The VI-2 platform was developed to meet all demands on valve amplifiers with a single flexible basis:
- High-end music reproduction
- Endurance and long maintenance intervals
- Compliance to national and international standards, especially regarding safety
- Safe and rugged design with visible tubes
- Flexibility for custom modifications
The platform features an external power supply unit feeding the amplifiers with filtered DC operating voltages. Thanks to the external supply there is no need for compromises regarding the amplifier supply. All enclosure frames are handmade from stainless steel while the covers are made from aluminium. The amplifier covers are made from glass to have the valves visible.
This design principle is the same for power amplifiers and preamplifiers. At the same time there is much room for various amplfier concepts and customized amplifiers.
The frames of the enclosure consist of stainless stell and are "handmade in Germany".
The design principle for amplifier enclosures and the power supply unit is similar. Power supplies have a glass front and custom made perforated sheets on top and bottom for optimized ventilation.
The essential characteristic is electrical safety and compliance to relevant standards. With the present design the valves stay visible without removing any cover thus without affecting safety.
Due to the separation of frame and chassis it is possible to vary in size and layout of the enclosures based on the actual needs.
Apart from electrical safety and apart from a rugged and exclusive optical design special attention was paid to the thermal management. All parts are operated significantly below their thermal specification yielding extended lifetime and reliability. The valves, especially the power valves keep their minimum distance between each other in order to avoid additional heating.
The surface finsih of all enclosres is chosen upon customer wishes. All usual treatments like sanding, polishing or anodization are possible.
The internal wiring of the amplifiers is done in a classical manner using ceramic solder strips. This provides free air circulation around all components improving their reliablility. The tube sockets are also handwired with the advantage of slightly movable socket contacts. This in turn reduces stress on solder connections and on the valves base. Various further advantages will be gained from ceramic strip wiring some of which are optimized three dimensional signal routing, easier maintenance and reduced smell from PCBs.
A good design is a simple design ?
This thesis maybe is partly true. However we like to move from the "good design" to the "better design". In fact we would want a design without compromises but this will not exist in a technical world. Technical requirements like lowest distortion, lowest output resistance, wide bandwidth and high output power will require compromises to be made. Luckily it is often the case that one part of a compromise is the effort taken in improving things. Accepting a higher effort and cost at some point will suddenly reveal further design headroom and new possibilities.
Mostly our amplifiers are unique as they are developed or manufactured according to customer wishes. Regarding amplifiers concepts we try to avoid dogmas in favour of plain engineering. When using the appropriate know-how and spending adequate care many concepts will lead to a good or even excellent amplifier. Dogmas like plenty of feedback or none, single ended or push-pull etc. seem counterproductive.
No matter if its a complex design or a simple design - some principles shall be always respected:
- Safety first - electrical sfatey for the owner, owner guests and children
- Stable specifications without manual interaction for new and used tubes
- Reliability and long maintenance intervals
- Use of high quality components and use of a wizard*)
*) The wizard was first introduced by Tektronix in their type 454 oscilloscope. In general a wizard will achieve circuit properties being otherwise impossible. Unfortunately wizards cannot be bought at the distributor neither are they replaceable. However we are convinced of those wizards and therefore we are going to use them now and then.
The output transformer is the most critical part of a valve amplifier. Most transformers suffer from low bandwidth, high losses and high distortion. While it is understood that we use very high quality parts in our amplifiers we spend significant effort to improve the properties of the output transformer in a multistep process:
- Calculation and simulation of the transformer
- Production of the core from high grade core laminate and mostly with air gap
- Measurement of the manufactured core and optimisation of the winding data
- Winding and fisnishing the transformer
It is evident that there is nothing like a "standard transformer". Parameters like primary inductance, winding ratio, core cross-section and others heavily depend on the actual amplifier design, the valves used and the desired output power.
With our development procedure we achieve high linearity, high bandwidth and low losses even before applying global feedback. At the same time we gain further design headroom for a eventual global feedback due to the excellent transformer properties. Any additional global feedback is then meant to improve an already excellent amplifier.
To a large extent the properties of the transformer are defined by the air-gap in the core. It is well known that push-pull amplifiers do not need any air-gap. However this is true for the DC magnetisation only. All other properties can also be improved by a suitable air-gap in the core. A bigger transformer will be the only drawback.
In addition to the regulated DC suppply modules the external power supply contains the control computer as well as appropriate stand-alone control circuitry. In the event of a failure the supply voltages will be switched off in order to prevent further damage.
All DC regulators are optimised for low hum and low noise. In addition care was taken to provide low resistance in the whole audio band and a benign transient response. The regulated solid-state supplies are optional. It is also possible to fit tube-only regulators or even unregulated supplies. Regulated solid-state supplies have the advantage of a clean and constant output independent of the mains voltage or the actual mains supply quality.
In case a more handy supply is needed it is feasible to delete the anode regulators. When a reduced power tube lifetime is acceptable also the DC filament regulators might be deleted. However keeping the prestage supplies as well as the bias and screen grid supply regulated is highly recommended.
The DC filament regulator provides virtually hum-free supply with low tolerance (i.e. +1/-2%). In addition the regulator is short-circuit proof and provides a soft-start functionality and internal fault monitoring. Keeping the filament voltage within +/- 5% is an essential requirement for constant operating conditions and long lifetime. With a unregulated AC supply this cannot be achieved as the mains voltage fluctuates by more than +/- 5%.
Speaking of high-voltage might not be correct in terms of the applicable standards. However for the purpose of this article high-voltage regulator denotes the anode and screen-grid supplies. The prestage regulators are optimised for low hum and low drift. The anode regulators for the power tubes equally provide regulated screen-grid supply. The screen-grid current is constantly monitored. In case of excessive screen current the controller is notified in order to switch off the power supplies.
A power amplifier might not need any user interaction other than switching it on and off. Excperience shows however that even the best valves obtainable might need an adjustment now and then. Furthermore the owner of the amplifier might be happy to be informed about the general condition of his amplifier or about eventual errors.
As a result we developed a controller with optional display especially built for the management of valve amplifiers. Controller and display are inactive during normal operation and therefore do not produce any radio disturbance. The controller is activated by user interaction or in the event of a failure.
Controller operation includes:
- Information on general operating conditions (operating hours, supply voltages, valve idle currents etc.)
- Measurement and monitoring of internal supply voltages
- Adjustment of power tube idle currents on user request (option for fixed bias concepts)
- Reaction on errors and user information
- Choice of class A or class AB operation
- Logging of internal values after startup, after user triggered measurements or adjustments
- Operating hour counter and power cycle counter
We spent huge effort in this concept and therefore we do not want to start saving at the details.
The appliance inlet is equiped with a rugged and reliable locking plug. With this certified plug loose contacts can be simply forgotten. A switched outlet for further equipment like a preamplifier is provided with the same quality.
Last but not least a power amplifier is meant to turn a input signal into a power output to the loudspeakers. The essential connectors are therefore the input connector and the output connection. Without starting to experiment with lower cost and inadequately specified RCA connectors we go straight to the professional XLR connector. It is lockable and provides symmetrical wiring as needed for a symmetrical amplifier. If really needed the XLR connector could also be used asymmetrically. As we like professional connectors we provide a Speakon connector for loudspeaker connections.