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Introduction

There is arguably no tone setup that better defines the classic blues/rock guitar sound than a germanium treble booster driving into a vintage all-valve amp.  This time-honored combination is the basis for the V-Stack signal processing architecture, shown diagrammatically in Fig. 1 below.



Fig. 1

The signal processing stages of the V-Stack, as represented above in Fig.1, consist of a treble booster section followed by a valve amp simulator consisting of three stages: preamp, tone network, and output stage.  The final processing stage is a guitar speaker simulator, which rounds out the amplifier tone and adds an extra degree of realism for direct-in recording.  Due to the large amount of available signal gain, a special high-isolation audio multiplexer circuit is required to handle the bypass function.   The V-Stack is based on a proprietary analog custom microchip (Fig. 2), which combines state-of-the-art high performance analog signal processing techniques with low power circuit design.  The low power consumption of the resulting circuitry allows the V-Stack to operate for typically 500 hours or more using a 9V alkaline battery.



Fig. 2 The ATD AC7303 Analog Custom Integrated Circuit: A Treble Booster and Guitar Amp Simulator combined on a single chip.

The V-Stack Treble Booster

Whether you like to play "clean" or prefer an overdriven amplifier sound, a treble booster can add an edge to your tone.  In the vintage era of guitar amplification, treble boosters were used by many guitarists to boost their guitar signal to a sufficient level to overdrive the valves in their amplifiers, pushing them into a smooth and harmonically rich kind of distortion.  Apart from providing a useful amount of signal gain, a treble booster has a frequency characteristic that is also very useful in terms of its contribution to guitar tone.  Due to the physics of the interaction between the guitar strings and the pickups, the signal level of each string diminishes with string diameter.  This means that if you play a full chord, for example, the strength of each note in the chord diminishes with the increasing pitch of the note.  While this can produce a natural and pleasing result when played through a clean amplifier, the result when playing through a heavily overdriven amplifier can be a loss of detail caused by the higher pitch notes being overpowered, or "blocked", by the lowest pitch among the group of notes.  This is illustrated in Fig. 3(A) below:  The green trace represents a typical guitar signal when two notes are played together.  The red trace is the same signal after being passed through an overdriven amplifier.  The loss of detail in the overdriven guitar signal compared to the original signal is very apparent in this case.



Fig. 3 Illustrating Overdriven Note Separation With and Without a Treble Booster

A treble booster has a gain characteristic that increases with increasing pitch (typically starting with a gain of between 1 and 2 at the fundamental pitch of bottom E, and doubling every octave within the spectrum of the guitar signal.).  Since this characteristic is the opposite of the natural characteristic of the guitar signal, as described above, the end result is that, for a group of notes played together, the treble booster causes each note in the group to produce signals that are more nearly equal in strength at its output, and the "note blocking" evident in Fig. 3(A) is prevented, as illustrated in Fig. 3(B).  In summary, therefore, a treble booster provides two important functions: gain; and enhanced note separation. Yet another important purpose of the treble booster is to ensure that there is ample signal level when soloing in the higher registers to keep the amplifier in deep overdrive, thus producing the characteristic "screaming lead" sound.

The classic treble booster, perhaps best typified by the Dallas Rangemaster, is a simple one-transistor device. Fig. 4 shows a simplified schematic of a Rangemaster type treble booster.  Despite its small size and outward simplicity, when used in conjunction with an all-valve amplifier, the effect it has on guitar tone can be spectacular.



Fig. 4 A Minimum Configuration "Rangemaster" Type Treble Booster

The Rangemaster treble booster has been produced using several different transistor types over the years, each of which alters the tonal qualities to a greater or lesser degree.  Most experts on the subject agree that the original version was built using the OC44 PNP germanium transistor, which, being actually intended for radio frequency amplifier stages, produces the brightest and clearest sound of all the variants.

Keeping true to the vintage era reproduction design objectives of the V-Stack, the original OC44 version of the Rangemaster was chosen for the V-Stack treble booster section target specification.  Great attention to detail was paid in the design of the V-Stack treble booster section, ensuring that both the small and large signal responses matched closely to those of the Rangemaster.  Computer simulation results, shown in Fig. 5 below, demonstrate the high degree of accuracy with which the V-Stack treble booster section reproduces the frequency response of an 'OC44' Rangemaster.



Fig. 5 Computer Simulation Results: Comparing the Frequency Responses of the V-Stack Treble Booster and an "OC44 Rangemaster"

By the application of modern microelectronics design technology, two important improvements have been made in the performance of the V-Stack treble booster over the original Rangemaster design.  Firstly, the electrical circuit self-noise has been reduced, and secondly the frequency response characteristic has been made largely independent of the guitar volume control setting.  This latter attribute is an essential part of the V-Stack "harmonic signature" amp modeling approach, in which the guitar volume control is used to control the amount of preamp overdrive.

The V-Stack Amp Simulator

As part of the design process of the V-Stack amp simulator section, a number of contemporary vintage-era valve amplifiers were studied both at the schematic level and by use of computer simulation.  The end result of this study was the composite amplifier schematic of Fig. 6.  This theoretical amplifier has a great deal in common with many amplifiers from the same era, while not being exactly like any one in particular.  The import aspect of this composite is that it has the ability to produce, in controllable amounts, both preamp and output stage types of harmonic distortion.  This generalized approach to controlling both the degree and type of amplifier distortion allows the V-Stack amp simulator section to approximate the "harmonic signature" of most amplifiers that use push-pull output stages (which practically all performance grade valve amps do).



Fig. 6 A composite of contemporary vintage valve guitar amplifiers

Although not exactly like any one of the amplifiers used in the study, with the negative feedback removed (shown dashed in Fig.6), the composite amplifier of Fig. 6 is most closely related to the normal channel of the venerable Vox AC-30.  For this reason, the design specification of the V-Stack amp simulator section is based on a combination of the schematic of Fig. 6, together with a gain and frequency response specification taken from a typical AC-30 normal channel.  In translating the objective circuit of Fig. 6 into an equivalent circuit in silicon microchip technology, great care was taken to preserve both the asymmetrical harmonic distortion characteristics of the triodes in the preamp stage, and the electrical symmetry of the output stages.  For the triode preamp simulator, the widely used 12AX7/ECC83 dual-triode was chosen as the design benchmark.  Conventional op-amp design techniques were set aside in favor of custom amplifier stage design, producing a level of sonic quality not otherwise possible.  Such is the inherent linearity and symmetry of the output stage simulator, it can operate with very little negative feedback.  This means that at higher drive control settings (e.g. 7 - 10) it behaves similarly to the class A push-pull output stage of the AC-30, while at lower drive control settings, it behaves more like the more typical class A/B push-pull output stages of (e.g.) a Marshall or Fender.

The V-Stack amp simulator section also includes two amplifier bandwidth modeling controls, BASS, and CUT, to provide control of the lower and upper amplifier cutoff frequencies respectively.  When used in conjunction with the guitar volume control and the V-Stack DRIVE control, a wide range of amplifier tones are possible.  Fig. 7 below demonstrates how, with the BASS control set at 4 and the CUT control set at 2, the V-Stack can reproduce very closely the combined frequency response of a Rangemaster treble booster and a Vox AC30 normal channel.



Fig. 7 Computer Simulation Results: Comparing the Combined Frequency Responses of the V-Stack Treble Booster and Amp Simulator Sections and an "OC44 Rangemaster" feeding into an AC-30 Normal Channel.  (Responses normalized to 0dB peak gain).

The V-Stack Speaker Simulator

The guitar speaker is an integral component of the sound of any guitar rig, influencing tonal balance and adding its own specific type of coloration to the overall sound.  The design objective for the V-Stack guitar speaker simulator section was to produce a simulation that was very convincing when used for D/I recording, or with full-frequency range sound amplification systems, and yet would not compromise sound quality if the pedal is hooked up to a guitar amplifier.  This was achieved in practice by adopting model parameters that simulate a speaker having a response bandwidth toward the higher end of the range of typical guitar speakers.  The result is a generic guitar speaker model having a frequency response that can be complemented to some degree by the V-Stack CUT control.  Working in a fashion analogous to a variable "power brake", the MASTER control of the speaker simulator section provides a "universal" output level control, which ranges from zero to a maximum output level of several volts peak-to-peak, without compromising noise performance or tone.  The range of control this provides allows the V-Stack to chain seamlessly with other effects, or interface with the inputs of practically any type of audio equipment.

Summary

Some of the key design objectives for the V-Stack are summarized below:

  1. Authentic vintage tone: design specifications based on actual examples of vintage-era equipment and amplification devices.
  2. Tone setup based on classic germanium treble booster driving into vintage all-valve amplifier.
  3. Output suitable for both D/I recording, and for use with a wide range of amplification equipment. 
  4. Simple and intuitive controls, based on "harmonic signature" amp modeling.
  5. Low power consumption for long battery life and true portability.
  6. Ultra compact, high performance, and high reliability circuit design using state-of-the-art analog microchip technology.
  7. "True Bypass" type audio bypass switching.

The above technical overview is intended to provide an insight into the V-Stack design process.  The origins of a number of key target specifications are discussed, and computer simulation results demonstrating the degree of compliance with some key specifications are presented.

 

Disclaimer:  All trademarks referenced in this article are acknowledged to be owned by their respective owners.  V-Stack.com is not affiliated with Dallas-Arbiter, Fender Musical Instruments, Marshall Amplification, or Vox Amplification.