The heart of any amplifier is the power supply unit (PSU), which changes the mains voltage to safe voltages that the amplifier can use.
Most of the highest quality amplifiers use very large and heavy 50 or 60Hz mains transformers depending on the country. These along with rectifiers and reservoir capacitors go to make up the usual type of power supply generally in use today. This standard style of PSU has been designed to absorb and store energy or charge at a rate of 120 times per second. Unfortunately the demands for reproducing most types of music are much greater and sometimes this means that conventional amplifiers are not good at delivering their energy at audio frequencies.
In order to cope with these musical demands the conventional amplifiers often use components that are unnecessarily large and this factor dictates the size of the amplifier. The normal use of linear voltage regulators associated with this type of power supply makes the situation worse still. Requiring the amplifier to dissipate much more power in the form of heat – the more power the amp has to dissipate as heat, the larger the heat sinks, the larger the design, the less compact the audio signal. Thus this type of amplifier is bulky, heavy, sometimes slow to respond, generally inefficient, causes high mains power distortion, can cause electrical and acoustic noise and requires a relatively long and exposed audio path. Likewise, with these types of amplifiers, a common problem is that frequencies below 120Hz are not well represented and feel inaccurately timed due to the capacitors slow recharge rate.
All Chord amplifiers have been designed around advanced ‘high frequency’ power supply technology, which was originally developed for use in aerospace, telecommunications and data processing where quality and power is needed but in a very space efficient design.
Some manufacturers recognised the benefits of this type of PSU but couldn’t perfect the design for audio applications and although some amplifiers (launched in 1980) were innovative they were also a commercial failure due to electrical noise, poor reliability, high engineering costs and other associated problems.
It took John Franks, founder and chief designer, of Chord Electronics around ten years of perseverance and innovative engineering to overcome the problems of the early power device components and the associated techniques before a true high end product could be developed.
The promise of an audio power amplifier that was compact, rugged, highly efficient, reliable with fast response, good mains input and load tolerance with low acoustic noise was not realised from this concept until 1989, and the launch of the Chord SPM900 stereo power amplifier.
Innovative Technology – Chord High Frequency Power Supply
The Chord power supply is a self-contained, self-monitoring module, which is common to all Chord amplifiers, and is fundamental to their exceptional performance. A sophisticated mains input filter ensures that not only is the power supply shielded exceptionally well from disturbed mains input, but also, and more significantly, the mains itself is completely untouched by emissions from the amplifier. The amplifiers comfortably exceed European Electro magnetic emission standards, which are acknowledged to be the toughest in the world.
In a high frequency power supply the incoming mains is filtered and then rectified to generate a very high voltage DC Supply. This is around 300 to 350 Volts DC, which is far too high to operate audio circuitry, and is still connected to the incoming mains. The resultant D.C. is stored in a bank of high voltage capacitors. It is then chopped using high voltage MOSFETS running at 80kHz. The resultant waveform then passes through a very special ceramic cored high frequency transformer, which is wound with individual multi strand litz wire to avoid copper losses associated with the ‘skin effect’ at these frequencies. The size of a transformer reduces as operating frequencies increases, so a transformer operating at 80,000 Hz is far smaller than one operating at mains frequency of 50 or 60Hz. The output from the transformer is then rectified once more and passed on to a truly innovative ‘Dynamic Coupling’ system before final storage in a further massive bank of high voltage capacitors.
On the output of the transformer, very fast rectifiers, a small coil and small capacitors filter and convert the high frequency waveform back to DC, ready for the electronics circuitry to use. By controlling the timing of the switches the output voltage can be held constant or varied, as required, without the need for further inefficient voltage regulators. This processing all happens well above the range of human hearing, which, in principal, should be good for audio circuitry.
‘Dynamic Coupling’ is a unique system whereby the positive and negative rails are mutually coupled tightly together by a strong magnetic flux. The result is that if a high demand is put on one rail, the required energy will actually be drawn equally from both rails. The rails will therefore stay completely and perfectly balanced at all times with the mid ground point and each rail in turn. Simply put, this system keeps the amplifier in balance dynamically and the reference point perfectly clean.
The power supply has been designed to be extremely ‘flexible’, and will absorb tremendous demands from the amplifier section without stress. Enormous reserves of power can be instantly reproduced to their full extents. The amplifier sounds simply effortless.
The amplifier section is a class AB sliding bias design which operates in class A during most normal use, with class B operation introduced only in the most demanding situations. The output stages are designed around metal on silicon MOSFET devices developed exclusively for Chord.
Mains transformers, rectifiers and capacitors are very simple. Designing a good switching device is not. The PSU in Chord products is a complex but ultimately reliable engine for our amplifiers. There are many thousands of Chord power amplifiers in existence throughout the world, many of them being used in the world’s most famous studios. Amplifier downtime means studio downtime, and at Abbey Road, where a days studio rental costs thousands of pounds, ultimate reliability and world class sonic performance have led to Chord amplifiers being installed throughout the entire complex.
Electrical noise with Chord PSU is not an issue. Both measured and subjective listening tests prove that high frequency ‘hash’, so common in our competitor’s products is negligible within Chord products. Ten years of research and development in this field pays dividends, with Chords products meeting the CE emissions, harmonics and immunity tests with ease.
To put the magnitude of Chords achievements into perspective, the latest PSU that has been developed can deliver 4000 Watts (mains outlet permitting) from its ferrite based transformers. To get this power from a conventional design you would have an iron based transformer the size of a small car wheel, only much heavier of course.
With Chord on its fifth generation of audio power supply, we are be flattered that other audio companies have copied our lead and have acknowledged that using high frequency power supplies are the best solution for audio reproduction of the highest fidelity. We currently have three PSU designs a 600 Watt, a 2000 Watt and a 4000 Watt, each one suited to its application in our product range giving us the flexibility and control of power that no other company has.
The Audio Circuit
Fundamentally there are two types of power device. Majority carrier (bipolar) transistor or metal on silicon (MOSFET) devices which are usually much more expensive to manufacturer.
It should be remembered that the first truly high volume, high power bipolar transistor, the ubiquitous 2N3055 was originally developed as a triggering device for an atomic bomb as of course it is a switch, as all bipolar transistors are. These devices are not good in their transition period between ‘on and off’ which, unfortunately for Bipolars, is where they are used in audio amplifiers.
The Chord output stages are designed around metal on silicon “MOSFET” devices developed exclusively for Chord electronics by a UK semiconductor fabrication house formerly involved heavily in the aerospace sector. Chord Electronics approached them to produce a 200 Volt/ 300 Watt device, which has been realised by use of a novel packaging technique, which places two silicon chips in parallel in the same TO3 style case. This technique has the significant advantage of ensuring perfect thermal matching of the devices, eliminating the need for temperature balancing resistors in the output stage, thereby substantially improving both efficiency and the overall stability of the amplifier. Our MOSFET design has been refined over time and is now in it’s fifth generation. So even our smallest amplifiers benefit from using our own bespoke silicon.
Why go to all this trouble?
Many of our competitors use Bipolar transistors and have to operate them in class A. This means that when the amplifier is not delivering its power into the load, the power is dissipated into itself, leading to larger heat sinks and a general scaling up of all related power components. The only benefit of the bipolar transistor is their good transconductance criteria, but the Chord MOSFET is now comparable.
The approach of the ‘A class’ designer is not to turn the devices off to overcome the problems of operating the devices in their transition region, this means high inefficiency and lots more heat. For designers who choose to operate Bipolar transistors in Class A/B amplifier designs other complications usually arise. The biasing control and timing within the local feedback loops attempting to control these devices give rise to inconsistent performance over a temperature range and between individual units.
Output Protection Circuitry
The Chord Output Protection System relies on a magnetic flux being generated between the power supply rails. We carefully sense minute flux density differences between the two rails over a given time period, simply put we use a mathematical algorithm to determine whether power should be delivered or withheld by asking the question ‘Is this audio power being required or not?’ If it is the amplifier will give it, if not (usually a fault condition, short circuit or a dc offset that could damage the loudspeakers) power is then withheld and the amplifier is switched to standby mode thus protecting the loudspeakers.
