Chord Amplifier Design

The heart of any amplifier is the power supply unit (PSU)...

The heart of any amplifier is the power supply unit (PSU) which changes the mains voltage to safe voltages that the amplifier can use.

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 beingrequired 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.

Our thermal management is based upon pure convection through good design. We do not believe electromechanical fans are acceptable in a domestic environment although in our professional range we have opted for forced air-cooling. These amplifiers are usually rack mounted out of the listening environment, and although our large dual 120mm ultra low noise fans are quiet we would not dream of designing a product for the home that needs this approach - as this would be totally wrong.

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