The C3g tube handles all the voltage gain needed. It uses a new generation or our choke rectified shunt regulated power supply. providing a stable noise free operating point.
The output stages power JFET cascodes form a bridge that when unbalanced by the input signal via the 2 transformer windings restores balance by adjusting their current conduction through the load. The 2 output stages actually enclose the load in their control loops and try to correct each others errors reducing the distortion to negligible amounts. Leaving only the “character” of the input stage.
To reduce the amount of heat generated the amplifier is biased so only the first few watts are in true class A. Music has a crest factor of 10 on average. This means that most of the time we listen with 1/10th of the peak levels. For most users this translates to under 5W. Peaks could reach over 100 watts during playback, but only for few milliseconds. For those peaks the amplifier switches to class B using only one leg of the output stage capable to deliver up to 250W/8ohm and 350w/4ohm.
Not sharing a common reference point apart from the load itself makes the second leg correct for any error produced by the first one. Result is an amplifier capable of extreme power wile delivering exquisite smoothness and low level resolution.
The driver stage is isolated by the interstage transformer and is not influenced in any way by the output stage maintaining tone colour and resolution at all powers and loads.
Having the output stages floating around the load prompted for the use of a unique biasing technique powered by solar cells. This was the quietest galvanically isolated power we could come up with. Using opto-electrical conversion we control the bias by adjusting light intensity in the control circuit.
Teres is an example of modern technology solving age old problems allowing us to go a step beyond.
The physical appearance of Teres is typical Thrax understated elegance. Exquisite finishing ads to the presentation of a solid block of metal like a bigger brother of Heros.
Blending this with stylish aesthetics, produced a beautiful sculpture - to be known as the Heros.
Operating all devices in Class A produces a fair amount of heat. The design of the chassis is such as to cope with the heat and yet to provide vibration free structure for the components. All the electronics are enclosed in separate compartments as not to interfere with each other.
When properly applied the rectifier produces no EMI/RFI in the chassis. The result is an amplifier free of grain and frizziness, giving new meaning to transparency and tone.
The design being fully differential is immune to power supply noise, but still we use our flagship current source shunt regulator for the input stage.
The power supply for the output stage uses a new breed of rectifier diodes using a new material called Silicon Carbide. They have no reverse current and switching loses.
Almost double the efficiency of a normal Class A design, no DC on output and many other benefits are taken advantage of in our implementation. Like a fully differential (balanced) architecture using an input phase splitting transformer and summing back at the push pull output transformer. This solution completely floats the inside of the amplifier isolating it from any external influences.
The Heros are an unusual two stage hybrid transformer loaded amplifier. Based on our design concepts and experience it features a tube input stage providing the necessary voltage amplification followed by a transformer loaded cascaded jfet/mosfet output stage taking care of current gain. No solid state amplifier designer would attempt to use a transformer for speaker output as the whole concept of solid state amplifiers is to avoid it. The numerous benefits from using an output transformer are usually ignored mostly due to cost reasons. Yet the result speaks for itself.
As tubes age and cannot be perfectly matched we have designed a circuit that keeps the current in both output tubes the same. High power versions of the current sourced shunt regulators are used for both stages. We use a microprocessor control to make sure all is switched in the right sequence and that all separate systems function as expected.
To achieve simplicity while delivering exemplary performance each device in the signal path had to be the best we could lay our hands on. The distortion of triode vacuum tubes compared to solid-state devices (transistors) when used for voltage gain is a few orders of magnitude lower by default. A special breed of tubes called Directly Heated Triodes (DHT) deliver an extra order of magnitude of better distortion performance compared to indirectly heated ones, but not all DHT's are the same. Emission Labs manufacture some of the very best tubes ever made. They were the obvious choice for this daunting task. Even then we decided to go further and run two of them out of phase in an attempt to cancel some of the negligible distortion they would generate. For this to work we had to design an even more linear driver stage that would supply the two output tubes with exactly the same signal in phase for one and out of phase for the other. The choice was clear-use another DHT. The tube used produces enough power and voltage swing to accomodate a two-fold overdrive and positive grid drive if needed. Ultra linear DHT tubes have considerably low voltage gain and require high input voltage for full power. We implemented an input solution with the cleanest possible voltage gain - a step-up transformer.
A power supply operates best when the load it's presented is constant. Following that thought, both stages of the amplifier were designed to use the same type of regulators namely, the constant current sourced shunt regulators. A constant current source is a device that passes the exact same amount of current no matter what happens on either end. This is followed by a shunt regulator that will try and keep a constant voltage across itself by changing its conductivity no matter what. At the voltages we operate in, this has become possible only very recently.
The Thrax Spartacus Power Amplifier is a two stage shunt regulated directly heated triode power amplifier featuring one of the simplest possible single paths with absolute zero feedback. (wire-vacuum-wire-vacuum-wire).