Thrax Spartacus Mono Power Amplifier

Two stage shunt regulated directly heated triode power amplifier Spartacus features one of the simplest possible signal paths with absolute zero feedback. (wire-vacuum-wire-vacuum-wire).

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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 few orders of magnitude lower by default. Aspecial breed of tubes called Directly Heated Triodes (DHT) deliver an extra order of magnitude 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 accommodate 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.

As simple as the signal path is, the rest of it is not. Spartacus has one of the most sophisticated and complicated power supplies. We use a very well-known but rarely used arrangement.
To reduce power supply noise instead of the normal arrangement of diode bridge rectifier and electrolytic capacitor bank we use a choke input full wave tube rectifier arrangement. This practically eliminates switching noise and anything that comes before the choke.

The other beauty of this topology is that the choke keeps the voltage on the capacitor bank quite constant. In our case this allows for a considerably small capacitor bank opening the possibility to use highest quality film capacitors for the job.

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.

Thrax rearviewblu 280px.jpegblkWe use a special technique to supply 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 the big difference.

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 microprocessor control to make sure all is switched in the right sequence and that all separate systems function as expected.

The chassis to house all of the above had to be a masterpiece itself. Milled from solid plates of different thickness 6061T651 grade aircraft certified aluminium are used for the construction. 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 centre 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 all the sensitive electronics. There are no screws visible anywhere on the chassis. And the chassis feels like a solid block.
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