Controls and Relative Signal Flow Overview
First it should be noted that the Classic control panel is laid out in the order of the actual signal flow from the input jack to the volume control.
Other than the fact that the Low, Mid, and High controls are actually one unit and thus one stage, the signal flows through the amp from left to right as the controls appear.
The Classic is a completely analog/solid-state device that performs a great deal of analog signal processing to accomplish it's
remarkable tonal and dynamic qualities. These circuit techniques can outperform digital signal processing in key areas of audio electronics and the Classic is a perfect example of this.
Major Signal Processing Stages
- Input Buffering/High Pass Filtering
- Dynamics Processing (Preamp, Compression, Limiting, Harmonic Processing)
- Tone Control (EQ)
- Guitar Speaker Simulation and Filtering
- Spring Reverb
- Output Level (Volume Control)
- Power Amplifier
The Classic instrument input is where the
guitar signal enters the system and the analog signal processing begins. The first step is buffering
which converts the relatively high impedance guitar output to a very low impedance and precise
representation of the signal for further processing. All of the following stages depend on the
quality of this signal and as simple as this function may seem, it is not at all trivial and
presents some technical challenges. The Classic
input buffer does not introduce significant noise when used with a typical guitar using passive
or active electronics and has a dynamic range of about 138 db.
The next stage is high pass filtering. Electric guitars of all types almost exclusively have
too much low frequency information. Some of it is completely unwanted subsonic thumps,
thuds and booms etc., and the rest is simply disproportionate bass levels.
The Classic uses a custom fourth
order filter for this function that is always in the signal path,
and is quite sonically transparent.
The Drive control is the user interface to a rather complex stage of analog signal processing.
This is where the Classic truly departs
from conventional guitar amplifiers of all types. The 6 position Drive switch changes three
important parameters at once via some creative circuits and a dual gang switch.
First, the high pass filtered signal fed from the input stages is amplified in the 6 steps determined
by the Drive switch. The second half of the switch simultaneously compensates for the resulting output
level change that would otherwise be too extreme. The switching increments also roll off the low
frequencies as the drive gain is increased which gives an appropriate equalization for the entire
range of overdrive.
Next the signal is fed to QES's unique transistor based implementation of a push-pull, class A preamp
stage that boasts zero negative feedback and a very nearly perfect hyperbolic tangent overload
characteristic. We call it the Solid-State-TubeTM(SST). The SST circuit has a dynamic
gain range of about 46 db and an absolute dynamic range of about 140 db. This is where
the Classic gets it's rather impressive
compression effect all the way down to it's cleanest mode. The transition into overload is
very gradual and gives the player a remarkably distinct feel of touch sensitivity. The SST
is also extremely tolerant of overload and allows the
Classic to be fed from a wide
variety of overdrive preamps.
Tone Control (EQ)
The output of the SST circuit is fed directly to the the tone controls of the
Classic. The Low, Mid, and High controls,
are actually part of a very high quality constant-Q graphic equalizer. The bands are centered at
62.5 hz, 500 hz, and 4 khz which gives a very ideal range for electric guitar. The noise and
distortion characteristics of the Classic EQ
are exceptionally low and in some regards very near theoretical limits. This level of circuit
precision may seem contrary to conventional thinking regarding guitar amplifier design, but
that conventional thinking is in part what prevented guitar amplifiers of this quality from existing
in the past.
Guitar Speaker Simulation/Filtering
The next step toward ideal guitar sound is the filtering and dynamic effects of the speaker.
The Classic accomplishes this in two stages.
The first is a virtual speaker and the second is the actual 12" guitar speaker. The virtual speaker
is an analog electronic filter that produces the filtering and dynamic characteristics that a
typical guitar speaker accomplishes by electro-mechanical means.
The second stage is the speaker itself, however to a lesser extent.
The Classic actually sounds nearly
the same at line level due to it's internal speaker simulation. This is also one important
reason that the sound of a given Classic
does not audibly vary from the next and also produces it's exceptional sound through an extremely
wide range of speaker types and PA Systems.
After the speaker simulation stage, the signal is sounding like the first class guitar
amplifier that the Classic is, but dry.
This is where it gets fed to the reverb circuits and this is the first reason that the
Classic spring reverb behaves
differently from a typical tube amp or other conventional amplifiers.
Imagine putting a microphone in front of a dry guitar amp and sending that signal to a reverb device.
The result is much different from what you would get from a conventional guitar amp that would
reverberate, distort, and then filter via a guitar speaker. In such an amplifier you simply
would not use the reverb if you planned to do much hard driving because what results is a sonic mess.
The Classic on the other hand produces
an effect that is very "studio". It does the overdrive and speaker simulation/filtering upstream
and then the full bandwidth clean reverb is amplified at very low distortion. This means that you
can have any level of spring reverb while cranking the drive gain and the result sounds very much
like a studio recording.
The electro-mechanical spring reverb unit in the
Classic is supported by QES's
very innovative circuit techniques for both drive and recovery. The very high quality
circuits used are based on extensive research and testing conducted in the QES labs.
The end result is a very quiet, clean, and controlled spring reverb system that gives the
impression of being too good to be based on an electro-mechanical device. Among it's many
other innovations, the Classic
takes spring reverb quality to a new level.
The mixing of the reverberated and dry signals in the
Classic is accomplished using a professional
quality 2 channel audio mixing circuit that exposes the wet signal channel on the control panel.
The Reverb control is fed into the mixer using an electronically logarithmic circuit that gives
the control it's feel of a professional fader as well as contributing to it's exceptionally low
noise characteristics. The quality of the reverb mixing is also affected by the careful matching
of the respective wet and dry signal filtering.
Output Level Control (Volume)
Yes, even the volume control of the
Classic is high end. It uses active
circuits to give an electronically derived logarithmic characteristic. This means that the
amplifier will not have 80% of it's power at "2" and do little more than overload from there
as conventional amplifiers typically do.
The Classic has very smooth fader like
control of the output level from silence to full power. The active gain control also has the
wonderful merit of taking the noise down with the volume. This means that while having very
quiet practice sessions, you would think that you were using a 1 watt or less amplifier, not
a 30 watt amp turned down.
The Classic uses a balanced power
amplifier system designed to give consistent high quality performance indefinitely. It has no
audible noise or distortion, distributes is power across 12 transistors for exceptional long
term reliability, and operates from a single polarity 24 Volt/2.7 Amp computer style external
power supply. The amplifier produces 30 watts @ 4 ohms of clean tone, but as much as 48 watts
while heavily over-driven.