Subject: How the LXH2 Fender/Marshall Simulators Work
From: "Saved by Internet Explorer 11"
Date: Thu, 16 Jan 2014 10:40:02 -0800

How the LXH2 Fender/Marshall Simulating Preamps Work
Added notes as of
11-30-05

A visitor to this website, Frank, from Germany, has built the fender amp and
cabinet simulator. He has suggested some modifications based on his own
listening tests. They appear in the main table of contents of this website. He
has included some mp3 files at his site which is linked to here. They sound
great!

Not shown on the schematic for the fender amp simulator is a pair of 68 ohm
resistors in line with the power supply leads of op amp "6". (one for the
positive lead and one for the negative lead). This will duplicate the effect of
a tube rectifier like type 5Y3. This will give that "sag" that many guitarists
seek.

Feel free to ask any questions you may have.

Table of Contents

Power amp
distortion theory - soft and hard clipping

Crossover
distortion, bias shifting, even harmonics, etc.

Effects of
the output transformer

Effects of
the amp's output impedance

Distortion
from overdriven speakers

Preamp tube
distortion

Making
op-amps clip normally

Pre-equalization

Power amp distortion theory - soft and hard clipping
A
characteristic of the best sounding guitar amps is power tube distortion. Unlike
transistors and op-amps, most power tubes do not clip sharply ("hard"), but
perform a severe "waveform compression". This is sometimes known as "soft
clipping". However, soft clipping alone does not make good distortion tone. It
is necessary to get hard clipping as the signal strength progresses well beyond
the threshold of soft clipping. In an actual tube amp, the hard clipping
typically results from the phase splitter stage or the last preamp tube before
the phase splitter.

The more desirable "Marshall" amps have a noticeable soft clipping rather
like that of a diode clipper. As the signal strength progresses well beyond the
threshold of soft clipping, hard clipping occurs. This can be simulated by
preceding a diode clipper with an op-amp stage that does good hard clipping.

In the LXH2 "Marshall" simulator, the op-amp stage before the diode clipper
has a +/- 9 volt split supply. The diode clipper is a pair of 1N914/4148 type
silicon diodes in parallel in opposite directions. A 10K resistor feeds one end
of the diode pair from the previous op-amp stage; the other end of the diode
pair goes to ground. This provides a very wide range between the thresholds of
soft and hard clipping, which we found to make a good "Marshall" tone. To have a
narrower range between the soft and hard clipping thresholds to more accurately
simulate most authentic vintage Marshalls, use more than one diode pair in
series. To have a different hardness of soft clipping, use a different diode
type. Silicon rectifier diodes generally clip more softly than 1N914 types, most
red LEDs clip harder. "Low-current" type red LEDs (gallium phosphide red such as
Radio Shack 276-044) clip rather hard.

"Fender" amps have only subtle soft clipping and primarily hard clipping. If
you monitor the output of a "Fender" amp with an oscilloscope, you may be
surprised to see the output clipped almost as hard as that of a solid state
amp. This is typical of push-pull tube power amps with 6L6 output tubes.

This clipping can be simulated with an inverting op-amp stage, with a series
pair of zener diodes in parallel with the feedback resistor. The 1N4733 5.1
volt 1 watt zener diodes work well. Lower voltage and lower wattage zener
diodes often don't clip hard enough. To add a touch of soft clipping, add a
series string of a large number of parallel pairs (opposite direction within
each pair) of both silicon and germanium diodes. This is to achieve a small
amount of "waveform compression" (the soft clipping) before hard clipping
occurs. The number of diode pairs needed depends on the exact type and brand of
diodes, since they can vary somewhat.

Crossover distortion, bias shifting, even harmonics,
etc.
The soft clipping/hard clipping described above is the form of
distortion generally needed for most good guitar tones. Other forms of
distortion, if implemented, make the tone more like that of an actual tube amp
but often not actually better. In most tube amps having adjustments that affect
these, minimizing these effects is usually judged to result in the best sound.

Crossover distortion is affected by any control used to adjust the bias of
the output tubes. Amps generally sound better if this control is adjusted for a
more positive/less negative bias, but without having excess idle current through
the tubes or the output transformer. A higher bias favors less crossover
distortion.

"Bias shifting" is included in some solid state tube simulating devices for a
more realistic sound, even though the most desired tube amps have less of this
effect.

As much as people talk about even harmonics and asymmetric distortion to
produce them, push-pull tube amps with any control affecting output tube
distortion symmetry (typically a bias symmetry control) are generally judged to
sound best when output distortion characteristics are most symmetric. Asymmetric
distortion generally leads to a rougher, more "hoarse" intermodulation effect
when chords are played.

Effects of the output transformer
When a tube amp is
running clean, the negative feedback circuitry in the output stage largely
corrects any effects of the output transformer. However, negative feedback only
does this while reducing gain. During distortion, the gain is either nonexistent
or less than the value the negative feedback circuit would reduce the gain to,
and the negative feedback does not work. This makes the frequency response of
the transformer become apparant during distortion.

In "Marshall" amps, the output transformer has a bass response that rolls off
gradually in the bass. In the "Marshall"-simulating LXH2 circuit, this is
accomplished by adding a .1 uF capacitor in series with the 10K resistor in the
diode clipper to achieve a distortion-dependent slight bass rolloff.

Some "Fender" amps have a slight treble attenuation from a loaded output
transformer, which the negative feedback circuit normally corrects. The onset
of distortion is accompanied or slightly preceded by a slight reduction in
treble response, known as "darkening". This can be simulated in the LXH2
"Fender" simulator's distortion circuit by adding a capacitor, perhaps with some
series resistance, across one or two of the diode pairs.

Effects of the amp's output impedance
A speaker sounds
different on a solid state amp than it does on a tube amp because the output
impedances of these amps are different. Using a higher output impedance shifts
the speaker's frequency response in the direction of its impedance-vs-frequency
curve. On a solid state amp, a guitar speaker often sounds "dead". On a tube amp
with higher output impedance, the speaker's response is increased at frequencies
where its impedance is higher. This generally greatly increases the speaker's
production of a range of bass frequencies, and slightly raises the speaker's
treble response.

When a tube amp distorts, its output impedance is even higher, since lower
output impedance is dependent on the negative feedback circuit functioning. The
high output impedance of a distorting tube amp results in a bassier speaker
sound, sometimes even resonant for closed box speaker cabinets.

To give a solid state amp a higher output impedance, simply add a resistor in
series with the speaker cabinet. A resistor with a value near or slightly above
the speaker's rated impedance usually makes the speaker sound best. This is not
quite perfect, since the output impedance of a distorting tube amp is not
constant and nonlinear, but this seems to work quite well.

Another trick is to use an equalization circuit, whose response resembles the
impedance curve of the speaker, using resonant means at any frequency where the
impedance rise is associated with a resonance. To adjust the bass
characteristics of a closed box cabinet, one can use a parametric equalizer.
Boost the resonant frequency of the cabinet, using a Q that matches the desired
Q. The degree of boost in dB would be 20*log of the ratio of desired Q to the Q
the cabinet has on the amp in use, OR whatever sounds best.

The LXH2 amp-simulating preamps have resonant
bass-boosting/cabinet-simulating circuits. These circuits are made to simulate
the low frequency characteristics of actual cabinets driven by amps of high
output impedance, for use with headphones or non-guitar speakers. These circuits
have been found to work well with guitar speakers on solid state amps and
non-distorting tube amps, despite the combined low frequency ultimate rolloffs
of the speaker cabinets and the cabinet simulating circuits.

Distortion from overdriving speakers
Guitar speakers
typically add their own distortion at higher power levels. This is mainly "flux
modulation" distortion, which is caused by the magnetic field from current
flowing through the voice coil adding to/subtracting from the magnetic field of
the magnet.

This distortion is a gentle, assymetric "even harmonic" type of distortion.
Although this generally does not detract from good tone, guitar speakers still
sound good with a properly distorted signal at any power level. Our listening
tests indicate it is not necessary to duplicate this distortion in order to get
good or realistic guitar tone.

Preamp tube distortion
Many distortion devices use
preamp tubes. One reason is probably the smaller size and lower power and
voltage requirements of preamp tubes. Another is their typically asymmetric
distortion characteristics, widely considered desirable due to the theoretically
resulting even harmonics. However, the distortion is "rougher", since one side
of the waveform gets almost entirely "hard" clipping and asymmetric soft
clipping tends to produce a "hoarse" intermodulation effect when chords are
played.

The more desirable forms of distortion require distortion like that of power
tubes.

Making op-amps clip normally
Ideally, op-amps clip hard
when the output voltage swings to within about 1 volt of the supply rails. This
can be used as a source of good hard clipping.

However, there are a few quirks some op-amps have.

1. Odd effects of input voltages going outside the "input common mode range".
If the voltage at either input goes outside the usual working range, the op-amp
may do strange things, including some sort of "clipping reversal". When well
into hard clipping, "reversal" can cause the op-amp's output voltage to
suddenly reverse, resulting in a bad distortion sound. To avoid this, have a
mild voltage divider before the stage to protect from this. This is especially
needed if the previous stage may clip and feeds the stage in question through a
capacitor, since the flat part of a clipped waveform can get "tilted" by the
capacitor and the leading portion of the "tilt" will have an extra high peak
voltage.

2. If you have lots of gain, be sure to use capacitors to block DC before any
offsets get amplified to significant levels.

3. If one op-amp stage in a multiple stage IC drives a heavy load, then the
distortion characteristics of all stages in that IC may be modulated by the
total output current or consumed supply current. Some op-amps have internal
protection resistors in series with the IC's supply leads. Please also beware
the protection is foolproof only at lower supply voltages.

4. The 324 op-amp has "crossover distortion". So may some other lower power
consumption op-amps and some op-amps designed for single supply use. This
distortion typically does not occur if the load is a resistor that terminates
at one supply rail; problems are largely associated with zero crossings in
output current.

5. Low power consumption op-amps may "slew" instead of clip, especially at
higher frequencies. The op-amp should have a slew rate high enough to provide
normal clipping, typically above a volt per microsecond. Low power op-amps may
also have low gain at higher audio frequencies. You should have open-loop gain
well above the wired gain at all audio frequencies for desired frequency
response and for the negative feedback to reduce distortion while the op-amps
are running "clean". Some low power type op-amps may simply have bad hard
clipping characteristics.

Pre-equalization
It was found desirable to slightly
attenuate the lower frequencies prior to distortion. Otherwise, the fundamental
content of lower strings can be excessively dominant during chords, resulting in
a "farty" distortion sound. Actual "Fender" and "Marshall" amps usually have
some such rolloff, typically 6 dB per octave below about 160 to 300 Hz. The LXH2
"Marshall" circuit as shown in this site has an even higher rolloff of 500 Hz.
Those who build solid state distortion circuits are encouraged to experiment
with this for the results that sound best to them.

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