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.
V-Stack Treble Booster
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"
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.
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.
V-Stack Amp Simulator
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.
V-Stack Speaker Simulator
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.