to be reverse engineered. This process was trickier than expected, since opening up a version 1 unit revealed that the whole circuit board was covered with dark grey epoxy, thus an inspection of the used parts was impossible. In version 2 only half of the board was covered with epoxy, but the markings and labels of the components visible were scratched away to conceal what components they were. Version 3 and 4 finally made a closer inspection possible - no epoxy was used and the labels on the components are intact. It seems that the reputation and success of the pedal has been established firm enough for Fulltone, so that concealing the circuit topology is not necessary anymore.

The overdrive/distortion portion of the circuitry is based on a simple opamp gain stage with negative feedback and with a pair of cross coupled clipping diodes at the output to clip the signal and thus create the distortion. A similar circuit topology is used in many other distortion pedals as well (e.g. the Proco Rat), but Fulltone uses a very clever modification that significantly separates this unit from most others. First off, instead of traditional diodes Fulltone uses NMOS transistors in diode configuration (gate and drain short-circuited), which already accredits for a great difference in tone. But the biggest difference is this: In the Proco Rat the two cross coupled clipping diodes are directly connected to ground, meaning that as soon as the signal exceeds the threshold voltage Vth of the diodes (usually 0.7V), the signal gets clipped resulting in an output signal with a peak voltage of +/-Vth independent of the input signal amplitude. This naturally creates the wanted distortion, but also loses the dynamics of the signal. In the OCD, instead of connecting the diodes to ground potential, Fulltone uses a floating bias point which is connected to the input of the opamp stage. This creates a clipping threshold proportional to the input signal amplitude, resulting in a strong dependency of the amount of distortion created to the input signal. This is the core idea of the enormous dynamics of the OCD.

In version 4 the circuit board has undergone numerous modifications, so a do-it-yourself upgrade from an older version to v4 will be hard to do. A significant tonal modification has been the addition of a germanium diode in the clipping stage, resulting in asymmetric clipping. This creates stronger even-order harmonic content, whereas symmetric clipping will result in mainly odd-order harmonics. The germanium diodes have a lower forward voltage threshold than their silicon counterparts, which means they will start soft clipping earlier than and sound more natural. Thus the effect of the added asymmetrical clipping is not as abrupt and obvious as it would be with silicon diodes, but it still adds even-order harmonic distortion products giving the sound a new flavor. The added diode increases the clipping threshold of that specific clipping branch, resulting in less distortion for equal output signal than with the previous configuration.

The Drive knob controls the amount of feedback of the opamp stage thus controlling its gain. A second opamp stage with constant negative feedback is used to buffer the signal after the distortion section, which then is followed by a parallel RC-pole for tone control (Tone-knob) and a series resistor (Volume-knob) to control the output signal level. The used operational amplifiers are a dual JFET opamp with huge voltage gain, thus providing the enormous voltage gain of the OCD.

The OCD operates with a supply voltage ranging from 9V to 18V. Using a higher supply voltage will affect the clipping threshold through the floating bias point and thus give more headroom to the signal. This effect depends strongly on the used setup and the signal swing at the input of the OCD - with the setup used for these sound samples the effect was too small to hear any difference, but if the OCD is driving a high power amplifier with lots of headroom in itself, the effect of raising the supply voltage might be obvious.

The circuit is a very clever design, although nothing revolutionary in the electronics world.
The OCD:s are hand built and look very professional with clean and smooth soldering, the components and closure are high quality and durable.


COMPARISON – Versions 1, 2, 3 and 4:

Fulltone OCD V1 V2 V3


The version of the OCD can be seen from the serial number. Here a short summary of the different versions, the modifications made and how they according to the manufacturer affect the signal:
  Version 1 Version 2 Version 3 Version 4
Serial numbers:
#000 - #4563
#4564 - #9473
#9474 - #16060 *NOTE
from #16061 upward *NOTE
Changes:  
-Volume-pot changed
(100K->500K)
-Drive-pot changed
(500K->1Meg)
(this modification is made to unit with serial numbers above #9127)
-Tone-capacitor changed
(0.1u -> 0.047u)
-Tone-pot changed
(25k -> 10k)
-Added germanium diode in clipping section
-Volume control pot tapering changed (logarithmic)
-Opamp feedback loop capacitor and tone control capacitor values lowered for decreased bass response
Sound:
-Lots of bass
-lots of high end
-Less bass
-More middle
-Slightly softer high end
-Stronger midrange
-Increased sustain
-Improved Tone-control
-Less bass
-Richer even order harmonic distortion due to asymmetrical clipping
-Improved Volume-control
Images: OCD Circuit Board V1
Circuit board V1
OCD Circuit Board V2
Circuit board V2
OCD Circuit Board V3
Circuit board V3
OCD Circuit Board V4
Circuit board V4
*NOTE: Some OCDs with a serial number below #16061 have been marked v4 on the interior, nevertheless they still have the v3 circuit board and do not have all the modifications of the v4. A rumor has been going around that the labeling of these v3:s as v4:s was a joke from the assemblers - weather or not they truly differ from the v3:s is not known to the author.


Sound differences:

The change from version 1 to version 2 is soundwise rather small. The Volume-control is slightly smoother, but the overall sound is affected very little. The high end is slightly softer and the bass maybe a little bit weaker, but all in all a very subtle change.

Moving on the next version the differences in sound are significantly larger. The amount of distortion and volume is increased, the midrange strongly boosted and a slight touch of compression is noticeable, whereas the shiny high end is reduced. The structure of the distortion is slightly smoother and rounder, not quite as gnarly and gritty as in the previous versions. The differences are not radical here either, but definitely very noticeable.

Version 4 is again not a dramatic but very noticeable change. The asymmetric clipping creates stronger harmonic content and makes the sound rounder and smoother. Compared to the v3, the v4 has now a higher clipping threshold resulting in a larger voltage swing before clipping occurs - the overall amount of distortion is thus slightly less giving a more transparent sound than the v3.



Summary:

All versions of the OCD are very good, high quality overdrive pedals. Its enormous dynamic range clearly distinguishes this pedal from the mainstream of overdrives and gives it a unique soundscape and flavor. Nevertheless, the large dynamics combined with a very transparent and articulate sound set high requirements on the playing and on the used equipment. Unlike with many other overdrive units, the OCD cannot be used to cover up a bad sound from your guitar or amplifier - on the other hand accidental unevenness in playing intensity will result in variations in the amount of distortion, which might sound strange (- of course this could be alleviated with a compressor in front of the pedal).
Due to its nature the OCD may not be the right choice of overdrive for everyone, although its tone and character are really great. For a beginning comping guitarist seeking for his/her first overdrive pedal, the OCD may be way too dynamic, but for a guitar player, who seeks after great tone and an overdrive that strongly interacts with the playing intensity, this pedal is definitely top of the line.
Version 3 of the OCD is slightly more forgiving in sound and dynamics and its stronger middle-boost and distortion make it more suitable for comping and riffing, but it lacks some of the crystal clear transparency and articulate character with enormous dynamic range. The asymmetric clipping of the v4 give it a slightly new flavor with richer harmonics compared to the previous version and again increased transparency from v3.


Links and additional information:

  • Fulltone Official website
  • Reviews at Harmony-Central


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