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Grübelzwang
Overdrive A Take On The Take On The Fulltone OCD (And Some New Findings) last update: Dec. 13, 2024 Copyright 2024 by H.
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| MAIN PAGE>MUSIC STUFF>GRUEBELZWANG Index  More
Disorder?The Best From East And WestThe Clipper Section And
Reference VoltagesA Look At The Distortion
Generators Treble
ManagementBuffering18V SupplyVerdictMore
ExperienceReferenceUpdate History
Back To Index More Disorder?
Writings on OCDs abound, so indeed this writing would have been superfluous if id did not contain anything new. It is probably aimed towards the experienced builder. Please take note that I do not supply a ready-to-use layout. I see my function predominantly as educational and an inspiration to learn. Recently I stopped by Guitartest to look at the Baldringer Drive. Over the years quite a respectable number of pedals have gone through the hands of this site' owner, so I trust his expertise to an extent. He thinks that Fulltone's OCD sounds quite similar to the Baldringer. We will come back to that claim during the course of this writing. Years ago I had acquired one of the OCD sound-a-like kits, and I DID NOT LIKE IT. Granted, those kits are a tinkerer's nightmare. They are a production unit in a way and do not lend themselves readily to modifications. I did what I could, but the outcome was not pleasing, so I gave it away. Re-fueled, I wanted to look at this again, but this time made my own PCB. I adopted Fred Briggs' Super-OCD schematic and added some simple buffering, which we will hear more on later. This is the basis upon which we will build. I had some quite surprising insights during building this unit, which may be of interest to the kind reader. The Best From East And West So I basically just used Fred Briggs'
super-OCD. I wired it as a
conventional bass control, which increases bass
clockwise. Although boost and cut is just a
way of looking at things, having a cutting control,
whose action increases clockwise while decreasing
the affected parameter, is confusing, despite the fact
that this was technically correct.
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The very type of control has appeared on numerous other pedals like Wampler's Euphoria and the Timmy. It is very useful to tame the low-end.
Briggs' version also employs a more effective tone control (treble control), which is relocated to the output. It is a variant of an actually very old type of treble control, which has been made popular by Jack Orman from AMZ under the name of "stupidly wonderful tone control (SWTC)". On the FSB thread somebody used a different variation for the super-OCD, which cuts to one side and boosts to the other side, but I found this unnecessary and unbalanced. Fred changed the second OPA to a variable boost, something I do not need. It also may have other side effects (more on this later). The S-OCD uses another of Orman's ideas, a saturation control for the MOSFETs.
In the following we will look into those sections with a microscope. The Clipper Section And Reference Voltages The circuit was working right from the start (erm, kind of...), while a few time constants needed time-consuming fine-tuning. Unfortunately, from a certain gain setting on, the circuit started motor-boating, which resulted in the hated "put-put-put" sound resembling an idling motor-boat engine. Its frequency increased with OPA#1 gain. The 'scope revealed huge fluctuations on the Vref rail. Now I've had this before, but only with valve amplifiers, where the cause usually was some supply filter cap time constants. I never had this on OPAs in conjunction with PCBs. And believe me, after so many years of PCB design I incorporated all known tricks of stabilizing circuits into my PCB. After visiting the usual suspects I found that there is quite some current dumped into Vref by the MOSFETs when they are conducting, enough to corrode the Vref voltage node. Some OPAs go into latch-up when they are hitting the rails, which means they are "hanging there" for an indefinitely small period of time. This may be part of the signature "sound" a certain OPA imparts to a circuit when clipping (more on this in a minute). Briggs also references the input's feedback return paths to Vref (like the lower leg of the gain setting R/Cs), which technically works, but also has the potential to corrode Vref. Several reputable designers strongly advise against that practice.
Since the original OCD works with a configuration like that, I attribute the malfunction to an coincidence of unhappy trace layout decisions on my PCB. Stuff like this is a bitch to track down. The cure for this was to reference the clipper section to GND instead of Vref via a large enough electrolytic capacitor (22µF). Alongside, all feedback return nodes are referenced to GND. Back To Index A Look At The Distortion Generators In the course of observing the waveforms with a 'scope I found that OPA#2 is contributing quite a lot of hard distortion by constricting the signal swing with its internal high and low boundaries. In my version I have added the Germanium diodes some units have in series with the MOSFETS. I have made provisions to bypass those with a switch selectively for evaluating the sonic impact of the resulting combinations.
The MOSFET body diode consumes 0.7 volts, and together with the circa 0.2 volts of a Germanium diode we end up with a clipping boundary of roughly +/- 1 volt, or 2V peak-to-peak. Using compliance will increase those values even more. Now the subsequent OPA stage has a gain of four in the standard version, which will invariable clip the input signal hard when slammed with two volts on a nine volt supply. This happens very early on the gain pot travel and is responsible for a certain gnarly tone (you will notice its absence when using a higher supply voltage; see later). Note that the driving OPA stage clips the latest (being first in the row), but will eventually clip too. OPA clipping has the reputation of sounding bad, but several well respected units like the MXR distortion + or the Crowther Hot Cake and many more use just that effect. The Hot Cake relies solely on op-amp clipping.
Fred Briggs talks
about using a "better" OPA for the OCD, but
consider that the TL082 was chosen with purpose,
much as the µA741 sounds best for the Hot
Cake. It may be worth trying a 1458 for
that reason.
I am ready to believe that Fulltone has wisely chosen the balance between the rather soft signal bounding by the clipper section (despite the fact that this topology is commonly called "hard clipping") and the merciless constriction at the boundaries of the OPA close to the rails by setting the latter's gain a certain way. Besides seeing no need of even more drive voltage (who needs more than a 9Vp-p drive signal?) the delicate balance that mark the transition from one distortion engine to the next may be impaired by fiddling with OPA#2 gain. Increasing OPA#2's gain will make this stage clip earlier.
Using a different type of OPA, as suggested by Briggs, may or may not be an improvement, particularly since the TL082 is not apparently obtrusive. This circuit appears purposefully designed around a low grade OPA. I would definitely stay away from "modern" high-bred OPAs in this respect. Again, a 1458 or something like that may work well. Incidentally, this is said to be a double µA741, the latter which is the heart of a Hot Cake. Note also, that different OPAs do vary hugely in
Let's look at the way the OPAs clip. Clipping a signal symmetrically or asymmetrically makes a huge tonal difference. A short aside:
Some older
OCD versions try to provoke this asymmetry
by adding a single germanium diode to one
of the MOSFETs. However, its contribution gets
swamped by the vastly dominant asymmetrical
distortion of the subsequent OPA, even if its
purpose were to try to mitigate the strong
asymmetry. Indeed,
toggling the "hard clipper" section (MOSFETs
etc.) between symmetric and asymmetric
(as the Grübelzwang allows for) does
not have an effect as big as you may expect,
which is why it can be assumed that most
incarnations of the OCD sound very
similar, as is frequently expressed. Yet with
a very small change we can make the OPAs
clip symmetric at will: we raise the
reference voltage slightly.
I
followed the path Crowther has taken
by elevating the bias voltage of the OPAs a
few 100 millivolts or so. The exact value
has to be determined trimming a pot while
watching the scope for symmetric clipping. I made
this switchable, so that I can quickly jump
from asymmetric to symmetric. In doing so I
found that there is some component that
withdraws into the back in symmetric mode.
It sounds less distorted overall, although
there is the same amount of distortion
present, just because of a different spread
of harmonics in the spectrum. With this
active, turning the MOSFET assembly switch
to "asymmetric" really makes a noticeable
different. There is the interesting
aggressive component but transiting into
something smooth. Very pleasant. I was
astounded, what huge influence that has, and
how soft an OPA in distress can sound. Note
that R.O.G. have deliberately added
(symmetric) hard clipping sections to their
Thunderbird
Marshall emulator. Obviously those are
needed and indeed, some places in a tube amp
are hard clipping. Note also that an
output stage of a tube amp is built
symmetric, and as asymmetric the preamp
stages may clip, by the time the signal hits
the power stage, it is mostly symmetric
again (at least for push-pull amps). So pumping a stock OCD (with asymmetric clipping) into a smoldering amp may sound perfect, while a typical bedroom setup with a clean amp and higher settings of input gain may profit from a symmetric OPA clipping. Activating one of the GE diodes at the MOSFET can bring back the picking articulation and thrill.
Back To Index Treble Management The usual
resistor/capacitor series shunt arrangement is not
very useful. The SWTC
(first drawing) is much more effective. Indeed it
incorporates all of the tones the funny OCD tone
switch settings provide, but with all in between
nuances too.
Back
To Index
However, the third circuit shown in SWTC , as suggested on the FSB thread, did not work well right off the bat. It was too shrill and had limited adjustment range towards reducing treble. This could have likely been remedied with some more tinkering, but adding more treble to this circuit seemed superfluous in the vicinity of a bass cut control. Remember, less bass lets a sound appear more trebly due to the yin-yang of tone. Moreover, cutting bass in a distortion mechanism usually yields better results than boosting treble. From my past Björn Juhl experiences I remembered the extended feedback modes he uses in the Honeybee and its siblings. Unlike gazillions of designers before him, he feeds back higher frequencies from the point of their occurrence, such as the clipper stage. This is not the same as dealing with higher harmonics at a spot after or before. It feels like a dynamic thing, like a dynamic equalizer. I left the first OPA's HF limiting capacitor with 100pF (original: 220pF), but fed back from right at the junction of the 10k resistor and the clipper(s) with another 100pF cap. In this case I prefer ceramics. For HF limiting purposes they are unsurpassed. I considered a second feedback node from right after the SWTC, but this seemed unnecessary. The 1nF cap at the clipper stage is not doing much. Together with the 10k resistor this yields a cross-over frequency of 15kHz. So if we wanted to hear any change we would have to add something like 3.9nF for a total of 5. Changing the feeding resistor, as Briggs suggests, not only changes the R/C time constant (and not much anyway), but also changes the way the MOSFET section behaves. I recommend to leave this alone. The tone pot works well with the starting values Orman recommends. At its center position, the unit sounds like the unaltered bypass signal upon soft strums, and this is how I set it. Higher gain settings require some backing off. Back To Index Buffering Input impedance may be acceptable as is, but output impedance is certainly on the high side. Many effects are made this way, and you usually get away with murder. Most time this decision is buck-driven. Briggs conceded in a forum discussion that of course buffering would be the proper way to go about. I took the buffering scheme from AION's Xotic AC Booster. This is noteworthy because of the way they derived the bias voltages for the transistors. On the output, they just used Vref, which is exactly at half supply. The buffer's output is a diode drop lower, which would theoretically limit the output swing. However, the signals are likely small at this place. On the input they use an elevated bias voltage, which is produced by adding a single resistor into the divider chain. The resulting voltage Vref2 is about a diode drop higher than Vref, so the input can swing symmetrically for a hypothetical +/- 4 Volts. Very likely unnecessary, but cheap and nice. In my opinion, they should have added some filtering too, like it is applied on the first transistor on the Roger Mayer Axis Fuzz. I used exactly the same values, albeit derived from Vref2 and not VCC. I came up with this during my endeavors to track down the motorboating issue. This pretty much isolates Vref2 from any variation on Vref. Some of the myriad of OCD clones have a (transistor) output buffer incorporated, and somebody was on a mission to "replace the horrible sounding transistor buffer". I do not share this opinion, in fact I feel it is ridiculous to complain about the influence of a transistor that buffers a circuit that produces nothing but distortion. It is also ridiculous considering how much the subject of proper impedance matching is neglected in general. Adding symmetry to the "output stage" (OPA#2) is most easily accomplished by amending above mentioned reference voltage divider. Adding a 10k trim pot temporarily across their 6.8kOhm resistor helps to determine the value required to achieve evenly clipped lobes, while monitoring the output of OPA#2 with a scope. This will work on any supply voltage; the divider is ratiometric. I you prefer to have this option switchable, so a simple spdt toggle switch can toggle between symmetric and asymmetric. Back To Index 18V Supply Because of what has been found further up on the way the OCD's "distortion generators" are stacked and interact, the charge-pumped 18V supply changes the distorted tone a lot (note that it is not really 18V - the circuit is lossy. The number is usually chosen for marketing purposes and for convenience). OPA#2 will be driven into clipping later. It modifies the balance between the rather soft clipping MOSFET stage and the brickwall hard limiting OPA stage. De facto, it takes OPA#2's clipping mostly out of the equation, which will refer the clipping action mostly to the MOSFET stage and also results in a considerable increase in loudness. Increasing the headroom of OPA#2 thus is the complementary action to increasing the gain of OPA#2, and it reduces the grit added by the op-amp clipping. This can be perceived good or bad. Not saying that his might not sound useful, but it may not be what constitutes a typical OCD overdrive sound, and it might not sound very tubey, if such a thing exists at all. This mode feels like extending the lower drive settings on normal mode. Switching the voltage while having the effect activated produces a magnificent "thump", so this should be avoided during a performance. The thump itself is inevitable without resorting to complicated measures, because it switches DC. If Fred Briggs likes the "extra headroom to the circuit to stop the op-amp clipping from fogging up the tone", he eliminates what constitutes the OCD. Back To Index Verdict Indeed this unit sounds and reacts different that the numerous other dirt boxes I have. It is largely devoid of the fizz others generate and it is addictive in a way. On a 1:1 comparison the Grübelzwang sounds noticeably unprocessed and natural. Time will tell which of the added options deserves to consume front panel estate or remain condemned to dwell in the hidden. I cannot say if the Grübelzwang truly bears resemblance to the Baldringer drive, but it definitely does not change the character of my amp, and it truly sounds like what the Six-Pack-Joe would call tubey.
The Baldringer is a much evolved design and allows for a lot more of tone shaping, so this is not a fair comparison, letting alone its price tag. The Grübelzwang on the other hand, as different to the stock (clone) I had, leaves nothing much to be desired, so I am content with what I have now. For a while. May this writing help others to select wisely. Back To Index More Experience I have been waiting a fair while for the delivery of the correct law pots and the case. Those DIY suppliers are becoming increasingly unreliable and slow. A nuisance. Housing the PCB and using the right law and pots makes it first possible to really play with the settings and evaluate things. At first glance the switches don´t change much, but, as mentioned earlier, setting the op-amp clipping to "symmetric" changes the generated overtones dramatically. There is a very obvious difference in bass, because there is no artificial bass frequencies generated. It is as if something retracted into the background. You need to play with a setting for a short while before flipping a switch, and then you feel like "ah, that is really different". If I were to suggest a certain mod for an OCD, than it is this one. The SWTC works excellent, and the bass cut works wonders too. I believe an unhappy choice of bass voicing in combination with the artificial bass makes up what people call "dark sounding". Bass has to be set judiciously. The bass setting appears to have a big influence on the perception of the overall tone. Bass is particularly detrimental if you are playing "Jimi style" by riding the volume pot. The bass mod is the second mod I can suggest without reservation. I find that I like the minimum setting of the "saturation" pot (maximum series resistance) best. This control works well too, cudos to Fred Briggs (although it is old hat stuff). Yet, although signal shape changes dramatically over the travel of the "saturation" pot if viewed on the 'scope, sound does not; but there is a clear difference in tone between the minimum and maximum settings. I prefer the minimum setting. My "SI - GE - HALF" switch makes the sounds most polished on the "GE" setting (Germanium diodes in series with both MOSFETs) and most compressed (as expected) for the MOSFETs only. The "half" setting subtly re-introduces some of the evil crunch that has been lost by making the OPA clip symmetrically. It is astounding how much such a small change impacts tone. The elevated supply (it is not really 18V...) as expected yields a much cleaner tone, less crunchy, with a touch of preamp tube distortion, and with less compression. This can be useful too for a nearly clean guitar piece with a slight edge, but this can be had from other units too. The box is very quiet overall. For a 125B enclosure, this build was borderline due to the sheer amount of controls and wires. I suppose this is inevitable for an experimental device, but if some of those controls and switches turn out to be static, they will be removed from the front panel as a "performance" control. Back To Index [1] Merlin Blencove (a.k.a. Valve Wizard) , Designing Tube Preamps For Guitar And Bass (2nd ed.), Chapters 1.11 on Harmonic Distortion and 1.12 Intermodulation Distortion, pp. 15-18 [2] Merlin Blencove (a.k.a. Valve Wizard) , Designing Tube Preamps For Guitar And Bass (2nd ed.), Chapter 14 on Preamp Topology, p. 254 Update History
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