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Op-amp And
Diode Clipping Stages Revisited A look at op-amps, diodes and distortion mechanisms in the lights of a few commercial pedals last update: May 14, 2011 Copyright
2010-21
by H.
Gragger. All Rights Reserved. All information provided herein is
destined for educational and D.I.Y. purposes only. Commercial re-sale,
distribution or usage of artwork without explicit written permission of
the author is strictly prohibited.The original units with
their
associated trade-names are subject to the copyright
of the individual copyright owner. The Author is by no means affiliated
with any of those companies. References to trade names are made for
educational purposes only.By reading the information provided here you
agree to the Terms
of Use.
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| MAIN PAGE>MUSIC
STUFF>DIODE_CLIPPERS Index   The Birth Of Component
MojoThe KoTīs Spice
IngredientsDiodes In Clipping Stages
Diode Bounding - A Perfect Approach
To Tube ToneMore Tube ToneSlew Rate
Limiting And Psycho-AcousticsMore
Possible Sources Of OPA DistortionReferenceThe Birth Of Component Mojo I
do not want to add to the list of the guys talking about the virtues of
a certain semiconductor "sound" and where to get that maybe long
obsoleted device.
Since everybody rants about those mojo components, they are readily available these days and I would be willing to buy them just for convenience, if there were not this uncomfortable feeling remaining, that I do not really know why they are preferable over some other type. Sometimes the choice of a certain component in a device happened accidentally, for no particular conscious reason. It was later that it turned out that the choice was crucial. The often quoted TS series op-amps for example were initially chosen for profane reasons only: availability and price. Later it is hard, if not impossible, to find out what made the device so special. But we can look at a few milestones at least. In the verge of building a King of Tone clone, I looked at the diodes used there and what allegedly makes them so special. Back To Index The KoTīs Spice Ingredients
It becomes obvious that they are not very special at all and it may thus appear a waste of time and money to pursue those exotic types. As the story evolved for the opaīs used in the tube screamers it very likely did for those diodes - they just were around in millions at the time (or place) the circuit was developed. (Note: I do not say the specific type of diode was chosen accidentally). Back To Index Diodes In Clipping Stages Diode clipping circuits can be roughly divided into two kinds of circuit:
The
original TS falls into the first category. Read about all its secrets
in R.G.Keens
article.
A pair of anti-parallel
diodes in the feedback loop limits the amount of voltage the output of
an operational
amplifier can produce. This interacts with the opaīs gain.
Allegedly it is even important if the diodes are in the feedback path
of a non inverting
amplifier rather than an inverting as some article
claims.
A series shunt type clipping circuit follows a driving stage (like an opa) and is fed by a current limiting resistor. This does not interact with the opaīs gain. The Fulltone OCD incorporates exactly that topology, and, indeed, the distortion section of the KoT does too. At the time of writing it is not known to the author, if there is any tonal difference between those two methods provided they are set up comparably and the headroom is sufficient. More investigation is in order. Both types of clippers on their own will limit the voltage across them to their threshold voltage. For a silicon diode, this is 0.6 to 0.7 Volts. In this region the magic happens and thatīs where all the mojo comes in, because diodes will differ in the way they start to cross into conduction. Somebody suggested the usage of the intrinsic (parasitic) diode of a MOSFET. While this basically works, they are comparably slow, although their on-resistance is low. Although it might sound good to somebodyīs ears, it will certainly not serve well to replace the exotic diodes asked for in this circuit. Similar things apply for standard rectifier diodes like the 1N400x series and even more so for LEDīs. I vastly appreciate Madbeanīs work, which was the basis for my tone queen project, but technically I disagree on his suggestions for diode subbing for the above mentioned reasons. It turned out that MOSFETS do sound good for the series shunt clipping part despite their obvious technical difference. Shottky diodes are fast too (even faster) and may work, but more of them would have to be stacked for experimentation, since they have a voltage threshold that is about 1/3 of a standard rectifier diode, otherwise the voltage balance will be impaired which may in turn impair the dynamic behaviour that makes this stompbox so unique. I am not claiming, that using those diodes is essential to the sound, but assumed it is, any of the listed replacement diodes will perform exactly the same and are readily available. And I am sure, there are lots more with the same specs. Anything beyond that... is mojo... [In retrospect, the 1N4148 do not sound overwhelming in a stock series shunt configuration as the KoT has it. The MOSFETS sound much better. See my article dedicated to the tone queen...] Back To Index After
all, with tube screamers and all the clones in its wake, we are after tube tone.
A
tube clips much more graceful and gradual than a solid state rectifier,
even germanium.
Hard limiting also means heavy compression. Any amount of input
signal is limited to a certain value. This is not tube like.
Letīs look at the way a tube diode conducts.
If we apply this method, we can therefore expect some hefty compression.
To give an example how much signal variation this method introduces, look at the following table that has been experimentally determined in a test conducted by Kevin OīConnor:
Source: Kevin OīConnor, 1995, The Ultimate Tone, Power Press Publishing, London, Ontario, p. 5-16 It can easily be seen that with rising series resistance the characteristic of the diode gets increasingly swamped, even although the threshold voltages are so vastly different. And just for completeness:
[To
understand the above quote, it must be known that all bounding circuits
are to be preceded by a series current limiting resistor...]
The
KoTīs distortion section (shunt clipper) however, depends entirely
on the characteristics of the diode selected. It has no compliance
whatsoever and can be expected to clip hard and with heavy compression.
Consequently, this has not been generally accepted by musicians as well
as the other modes. Also,
a volume step goes with it.It should be mentioned, that some diodes have inherently high capacitances, such as the 1N400x[2] (15pF). It is interesting in this aspect, that the KoT uses deliberately low-capacitance diodes. So if we recapitulate, the TS as a protagonist for endless copies uses a hard limiting diode pair with capacitive compliance and does some palliative measures to remove the harshness and achieve a wanna-be tube tone. The KoT in the series feedback diode pair at least, uses resistive compliance to achieve some much more tube-like tone with little compression. This leads further to the conclusion that the type of diode used is less critical, since its behaviour in the turn-on region is vastly swamped by the series resistance. Adding a little compliance by means of a series resistor would not only
More Tube Tone There
also exists some indication that a tube
amp limits the signal asymmetrically. This has been
discussed in a DIYstompboxes
thread.
Fulltone lives up to this by adding a ge diode in series with one of
the MOSFET diodes in both their OCD and Fulldrive (MOSFET ed.) pedals.
This gives of course different bounding for the two devices. One could
alternatively use two different bounding resistors, but this
would shift the whole "feel" and response away from bold distortion.
Too big a compliance would eventually result in no bounding at all, at
least not with the headroom available in typical battery-driven stomp
boxes like that.
Back To Index Slew Rate Limiting And Psycho-Acoustics Now a few thoughts on
op-amp choice.
Hi-Fi Guru John Linsley Hood, now defunct, spent a lifetime to study the perception of sound by the combination of the human ear and brain. From him we know that the ear can tolerate several percent of harmonic distortion without noticing, particularly second order distortion. However, the ear/brain seems to be very intolerant against distortion of the shape of a pulse. Now, the Hi-Fi guys did not learn from him too much, they still test their amplifiers running at full throttle on sinusoidal waves. This is not a criterion. Amplifiers that are tamed with heavy negative feedback usually exhibit low THD, but are made slooooow. This means that a fast transiting signal will be noticed as distorted, which is allegedly far more obnoxious than spectral distortion. One of the most overlooked reasons why valve amplifiers sound good for musical instruments is that they are very fast due to their voltage driven nature and their graceful response to overload. In the fight for low THD
figures there are
HF-compensation techniques applied in solid state amplification, that
yield unnecessary low THD
figures, but slow the amp down.
This effect is called slew
rate limiting,
the amplifier cannot follow (slew) a fast signal on the input and it
chokes for a fraction of a second. The nature of such slopes is
transient, but particularly musical instrument signals are of pulse
nature
and a mutilation thereof is obviously audible and obnoxious.
JHL postulates (amongst others) that this leads to a phenomenon called "listening fatigue", in other words, after a short time of exposure to such a system you find it unpleasant and want to turn it off. The goal must therefore be to prevent signals coming into the amplifier that are too fast for it to follow. This is of course a function of amplitude, gain and frequency. One of the techniques to prevent such signals ever entering an amplifier is to band-limit the signals coming into its input. Fast transits or even RF frequency will noticeably distort the wave shape. A simple RC filter on the input that limits the bandwidth to the audio range will cure all that. There is of course no such a thing on the KoT, so one will be added. For a discrete power amp there are of course additional compensation points needed that can be optimized for speed, but since we have no influence on that (unless we use an opa that is externally compensated...) this is not applicable for our situation. A private discussion with R.G. Keen over e-mail brought up the point, that the diodes inside an opaīs feedback loop present such an unexpected jump in gain that it probably chokes for a fraction of a second. This is probably worse, the faster the diodes are... Not much investigation has gone into this subject, but I postulate, that what holds true for a discrete power amp also holds to an extent true for an op-amp in this special application. Op-amps are usually very fast, because there is little power involved, but due to their limited gain-bandwidth-product they will invariably slew at some point. Although this measure may appear overdone at a first look, it is not exaggerated to suggest a fast opa for this application due to the system-immanent fast transits. Also not yet addressed has been the subject, that some opaīs tend to latch up and "hang" for an unpredictable time, when they slew. A 5532 for example, has diodes across its inputs to limit the voltage between them to counteract exactly that case. Now normally, both inputs are on the same virtual level in a standard gain application, but nobody can say what happens when things become non-linear for an infinitely small time and how the ear likes that. This may explain why some opaīs perform better in those circuits than other. Hard to say, how the original opaīs perform in this respect (I would not even know how to measure such phenomena, I suspect it is not so easy...) A LF353 would, by matters of speed, certainly outperform any of the above. Interestingly, it sounds very trebly in a KoT circuit and it was discarded. The NE5532 I would avoid in feedback-diode clippers. You may not listen to the clipping after the opa, you may hear the internal diodeīs clip. Apart from that, the 5523 is a current eater par excellence. There is not much we can do to limit the gain step inside our circuit, because this is part of the tone we want. Interestingly, the King Of Tone deliberately asks for high-speed diodes on both clipping sections, that are even faster than the common variety. Back To Index More Possible Sources Of OPA Distortion The question arose, what load a garden variety opa is able to drive. Although not too many datasheets of commonly used specimens display this figure, a figure that appeared frequently was 25-40 mA, so a load greater than 1k @ 9V (as the KoT has it...) should be well within the mark. Back To Index It is not proven that any
of the above applies to stompbox circuits, but it may well be, so it is
Referenceworth contemplating. It may make future decisions on component choice easier. [1] Fairchild Semiconductor Datasheet 1N4148/1N914 [2] General Semiconductor Datasheet 1N400x [3] NXP (Philips) Datasheet BA278 Back To Index Update History
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