Williamson amplifier
Encyclopedia
A Williamson amplifier refers to a type of vacuum tube (valve) amplifier
whose circuit design is similar to that originally published by D.T.N. Williamson.
published a pair of articles by D T N Williamson, under the title "Design for a High-quality Amplifier." The design, later referred to as the "Williamson Amplifier" was, perhaps, one of the first examples of a high fidelity
audio amplifier (although the LEAK
"Point One" Type 15 amplifier of 1945 pre-dates it). The 1947 articles aroused a level of interest that stimulated the publication of an updated version in 1949 (see references below). The detailed descriptions inspired many builders to construct copies, both for their own use, as well as for sale, and the design became widely known for the notably high quality of its audio reproduction.
design and used negative feedback
and a specially designed output transformer to produce lower levels of distortion
than previous designs.
The design used triode
s as phase inverters and drivers. The output stages were triode-connected KT66
tetrode
s, although a 6L6
, with slightly lower output, could also be used.
A notable characteristic of the design was the use of a negative feedback loop enclosing the whole amplifier, including the output transformer—in contrast to the use of a number of separate smaller feedback loops around individual stages of the circuit. Care had to be taken to avoid excessive phase-shift around the feedback loop, to avoid the feedback becoming positive at any frequency, which would cause oscillation.
Earlier designs used transformers to couple the output signal of one stage to the next. In particular, it was very easy to use a transformer with a centre-tapped secondary to produce from the single-ended signal output by the earlier voltage amplification stages a pair of signals in antiphase to drive the two output valves.
Signal transformers are a source of distortion; if the transformers are not included in the feedback loop, this distortion is not corrected. However, transformers produce large phase shifts at higher frequencies, which makes the use of feedback loops including the transformers, or overall feedback, problematical.
Williamson avoided all transformers except the output transformer by using a split-load valve voltage amplifier stage ("phase splitter
") to provide antiphase signal inputs to the symmetrical push-pull output stages. This permitted significant amounts of negative feedback to be used without instability. (See Linsley Hood (1993) for discussion of characteristics of the Williamson amplifier design and its performance compared to earlier and later designs.)
The Williamson design demonstrates the maturity that tube amplification reached in the late 1940's. Other than the British Mullard 5-10
circuit and David Hafler's Dynaco
ST-70, there was little improvement in the fundamentals.
Even two decades later, the Williamson amplifier's performance became a standard of comparison for innovative successor developments, including well-known semiconductor audio amplifiers (see for example Linsley Hood (1969)).
Valve amplifier
A valve amplifier or tube amplifier is a type of electronic amplifier that makes use of vacuum tubes to increase the power and/or amplitude of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and...
whose circuit design is similar to that originally published by D.T.N. Williamson.
Explanation
In April and May 1947, the British magazine Wireless WorldWireless World
Wireless World was the pre-eminent British magazine for radio and electronics enthusiasts. It was one of the very few "informal" journals which were tolerated as a professional expense.- History :...
published a pair of articles by D T N Williamson, under the title "Design for a High-quality Amplifier." The design, later referred to as the "Williamson Amplifier" was, perhaps, one of the first examples of a high fidelity
High fidelity
High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
audio amplifier (although the LEAK
LEAK
LEAK is the brand name for high-fidelity audio equipment made by H. J. Leak & Co. Ltd, of London, England. The company was founded in 1934 by Harold Joseph Leak and was sold to the Rank Organisation in January 1969. During the 1950s and 60s, the company produced high-quality amplifiers, radio...
"Point One" Type 15 amplifier of 1945 pre-dates it). The 1947 articles aroused a level of interest that stimulated the publication of an updated version in 1949 (see references below). The detailed descriptions inspired many builders to construct copies, both for their own use, as well as for sale, and the design became widely known for the notably high quality of its audio reproduction.
Description
The Williamson amplifier was of symmetric push-pullPush-pull output
A push–pull output is a type of electronic circuit that can drive either a positive or a negative current into a load. Push–pull outputs are present in TTL and CMOS digital logic circuits and in some types of amplifiers, and are usually realized as a complementary pair of transistors, one...
design and used negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
and a specially designed output transformer to produce lower levels of distortion
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
than previous designs.
The design used triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
s as phase inverters and drivers. The output stages were triode-connected KT66
KT66
The KT66 is a beam tetrode/kinkless tetrode vacuum tube for audio amplification.KT66 is the designator for a vacuum tube introduced by Marconi-Osram Valve Co. Ltd. of Britain in 1937....
tetrode
Tetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
s, although a 6L6
6L6
6L6 is the designator for a vacuum tube introduced by Radio Corporation of America in July 1936. At the time Philips had already developed and patented power pentode designs, which were fast replacing power triodes due to their greater efficiency...
, with slightly lower output, could also be used.
A notable characteristic of the design was the use of a negative feedback loop enclosing the whole amplifier, including the output transformer—in contrast to the use of a number of separate smaller feedback loops around individual stages of the circuit. Care had to be taken to avoid excessive phase-shift around the feedback loop, to avoid the feedback becoming positive at any frequency, which would cause oscillation.
Earlier designs used transformers to couple the output signal of one stage to the next. In particular, it was very easy to use a transformer with a centre-tapped secondary to produce from the single-ended signal output by the earlier voltage amplification stages a pair of signals in antiphase to drive the two output valves.
Signal transformers are a source of distortion; if the transformers are not included in the feedback loop, this distortion is not corrected. However, transformers produce large phase shifts at higher frequencies, which makes the use of feedback loops including the transformers, or overall feedback, problematical.
Williamson avoided all transformers except the output transformer by using a split-load valve voltage amplifier stage ("phase splitter
Phase splitter
A phase splitter is a device that separates a signal into multiple phases .The term is most often applied to amplifiers that produce two "balanced" voltage outputs: of equal amplitude but opposite polarity , but sometimes is used to refer to the generation of quadrature signals...
") to provide antiphase signal inputs to the symmetrical push-pull output stages. This permitted significant amounts of negative feedback to be used without instability. (See Linsley Hood (1993) for discussion of characteristics of the Williamson amplifier design and its performance compared to earlier and later designs.)
The Williamson design demonstrates the maturity that tube amplification reached in the late 1940's. Other than the British Mullard 5-10
Mullard 5-10
The Mullard 5-10 was a circuit for a valve amplifier designed by the British vacuum tube company, Mullard in 1954 at the Mullard Applications Research Laboratory in Mitcham Surrey UK, part of the New Road factory complex, to take advantage of their particular products...
circuit and David Hafler's Dynaco
Dynaco
Founded by David Hafler and Ed Laurent in Philadelphia, Pennsylvania in 1955, Dynaco was an American hi-fi audio system manufacturer popular in the 1960s and 1970s for its wide range of affordable, yet high quality audio components..-Early company history:...
ST-70, there was little improvement in the fundamentals.
Even two decades later, the Williamson amplifier's performance became a standard of comparison for innovative successor developments, including well-known semiconductor audio amplifiers (see for example Linsley Hood (1969)).