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|Guitar Amplifiers - Overdrive & Distortion
The overdriven sound of a valve power amplifier is highly desirable, with many
different output stage designs to produce the variety of trademark sounds heard on modern
recordings. The only problem is that a valve power amplifier is only capable of
producing this sound at one volume (usually, fairly loud!).
Dummy speaker loads (the good ones are not just resistive, they need to simulate the
reactive load of a speaker) allow a player to use one amplifier in a variety of playing
situations and styles by running the amplifier at the desired level, and using the dummy
load to regulate the volume level. Another option for the playing musician is to
use a variety of amplifiers, however, this approach appeals only to rare wealthy
There are probably 3 distictly identifiable types of valve power amplifiers used:
- Leo Fender's classic early designs used 6V6 tubes, and later, the higher powered 6L6's.
This gave a characteristic full and punchy sound, suitable for many styles of the
day, and later. Steel and country players like the chime-like clean sounds, and
blues players were quick to discover the classic way it breaks up when pushed hard.
At really high overdrive, though, the sound becomes quite dirty, with bass in particular
- Marshall designs started as Fender copies, but soon switched to EL34 output tubes,
possibly for local supply reasons. Anyway, the rest is history. These tubes
exhibit a softer overdrive transistion, and maintain clarity even at high overdrive
levels. They also have a limited middle response, giving rise to the famous
Marshall crunch sound. The lower powered EL84 tubes have similar characteristics.
- Vox AC30 (and the more popular top boost model) uses a Class A power amplifier design,
giving a sweeter overdrive. Listen to Brian May's sounds for plently of good
examples. The Fender and Marshall designs use class AB for their output designs,
which is more efficient (more watts per tube), and better for tube life. Without
any guitar signal, both tubes are very nearly 'off', but when you play, tubes take turns
handling each half of the signal. This leads to some (unwanted) distortion as the
tubes cross over. Class A designs have the tube operating at half power, with no
signal applied. When you play, the tube fluctuates between full and no power, so
there is no switching to add unwanted distortion. This is a very superficial
explanation; please read elsewhere on the Internet for more detailed descriptions.
Boutique amplifier builders offer composite designs, offering characteristics of all
designs. This can be done dynamically (by responding to picking strength and volume
settings), or with various switching schemes. Mesa Boogie has built it excellent
reputation for tube preamp overdrive and tone shaping designs, used in conjunction with
high quality tube power amplifiers. For most of us, we can use a wide range of
effective stomp boxes for our overdive and distortion sounds.
||There is much history on the attempts to recreate the desirable overdrive sounds with
various non-linear preamplifier designs. When a player tests one of these devices,
the first impression is usually formed on the type of overdrive character and tone
produced, and players will be looking for sufficient flexibility in the controls to tailor
this to their personal tastes. The basic types of overdrive are generally
classified as soft and hard clipping.
Soft Clipping: This is
usually marketed as "overdrive", where the gain is inversely proportional to the
input signal level. This is typically produced either with back to back silicon
signal diodes in the negative feedback path of an op-amp, or with germanium diodes or LEDs
back to back in a shunt to ground.
||Hard Clipping: Usually marketed as "distortion",
where the signal level is restricted within a range. This is typically produced
with silicon diodes back to back in a shunt to ground. This is the same as the
circuit above, using silicon instead of germanium/LED diodes.
Here's a picture of what
soft and hard clipping do to your guitar signal:
||There are some other criteria which players will notice (but maybe not immediately)
when using these circuits:
The ability to retain timbre.
Different guitar pickup combinations produce recognisable signature sounds of the
instrument used. By its nature, overdrive will mask this timbre to some extent,
however, many musical styles prefer to retain as much of the original character as
Inter-modulation distortion. Again, by its nature, overdrive
will produce inter-modulation distortion when two or more notes are played together.
For just two notes played, inter-modulation distortion produces an additional note
with a frequency of the difference between the original two notes. For chords,
where up to 6 notes are played, the combinations of note pairs can produce an
unrecognisable mess of distortion.
On the other hand, this is actually desirable in musical styles which use mainly power
chords, because in this case, the inter-modulation distortion adds a note which is tune
with the chord. For other styles, where a player may want to hold one note and bend
(change the pitch of) another, a slurring bass note occurs which is generally quite
undesirable. This can be minimised to some extent by limiting bass response.
Sustain vs Dynamics. One of the key desirable features of
overdrive is the sustain produced, however, too much sustain will destroy the dynamics.
Players will also want to use the overdrive sound for single note solo work, and be
able to turn down their guitar volume (effectively reducing the gain of the overdrive
preamplifier) to clean up the sound for chord work. Some designs are better than
others in this ability to compromise sustain and dynamics. Designs that give the
impression of 'switching' from overdrive to clean as a note fades are usually perceived as
Frequency compensation. Because preamplifiers are generally
connected between the guitar and the amplifier tone circuit, there is no pickup
equalisation to compensate for reduced treble response. Consequently, it is usual
to limit the bass response before the overdrive section. While it would be logical
to boost it after the circuit, this makes the inter-modulation distortion more noticeable,
so this is rarely done.
The overdrive circuit itself adds higher frequency components to the sound simply
because the overdrive circuit is non-linear. These must be cut to preserve some
tone similarity with the unprocessed sound, and to also remove unwanted high frequency
components. Most players prefer this to be adjustable, to suit their own tastes.
||Facts and opinions In writing these pages, I have tried hard
to avoid giving my opinions; instead I've tried to give you just the facts so you can draw
your own conclusions. I'll let you decide how successful I've been. In
talking about some classic overdrive pedals, though, I think I can add some value by
giving you my impressions of how these pedals sound to me. I've also shown portions
of the schematics of these pedals to explain their unique fetaures. These
schematics are not complete; they show only the effect signal path, and not all component
values are shown. Please don't email me asking for these values, because I don't
know what they are. Of course, if you do know, and want to tell me ...
Although labelled as distortion, this is a soft clipping device, using germanium
diodes. It's a good example of how little you need for a good basic sound.
You could easily swap (or switch) these diodes to silicon types for hard clipping.
||ProCo Rat Distortion
Not necessarily the next pedal chronologically, but look at how similar this design is.
It uses 2 silicon diodes for symmetrical hard clipping. I would also expect
that at high gain settings, the IC also clips to the supply rails.
||Ibanez Tube Screamer
No discussion on overdrive pedals is complete without looking at the Ibanez Tube
Screamer. There have been several minor variations of the pedal released by Ibanez,
and a larger number of variations sold by boutique pedal manufacturers. As our
guitar heroes die, it seems the equipment they used somtimes takes on a mythical status.
In my opinion, this is the case with the genuinely legendary Stevie Ray Vaughn and
the Tube Screamer. This results in some silly prices for original pedals, and a
lively market to convert different pedals to Stevie's model.
Nevertheless, the green Ibanez box is a very smooth sounding pedal that retains the
guitar timbre well, and for that reason works well with single coil guitars. There
is not an enormous amount of drive available, and the tone control is subtle. Like
many overdrive pedals, there is some middle boost, caused by the bass cut before
overdrive, and treble cut afterwards.
Another common use for these pedals is as a middle booster to drive a valve amplifier
harder. This is done by setting little or no drive, but with the level set high.
In the schematic, you can see two silicon diodes, back to back, in the negative
feedback path of an op-amp. This arrangement gives symmetrical soft-clipping.
||Boss Super Overdrive SD-1
These were originally sold without the tone control. The design is nearly
identical to the Ibanez Tube Screamer with 2 important changes. More boost is
available, but is partly offset by using 2 diodes in one direction and only one in the
other. This produces asymmetrical soft clipping, meaning that one side of the
waveform is clipped more severely than the other. A more common implementation of
assymetrical clipping is to use 2 silicon diodes, with a germanium diode in series with
one of them.
There is lively debate on the Internet about whether this sounds more natural, and
whether it better emulates some asymmetric valve phase splitter designs. In any
case, I think it does add a little character, and therefore suits humbucker guitars well.
||Marshall Pedals: Blues Breaker, Drive Master & Shred Master
These three pedals were released in the early 90's, and use different clipping and tone
shaping techniques to deliver different sounds.
The Blues Breaker uses silicon diodes in series with a resistor, in
the op-amp feedback path for very soft clipping. It's therefore a very subtle
pedal, with warm sounds at low to medium overdrive, but can sound a little fuzzy at high
gain. Retention of guitar timbre and dynamics is good, and intermodulation (read
above) is acceptable.
||The Drive Master uses LEDs shunting to ground for symmetrical soft
clipping. I like this pedal for its howling Marshall stack-like qualities with
single note solos and power chords. Dynamics are good at high drive levels,
retention of timbre is excellent, but intermodulation is a problem for anything but simple
||The Shred Master is not quite the animal its name implies. It
uses silicon diodes shunting the signal to ground, for symmetrical hard clipping.
Bass and treble controls, and a contour control offering middle boost and cut sounds give
a wide range of usable sounds, although I'm not convinced shred is one of them.
Retention of dynamics is good, intermodulation is OK, and retention of timbre is good at
low drive settings.
||Here is a circuit that combines many desirable features. Feel free to
experiment with the component values. For example, using lower value capacitors
around the tone control will give a brighter sound, and vice versa. The capacitor
on the left sets the tone at fully clockwise, while the one on the right sets the minimum
This circuit has been updated for 2002. A buffer mode switch has
been added - see the notes below (normally you would just wire this permanently the way
you want to use it.
The circuit features are:
- Battery power is connected by inserting a mono guitar plug into the input socket.
This power supply uses a voltage divider to provide half-supply voltage bias to the
- Input over-voltage protection (the 1K resistor and 2 x rectifier diodes)
- High impedance unity-gain buffer (the BC549 transistor) to interface the high output
impedance of a guitar with the following circuitry
- High pass filter (the 2.2K resistor and 0.15uF capacitor) to compensate for the natural
low-middle emphasis of guitar pickups
- Soft-clipping non-linear amplifier (the 1st half of the TL072 and the 4 diodes in the
feedback path) with variable gain control
- A switch to use soft-clipping (overdrive), or apply hard clipping (distortion, with the
2 diodes to ground)
- Low pass filter to compensate for the high harmonics added in the clipping stages (the
- The Tone control is a variable low pass filter (50K pot and a second capacitor) to allow
you to customise the amount of treble cut
- An output buffer with 6dB of gain to provide a low impedance output
- A Level control to allow you to use the pedal to boost or match normal guitar levels (or
use as a middle booster with low gain and high level settings)
- A footswitch to use or bypass the circuit
- When bypassed, the overdrive effect is shorted, so no background "fizz" bleeds
into the clean signal
- LED indication to show when the effect is on, used with a Zener diode to restrict
available voltage to the LED to give early indication of battery failure
- Bypass mode switch - use hard bypass to preserve original tone (and for bypass to work
even if the battery is dead), or use buffer mode to drive long leads without treble loss,
or to drive other effects such as volume pedals without tone loss
||The Op Amp should be any dual low noise device, such as a TL072. The 1N4148
diodes can be any small signal silicon diodes. The 1N4004 diodes can be any 1 amp
rectifier diodes (eg 1N4007 is OK also). The input buffer transistor should be any
high gain low noise device, such as a BC549.
The top left portion of this circuit
supplies 9V power and 4.5V bias to the rest of the circuit. Connect all the 9V
points together, and connect all the 4.5V points together.