Thrax Spartacus


Spartacus features one of the simplest possible signal paths with absolute zero feedback (wire-vacuum-wire-vacuum-wire). To achieve simplicity while delivering exemplary performance, each device in the signal path had to be the best possible. 
Emission Labs and KR Audio manufacture a special breed of tubes called Directly Heated Triodes. They are the most linear amplifying devices and some of the very best audio tubes ever made. 
It is a fact that the distortion of triode vacuum tubes compared to solid-state devices is orders of magnitude better.


Apart from the low distortion level, the type of distortion they produce is 2nd harmonic. To go further we employ a technique to eliminate the little distortion they would generate by running two of them out of phase so that they cancel out each other's distortion. For this to work we designed a 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 by using another super linear DHT - the PX25. It is loaded with a perfectly symmetrical phase split transformer with independent windings providing separate bias to the output tubes. Not a popular approach to design, at the voltage and currents involved, as it is extremely difficult to produce. But rewarding when done. 
The most linear DHT tubes have considerably low voltage gain and require high input voltage for full power.


Again we implemented an input solution with the cleanest possible voltage gain - a step-up transformer with switchable ratio.
Spartacus has one of the most sophisticated power supplies. 
To reduce power supply noise we use a choke input full wave tube rectifier arrangement. This eliminates switching noise and this topology keeps the voltage on the reservoir capacitor bank constant. In our case, this allows for a considerably smaller capacitor bank using of the highest quality film capacitors. Power supplies operate best when the load presented is constant - namely, with 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 by varying it's voltage and a shunt regulator that will try and keep a constant voltage across itself by changing its conductivity no matter what follows this.


At the voltages we operate it in, this has become possible only very recently. 
We use a special technique to supply power to the heaters (filaments) of the tubes. Following a separate winding, rectifier and capacitors is a voltage regulator supplying an electronic choke isolating the filament from the DC source and counteracting modulation of the heater current by signal going through it. It is complicated but small things make a big difference. 
As tubes age and cannot stay perfectly matched we designed a servo circuit that keeps the current in both output tubes constant and matched to a predefined reference. 
We use microprocessor control to make sure all is switched in the right sequence and that all separate systems function as expected.


In case of malfunction in any of the subsystems the amplifier shuts down to protect the tubes. 
The chassis to house all of the above had to be a masterpiece itself. Milled from solid aluminium billets. 
We have the power supply mounted on a mid level plate of this structure with the control board on top and the transformer and choke hanging below bringing the center of gravity low enough so the signal transformers can be mounted on the top plate to avoid magnetic interference. 
The top and mid plates link the front and back plates forming a protected cavity housing for all the sensitive electronics. 
There are no screws visible on the chassis. And the chassis feels like a solid block.