(C) The Valfet, "Bootstrapped Transformer", Universal Power Amplifier.
(C) The Valfet,"Bootstrapped Transformer,"
Universal Power Amplifier.
The improvements are in the valve signal amplifier and the buffer, the buffer uses low voltage small signal, physically small, high gain transistors: in all the circuits driving the output transistors. The advantage here is the much faster processing of audio signals.
This is achieved using bootstrapped power supplies to the new buffer design, via a dedicated separate power supply. The small 3 V/A transformer for this separate power supply is termed in this new design as the, "Bootstrap Transformer", The topology of this new design is published below.
Note. The high voltage, high current, power supply for the output transistors: top right in the diagram, is completely separated from the low voltage power supply below it. Further Note, the common connection for the +/- supply from the low voltage power supply is connected to the loudspeaker terminal. Therefore, all the processing and driving circuits float up and down: are bootstrapped, to the loudspeaker terminal.
The input is referenced to ground via a servo that is also bootstrapped to this loudspeaker terminal. The DC voltage offset on the loudspeaker terminal under no signal conditions is +/- 2.5 millivolts.
The design is further improved by the use of a different much faster thermionic valve high voltage signal amplifier stage driving this new buffer design, and the whole new power amplifier comprising of valve signal amplifier and buffer is very fast: and has a flat frequency response from 5HZ to 100 KHZ.
There is a completely new conventional chassis design using fully shrouded EI transformers, the new design looks far different to the old closed cabinet design. The new amplifier is displayed further down this page.
The schematics and circuit description and evolutionary theory for this evolved far superior power amplifier design are now included in the now new extended Valfet Audio Power Amplifier book: that can be down-loaded from this page.
Various models can be made from this design. One of the models is class A/B and class B selectable, the other model is class A and class A/B selectable. Many pairs of output transistors can be driven by this design allowing for loads down to two ohms and lower.
Many class A all valve amplifiers need their expensive output valves changed annually, this is very expensive. This new Valfet design that is a sonic match if not an improvement on all valve class A amplifiers uses only two cheap valves per monoblock amplifier, ( not inferior sounding ), so the maintenance costs are far less.
Initially the design was lashed up loosely on a breadboard, this lash up was very stable and allowed the listening tests to ascertain the sound quality that was found to be far superior to my original valfet design. The printed circuit boards were then designed as well as the chassis.
A pre-production model was then built to iron out any other problems that could possibly arise. Photographs and explanations are now published below of this pre-production model. At the back of the first photograph is the larger power output power supply transformer. The smaller transformer in front supplies most of the other power supplies. Two transformers are used to maintain isolation between power drain and signal circuits.
Moving towards the front of the chassis to the valves that are currently manufactured, the large valve on the left is the 5U4GBEH rectifier, the smaller valve to the right is the 6SN7EH signal amplifier valve. The front panel is not designed yet, there are holes in the front of the chassis for controls that can be fitted to some models. The two holes in a vertical line on the left can be used for fixing the front panel to the chassis.
To the right of the two vertical holes is a single hole, this can allow for a power output switch to be fitted to suit different power loudspeakers. To the right of this power output switch hole are three tricolour leds to indicate modes of operation, below the leds are two switches, mute and class a/b select. To the right of this group of leds and switches is another hole that allows for a valve gain selectable switch to be added to suit different pre-amplifiers.
To the right of the valve gain switch hole is the volume control with a volume control by-pass switch fitted above this control. The two in line vertical holes to the right of the volume control fix the right hand side of the front panel to the chassis.
Front Panel Not Fitted.
Looking at the back view there are two phono/rca sockets to the left, either socket is an input using the other socket for daisy chaining to other equipment. The two heat sinks are for the output and power supply transistors respectively. To the right of the heat sinks is the mains power input socket, on off switch, and power fuse tray. The earth and loudspeaker terminals are seen on the right hand side.
The amplifier comprises of four printed circuit boards. 1. The signals board that switches from class A to class B, this is a plug in board to the motherboard that can be fitted to some models. 2. The mother board. 3. The power output board. 4. The power supply board. All the boards are plug in boards.
The Signals Board.
In the top left hand quadrant is the final HT regulator and circuitry for the 6SN7EH valve and the circuitry that drives the indicator leds on the front panel. Top right quadrant is the buffer and temperature control circuit. In different models the buffer circuitry can drive up to ten sets of output transistors.
The Output Board.
The Power Supply Board.
The Underside View