• Retrofier Classic

    Controls and Relative Signal Flow Overview

  • First it should be noted that the Retrofier 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 Retrofier 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 Retrofier 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
    • Speaker
  • Input

  • The Retrofier 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 Retrofier 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 Retrofier Classic uses a custom fourth order filter for this function that is always in the signal path, and is quite sonically transparent.

  • Drive

  • The Drive control is the user interface to a rather complex stage of analog signal processing. This is where the Retrofier 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 Retrofier 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 Retrofier 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 Retrofier 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 Retrofier 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 Retrofier 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 Retrofier 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 Retrofier 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.

  • Spring Reverb

  • After the speaker simulation stage, the signal is sounding like the first class guitar amplifier that the Retrofier Classic is, but dry. This is where it gets fed to the reverb circuits and this is the first reason that the Retrofier 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 Retrofier 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 Retrofier 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 Retrofier Classic takes spring reverb quality to a new level.

  • Mixing

  • The mixing of the reverberated and dry signals in the Retrofier 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 Retrofier 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 Retrofier 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.

  • Power Amplifier

  • The Retrofier 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.