Viola Forte Mono Power Amplifier

Viola Labs

Type: Amplifiers

Audio Design

The signal path in the Forte is fully balanced throughout the voltage gain stages to ensure the maximum immunity from external sources of noise. To minimize the effects of the strong magnetic fields that are a consequence of the amplifier's high output current capability, painstaking attention has been paid to the physical layout of the amplifier. There is an absolute minimum of internal wiring and the unit's compact size enables the signal path to be kept very short.

Particular care has been given to the overload conditions that are often encountered under normal use. Sophisticated soft clipping circuitry allows the Forte to be driven into overload gracefully, without the introduction of high levels of harmonic distortion that other designs produce. This sounds better and protects your speakers. This circuit has absolutely no effect on the sound quality below the clip threshold. Many other soft clip circuits affect the sound before clip is actually reached.

Bandwidth control results in nearly identical small signal and large signal frequency responses. This approach avoids slew rate limiting.

The Forte has been designed to have inherently very low distortion. This allows the use of very little negative feedback and gives two important benefits. Firstly transient inter-modulation distortion is minimized. On complex musical passages this form of distortion masks low level detail. With the Forte, even when the music gets loud and complex, the quieter and more delicate instruments remain easy to follow. The second advantage of the Forte's inherently low distortion is that it is stable into all loads.

As is standard in Viola designs, great efforts have been made to minimize distortion at lower frequencies, because the majority of energy in most music is encountered at these frequencies.

To improve transparency and the resolution of low level signals, the Forte's output stage has been designed to operate with a relatively high quiescent current.

Heavy oxygen-free copper bus bars are used for power supply distribution within the amplifier, because they eliminate variations that would be introduced by the use of the wiring harnesses commonly used in high performance amplifiers.

Another standard Viola feature is the use of a very high input impedance. In the case of the Forte this is 1M ohms. This has three important advantages:

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.

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.

Film capacitors are used to provide the high frequency bypass for the power supply and ensure that all sections of the amplifier see a uniformly low power supply impedance. This is key to the Forte's ability to deliver large amounts of power whilst preserving all the subtle aspects of the music signal.

Low Noise Power Supply

The heart of a power amplifier is its power supply. Like the Bravo and Symphony, the Forte uses a choke input filter type power supply. This produces far less Electro-Magnetic Interference than the capacitor input designs most commonly used in power amplifiers.

It also avoids the high peak current demands that would be placed on the mains supply by a conventional power supply and therefore minimises inter-modulation distortion on the power supply “rails” and ground returns.

In addition, the Forte's power supply design completely avoids the generation of the high frequency noise that often occurs when switch mode power supplies are used. It also places less stress on the power supply components, leading to higher reliability and longer life.

Expandable Power Output

If more power is required, it is possible to connect two Fortes together in what is referred to as a bridge connection. This forms a mono amplifier with approximately 4 times the power of a single Forte. A second pairs of units similarly connected is then required for stereo system. Two bridged Forte amplifiers will deliver:

300W into 8 ohms
550W into 4 ohms (Limited by mains fuse)

Sophisticated Protection Circuitry

The Forte employs sophisticated protection circuitry to shut itself down should it detect any one of a range of fault conditions that might damage either the amplifier, or the speakers it is driving. These conditions include:

• The demand for excessive current at the output
• Under-voltage conditions on the AC mains
• Unsafe operating temperatures within the amplifier 

In the case of an over-current condition, the amplifier will shut down to protect itself and the speakers. Normal operation may be quickly restored by removing the cause of the fault and switching the unit off then on again.

If the AC mains voltage is too low for safe operation, the amplifier will automatically shut down. The amplifier will not turn back on until the AC mains voltage is within the acceptable range for normal operation. A 120V unit will operate between approximately 85-140V; a 240V unit will operate between approximately 180-270V. Outside these generous limits, the amplifier will shut off.

Should any one of the amplifier's internal sensors detect an excessive temperature, the amplifier will automatically shut down. Once the fault condition has been removed and the unit has cooled down sufficiently, the amplifier can be turned on again.

High Quality Chassis

Great care has been given to the design of the Forte chassis. The 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.


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.

Viola Forte Specifications
Output Power

75W per channel into 8 ohms
20Hz to 20kHz , THD < 0.25% 
150W per channel into 4 ohms
20Hz to 20kHz , THD < 0.5% 
All power figures quoted are continuous average power
Inputs 2 x XLR in parallel
Input Impedance 1M ohm 
Outputs 1 pair of WBT terminals per channel
Frequency Response

10Hz to 20kHz ± 0.15dB at 1W into 8 ohms
5Hz to 100kHz -3dB at 1W into 8 ohms
Power Bandwidth 5Hz to 100kHz +0, -3dB
Intermodulation Distortion (SMPTE) 1W to 75W into 8 ohms <0.075%
1W to 150W into 4 ohms <0.1%
SN Ratio -105dB @ 1kHz 75W, C weighted
Dimensions 22.5cm W x 11.0cm H x 42.3cm D
Weights 15.9 kg

Related Items