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The Viola Quartet is a high quality mono electronic cross-over intended for use with speaker systems with up 4 bands.  In its standard configuration the channel frequencies may be set by the user as below.

Low pass:

Low band pass:

Band pass:

High pass: 

Other frequencies are available to special order and the use of plug-in filter cards allow the unit to be quickly and easily reconfigured. Each band uses up to 4 filter cards. These cards are fully discrete class A designs and have a cut-off rate of 12dB/octave.  Depending on how many cards are fitted to a band, the cut-off rate  may be set to 12, 24, 36 or 48dB per octave. 

Low noise hybrid type potentiometers are used for the output level controls.  Each control is fitted with a 10 turn vernier dial allowing extremely accurate setting.  The vernier is lockable for long term stability. 

A choke input type power supply housed in a separate chassis is used to provide a very clean and stable power source for the pre-amplifier. It also provides isolation from Electro-Magnetic Interference. 

Audio Design

The Quartet uses Class A  discrete circuitry and is fully balanced throughout. The main audio circuitry is constructed on a single large mother board which accommodates the local power supply sections and input and output connectors.  This construction gives the shortest possible signal path and eliminates the variations in performance that can occur when external cable forms are used.

The amplification stages use Viola's custom designed OTA gain modules. These are fully balanced and approximate to an idealized gain stage.  They are constructed using discrete components, including ultra-high precision (.01%) thin film resistors.  These are employed because of their very low noise characteristic.  Three types of module are used, each optimized for a specific application within the circuit.  Careful design makes them unaffected by variations in component values. 

The modules use a dual monolithic FET input and cascode topology, which gives a very large dynamic range and prevents any effects from power supply variations. Another advantage of the Viola OTA modules is that they can be easily changed to allow for quick servicing and this also allows for easy product upgrades.  This is not possible with non-modular designs.

The input section has been designed to have an input impedance of one Meg ohm.  This has three major benefits:.

1.   A high input impedance preserves the balanced input’s ability to reject interference if the output impedances of the signal source are not perfectly matched. The ability of a differential amplifier circuit to reject interference is referred to as its common mode rejection ratio or CMRR. Many source components have small differences in the output impedances of the two halves of their balanced outputs. When used with a balanced input with an input impedance in the conventional range of 10k ohms to 50k ohms, a mis-match of just 1% in the two halves of the source device’s balanced output can lead to a significant reduction in the pre-amplifier’s CMRR and a consequent rise in the noise level.

2.  High impedance inputs reduce errors at connector contact junctions.

3. High impedance inputs conserve the source unit's output drive current. Consequently more of the source unit's output current is available to drive the capacitance of the interconnecting cables, thus improving the high frequency performance and transient response.

The modules in the output stage us a special form of compensation which preserves the phase integrity of the balanced output signal across the audio spectrum.  This is important in order to ensure timbral accuracy and maintain a stable and 3 dimensional stereo image.  To avoid transients, the outputs of this section are muted during power-up and power-down cycles.

Quartet filter modules

Like the OTA gain stages, the filters are also built as plug-in modules. They too are fully balanced, class A designs and are constructed using discrete components, including ultra-high precision (0.1%) thin film resistors.  Also like the OTA gain stages, they use a  linear phase design to ensure that the phase integrity of the signal is preserved across the audio band. 

Heavy oxygen-free copper bus bars enhance the efficiency of power distribution within the amplifier and eliminate variations that would be  introduced by the use of the wiring harnesses commonly used in high performance amplifiers.

Click here for a view of the inside of the Quartet.

Low Noise Power Supply

The Quartet uses a choke input filter type power supply. This produces far less Electro-Magnetic Interference than the capacitor input designs most commonly used in pre-amplifiers.  It also places less stress on the power supply components, leading to higher reliability and longer life.  In addition, the Quartet's power supply design completely avoids the generation of the high frequency noise that often occurs when switch mode power supplies are used.

The Quartet's choke input power supply also virtual eliminates high peak ripple currents, thereby minimizing inter-modulation distortion on the power supply “rails” and ground returns.  This further enhances the sound quality by reducing both noise and distortion.

Ease of Upgradability

The use of plug-in modules for the gain and filter stages allows for easy upgrading should improved versions become available in the future.

High Quality Chassis

The Quartet's front panel is machined from a solid aluminum slab and the side panels are custom aluminum extrusions. These are brushed to a super-fine finish and then anodized. They are mated with heavy gauge top and bottom panels to produce a structure that is extremely strong and rigid.

Warranty

Products of Viola Audio Laboratories are warranted to be free of defects if used under normal conditions for a period of five years from the date of manufacture.