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A30 Line Integrated Amplifier
The pure sound A30 amplifier was developed to meet the demand for an affordable Class
A amplifier that would be compatible with all types of loudspeakers. The A30
is a line level integrated design that allows for the selection of up to 3 different source components and control of the
volume by a high quality ALPS Blue Velvet potentiometer. Class A operation was
chosen because it gives lower distortion and a more relaxing, natural sound.
NB. The factory that makes the A30 also makes a 60 Watt Class B amplifier with different output
transformers, a different mains transformer, reduced bias current, a different feedback circuit and several different types
of component. All pure sound products are specifically made to suit our particular requirements.
The amplifier uses the very robust Electro Harmonix 6550 output valves in an Ultra Linear Push Pull configuration. This arrangement gives a combination of power and clarity that is difficult to beat. The amplifier uses an Auto Bias system, which means that replacement output valves
should be fitted as matched pairs. There will be no need to reset or monitor
bias conditions.
Very high quality components are used throughout including selected carbon film resistors
and SCR polypropylene capacitors. The power supply makes use of twin valve rectifiers and choke smoothing.
The output can be configured for use with 4 or 8 ohm loudspeakers.
The output stage can also be switched to triode-coupled configuration.
This further linearises the operation of the valves at the expense of some output power. It may be preferable with higher sensitivity loudspeakers.
The A30 may also be used as a very high quality stereo power amplifier if the volume control is turned up fully.
When partnered with appropriate ancillary equipment the A30 is capable of very satisfying performance.
Why this amplifier? Why now?
With the recent interest in lower powered Single Ended triode amplifiers has come the realization that to get
the best from these, the speakers used have to be both sensitive and quite large to give truly satisfying results. There are
also not that many good, high sensitivity (>97 dB/W) designs available at any price never mind at price levels the majority
of music lovers can afford.
However, there are some very good loudspeakers currently being made with sensitivities between 88 and 94 dB/W.
These are sufficiently revealing to make the virtues of simple, high quality amplification readily apparent, although in most
cases the output of a typical single ended amplifier is insufficient to give the scale and drama many types of music demand.
A good amplifier with 25 -30 watts of output power is more than enough to drive this type
of loudspeaker to sensible levels even in quite a large room. For simplicity a valve amplifier is preferable. Recently, there
has been an increase in the number of relatively affordable valve amplifiers
made in China.
Many of these are compromised by the quality of their transformers, components, layout and because they operate in Class AB or B to give
a higher power specification. They often also use a lot of negative feedback in an attempt to compensate for these shortcomings
with a consequential impact on clarity and low frequency control.
For some 10 years there has been a lack of affordable Class A valve amplifiers.
The pure sound A30 operates fully in Class A up to its rated output so it does not
change character at any signal level. There is none of the distortion caused by output devices switching on and off and therefore
no requirement for excessive feedback to correct for it. The 6550 valves supplied are the extremely robust Russian Electro-Harmonix
variety and are quite content with dissipating the power required to maintain Class A operation up to 30 Watts.
The valve rectifiers used keep the high voltage power supply clear of the noise that can be introduced by silicon
rectifiers. A substantial choke is also incorporated to keep the power supply impedance low.
The output transformers also give extremely wide bandwidth allowing very solid low frequency performance as
well as exceptional reproduction of timbre.
These design features combined with the use of high quality passive components, octal based input and driver
triodes and an elegant but simple internal layout results in an amplifier which sets a new benchmark for performance, build
quality and value.
Features
Class A operation
3 Line level inputs
30 Watts per Channel Output
4 and 8 ohm output taps
Twin valve rectifiers
Choke smoothing
ALPS potentiometer
SCR coupling capacitors
Carbon film resistors (for better sound quality)
ElectroHarmonix (Russia) 6550 output tubes
Very substantial construction including cast metal transformer shrouds.
A8000 CD Player
The pure sound A8000 CD
Player was developed as a high quality source to be used with the pure sound range of amplifiers. It can also be used in a wide range of other systems with great success.
The A8000 is built around the SONY 213Q high precision
mechanism, renowned for both its stability and reliability. A proprietary 24
Bit 192KHz D/A converter handles the data stream, which, once converted, is filtered using OPA2604 Op amps. The powerful output
stage comprises a pair of 6N3 double triodes. Signal coupling here is via MIT capacitors while the output stage power supply
utilises the German MCap capacitors.
The incoming mains supply is filtered by a choke
regulated circuit which in turn feeds 8 separate regulated supplies each of which uses Silmic and Rubycon capacitors. These
deliver power independently to various parts of the circuitry.
At switch on, the CD Player counts down for 20 seconds
to allow the valve circuitry to warm and stabilise before the machine can be used.
The A8000 also offers the facility to switch the upsampling
frequency from 44.1 KHz to 88.2 or 176.4 KHz.
The substantially constructed chassis includes a
10mm anodised aluminium front panel, high quality gold plated RCA sockets for L-R and digital outputs.
The remote control is a sleek, anodised design machined
from solid aluminium.
Many CD Players using a valve output stage have an
overly euphonic, almost ‘syrupy’ quality which compromises the reproduction of detail and drive. The A8000 has
no such obvious characteristic. However, it does reproduce instrument timbre
and dynamic range in a plausible way providing a lively yet unfatiguing presentation of music not dissimilar to that which
can be heard at a live concert.
2A3 Line Integrated Amplifier
The earliest audio amplifiers made
use of small, 3 element valves known as directly heated triodes. For many, the simple circuits that can be built around such
devices better preserve the structure, colour and feeling within the music than any other circuit topology. One of the best
sounding triodes, the 2A3, is in production again and its characteristic linearity has been employed in a novel configuration
to create a particularly charming amplifier. The Pure Sound 2A3 is an 18 Watt
per channel line level integrated design that allows for the selection of up to 3 different source components and control
of the volume by a high quality ALPS Blue Velvet potentiometer.
There are many amplifiers now making
use of directly heated triodes but few which adequately address the particularly awkward issues involved in driving these
devices properly. The Pure Sound 2A3 amplifier features an unusual choke loaded driver stage which allows the 6SN7 driver
valves to deliver the full measure of signal to the push/pull output valves. This
rather exotic configuration usually results in a tremendous depth to the tone of instruments and the Pure Sound 2A3 is no
exception in this regard.
Very high quality components are used
throughout including selected carbon film resistors and SCR polypropylene capacitors.
The power supply makes use of twin valve rectifiers and choke smoothing.
The output can be configured for use
with 4 or 8 ohm loudspeakers.
The Pure Sound 2A3 may also be used
as a very high quality stereo power amplifier if the volume control is turned up fully.
A10 Line Integrated Amplifier
The A10 is a new, small line integrated amplifier from pure sound. Operating
in Class A and delivering 10 Watts per channel it is intended for users of high sensitivity loudspeakers or for
those seeking high performance with small speakers perhaps in a bedroom or study. The A10 accepts two line level inputs
and may be switched to drive 4 or 8 ohm loudspeakers. The circuit is based around the sweet sounding 6P14 a modern equivalent
to the EL84/6BQ5 family.
P10 Phono Amplifier
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| HiFi World February 2008 |
The P10 phono amplifier is the latest addition to the highly acclaimed
Pure Sound product range. It is intended to meet the requirements of audio enthusiasts who still enjoy the distinctive
strengths and high quality of reproduction that can be derived from vinyl records. It
is compatible with moving magnet and high output moving coil cartridges. Low
output moving coil cartridges will require the use of an additional step-up device or matching transformer. Full output from the P10 is sufficient to drive the line level inputs of any integrated amplifier or pre-amplifier.
The active amplification stages use double triodes in a simple anode follower
configuration. The phono equalisation is achieved using passive components and can therefore maintain its accuracy over time
without introducing the unacceptable levels of phase shift found in active equalisation stages. There is no negative feedback
in the circuit.
The P10 features a particularly elaborate HT power supply. After the initial rectification and smoothing, the supply is split and two separate, heavily decoupled
high voltage rails are established. These feed each half of the output valve while further decoupling of each rail allows
the cleanest and most independent voltage rails possible to feed each half of the input valve.
The absolute stability of this supply regardless of the demands made by the audio signal on the circuit, lends the
P10 a stability and poise which allows effortless reproduction of the most demanding source material. The valve filaments are also fed by a DC supply thereby ensuring constant operating conditions for the
amplifier.
The circuit makes use of close tolerance metal film resistors, polypropylene
signal coupling capacitors and selected valves.
The
gain of the P10 is 40 dB (x 100).
The P10 phono amplifier meets the very strict performance criteria required
of all Pure Sound products and will give excellent results with vinyl records provided that it is used with appropriate ancillary
equipment.
T10 Impedance Matching Transformer
The T10 mc transformer is now available. It
has been our experience that the best results are achieved when using moving coil cartridges loaded via a correctly matched
transformer. The T10 contains two wide bandwidth, custom manufactured transformers with MuMetal cores. These have a natural
unloaded ratio of 15:1. The T10 also includes the provision to alter the load
seen by the cartridge. It allows different secondary load resistances to be placed across the secondary winding of each transformer.
These, acting in parallel with the load resistance (typically 47Kohms) of a phono stage are reflected through the transformer
to the cartridge. An optimum loading for the cartridge chosen can be readily found by ear. The transformers have a
step up ratio of 1:15 and a provision to adjust the cartridge loading between 30, 50, 75, 100 and 200 ohms.
L300 Line Level Pre-amplifier
Following some fairly extensive r&d applied to line level amplification, pure
sound will soon be launching two line level pre-amplifiers, L10 and L300.
L300 has 5 inputs
and twin outputs (2 pairs single ended and 2 pairs balanced via XLR). The circuit uses output transformers and a sophisticated
full valve rectified, choke smoothed, regulated power supply including a 300B triode as the series element.
A Line Stage pre-amplifier
needs to offer the facility to select between different sources, to provide some means of attenuating the chosen source and
then deliver it in an uncorrupted form to the power amplifier.
Controlling the Volume
In recent times,
with the prevalence of digital source components, many audio enthusiasts have adopted the use of passive volume controls incorporating
resistive potential dividers and even transformer based attenuators. These are volume controls where attenuation is achieved
through the use of tapped transformer windings. On the face of it either of these
might appear to offer the purest approach. However, it’s not so simple. Resistive attenuators are generally of quite a high impedance so as not to load the
source driving them but in turn confer little drive capability themselves. In
some circumstances they can sound quite transparent but it’s a commonly experienced subjective impression, that such
attenuators leave the music with a lack of purpose and grip. Long connecting
cables may also blunt the high frequency response, a situation which may be worsened by using a following amplifier with low
input impedance.
Transformer volume
controls seem to address the issue of drive capability to some extent, but they have their own problems. A wide bandwidth transformer can be made such that, when using both windings in full, i.e. with no attenuation,
it transfers a pretty good facsimile of the source signal. However, as attenuation
is applied and the transformer has to transform more and more, difficulties start to arise.
By the time significant attenuation is applied, the performance of the transformer no longer looks quite as impressive
with differing patterns of resonance being visible at each step. The reproduction
of timbre suffers and the sound quality realized alters slightly with each change in level.
Auto-formers which attenuate by tapping off a single sided winding have actually proved to be the most transparent
passive controls we have tested and yet even they seem to lack a certain something.
The other issue with volume
controls based on a multi position switch is a sense that the right level for listening to a piece of music can never quite
be reached. If the steps of the attenuator are made close enough to suit a source with a given output or a following amplifier
with a particular sensitivity, it will be wrong with another source or another amplifier. A conventional potentiometer gives
a much finer range of adjustment.
In the L300, source selection
is via a high quality switch located near to the input sockets. The chosen source is routed to a high quality film potentiometer
and from there on to the audio circuit. This consists of 2 directly coupled triode
stages, the second loaded by an output transformer which steps down the amplified signal allowing it to be coupled to the
power amplifier at a manageable level and via a low impedance output. The output
stage of the L300 is impervious to long interconnecting cables or lower impedance power amplifier input stages. The use of
output transformers also allows the option of a true balanced output configuration if desired.
A low gain, low output
impedance stage is in itself nothing new. Many transistor pre-amplifiers are similarly specified. However, they usually achieve it by means of excessive negative feedback which is highly detrimental to
the subjective reproduction of music.
Power Supplied properly
The L300 also incorporates
an extremely sophisticated regulated power supply to maintain constant operating conditions for the audio circuitry. It includes
a valve rectifier, a choke filtered stage and then a series regulator featuring the 300B power triode as the passing tube.
A very elaborate error amplifier referenced to a gas voltage reference valve allows the circuit to precisely maintain the
high tension supply regardless of fluctuations in the mains supply and the varying current drawn by the audio circuit. Filament
supplies are also fully regulated to further reduce the potential for noise.
The L300 preserves the
verve and energy in recordings that almost all pre-amplifiers and passive volume controls otherwise lose. It does this without sacrificing transparency or introducing
euphonic colouration to the sound.
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