By Jon Wong

The tube power amplifier can make CD audio music sounds much more musical, beautiful and lifelike.  As such, tube amplifiers usually enjoy a ‘higher’ status than many transistor amplifiers.  From the point of electronics design, all designs are compromises. For tube amplifier designs, the amount of compromises can be more significant than modern transistor designs.  However, the tube has its advantages. Despite the compromises, it has higher tolerances and the sound is great.  

For pure tube amplifiers, due to the high internal impedances of the tube, require the use of output transformers in order to connect to the low impendence loud speakers.  It is this transformer that brings about a complex combination of compromises. Due to laws of physics, if the output transformer were to deliver good bass, the high frequency performance will suffer. Vice versa, if the high frequency were to be very delicate and refined, the bass performance will suffer.  This is a case of balance and choice, a compromise. 

Although the British has thought of ingenious windings for the transformer, the Japanese has made high quality, high saturation silicon steel for the transformer; the transformer is still a bulky, expensive and troublesome piece of equipment.  For many modern loud speakers, the impedance is dynamic and can drop to below 4 or 1 ohm and musical passages.  This is equivalent to a direct short circuit, which pose a serious challenge to many power amplifiers.  For some amplifiers, the end result could be charred components.  If we want to have a high power, high current amplifier, which has the refinement of a tube amplifier, is it possible? 

Modern MOSFET are high speed, fast switching and delivers high power.  The MOS device also mimics the vacuum tube, which allows the replacement of traditional push pull or single ended tube/transformer combo.  Actually, hybrid designs are not new. Many manufacturers have made famous amplifiers, such as the Counterpoint SA20, Musical Fidelity F15, F18, F20 and the Copland amplifiers.  Unfortunately, due to the inherent parasitic oscillation and instability of early MOS devices, some excellent sound amplifiers built more than 10 years ago are severely plagued by the poor first generation MOSFETs. However, modern MOSFETs or HEXFETs are built as strong as tanks. They have as good a frequency range as the tube, and the current capability than reach more than 20 amps. As such, MOSFET are gaining popularity in new power amplifiers. In Nelson Pass’s famous amplifiers, MOS are extensively used.

To have the best of both worlds, the author has thought of a 300W power amplifier called the HP300B. It should have the delicate and refined sound of a 300B, and also power like a power plant. 

To achieve this, this power amp uses a high quality, simple, single ended vacuum tube voltage driver coupled with a MOSFET stage that is realised by a push pull, source follower circuit that is capable of delivering continuous 300W RMS into 8 ohms load. 

This design will have the warm and refined sound of tube, and the capability to drive almost all loudspeakers in the market. The high and lower frequency performance is on par with the classics from Counterpoint or Musical Fidelity.  It drives the Magnepan III with ease.  It strongest merit is its stable and accurate biasing circuit.  The signal to noise ration is excellent (better than 85dB on good tubes).  With the ears placed next to the speakers, there is no hint of any noise, often mistaken that the amplifier was not turned on or the speakers .