The Smart Bias Technology

The Smart Bias™ Technology was developed to offer you a truly new music-listening experience as you can now adjust the bias with a simple switch.The bias becomes a configurable setting which brings about a new user experience.

The need for a smarter bias method

Traditional methods of biasing suffers from various drawbacks (please have a look at the Knowledge Center to read more)

The Smart Bias overcomes all traditional biasing methods drawbacks :

  • Stabilization of the biasThe current in tubes is stabilized in any conditions : line voltage fluctuations, temperature change, warming up and aging of the tubes ...

  • Failure detectionWhen a tube fails, the Smart Bias stops the amplifier and indicates which one needs to be changed

  • Damaging risksThere are no risks of damaging the amplifier with bias tuning since all is automatic

  • Current balanceThe balance of currents in Push-Pull output stages is constantly maintained which is critical to get the best sound (Read more)

  • Tube agingWhen a tube cannot supply anymore the required current due to his aging, the Smart Bias indicates which one is aged and invites you to switch to a lower bias setting. This lets you listen to your music even when a tube is over.



The Smart Bias advantages

The Smart Bias has many advantages and adds new features :

  • Easy useTo turn on of off your amplifier, just push one button ! no tube adjustements, no need to count hours. The Smart Bias tells you when you needs to changes tubes and tune them everytime at a perfect value.

  • On the fly biasing This is the ability to change the bias while playing music !

  • No manual tuning requiredThere is no need to open the amplifier and to tune manually every tubes. You just need to turn a knob to a predefined bias setting and the system will make the adjustements for you.

  • Biasing hot or coldYou can choose your bias setting for each music by simply by turning a knob ! This effectively changes the amplifier tone so you could adapt to any music.



Connexions of Smart Bias

Please be sure to have read at least the technical page How Push-Pull Work in the Knowledge Center before continuing read.

The Smart Bias connects to each power tube in Push-Pull like this :

  1. Grid voltage measurement Measuring the grid voltage

  2. Grid voltage supply Supplies and regulate the grid voltage

  3. Cathode current measurement Measures the current flowing through the tube with the aid of a small value resistor

To be efficient, the Smart Bias needs to have full control on the amplifier. Therefore it is a global system connecting not only to the tubes, but also to the high-voltage supplies and user's knobs.

This is done via 3 electronic boards :

  • The biasing boardThis board controls the bias of all four tubes,

  • The high-voltage boardThis is the high-voltage power supply. It regulates the voltage, controls the starting, running value, stopping,

  • The control boardThis board reads the user's input such as bias setting button. It also communicates with the user via audio signals and lights.


Following is a diagram of the global connexions :

SmartBias ConnectGlobal legende
  1. Push-Pull ChannelsConnects to the two channels (left and right).

  2. High-Voltage supply Communicates with the High-Voltage supply by sending start and stop commands.

  3. Biasing boardControl the bias and grid voltages of all four tubes. Monitors constantly voltages and currents.

  4. Measurements and drives Measures a total of 8 currents, generates and measures 8 polarization voltages.

  5. Control board This board is responsible of all sequences, gives orders to and receive status from the two other boards. For example, it gives the "changing bias" order to the Bias board when user selects another bias value.

  1. Main switchThis is the global power swith that supplies all boards on the amplifier. The control board is therefore able to power off the amplifier.

  2. Power button Manages the starting and stopping of the amplifier according to the ON/OFF button.

  3. Volume control knobLowers the volume when changing bias, stops the sound when starting or stopping the amplifier.

  4. Bad tube indicators Indicates which tube has failed, needs to be changed or require a lower bias value.

  5. Bias selection knobThis is the button to select the bias value.

Internal structure of the biasing board

We shortly describe here one of the electronic board of Smart Bias. The biasing board is responsible for tuning, monitoring and changing the bias of power tubes. It is composed of a microprocessor and many analogs circuits to measure, communicates and drive signals:


SmartBias InternalStruct legende
  1. Internal power supplyThis is the power suppy for all electronics on the board. Is supplies the microprocessor, analogs circuits and biasing circuits.

  2. Negative Level ShifterThis subcircuit converts the voltage from the Digital to Analog Converter to a negative high voltage applied to the control grid of output power tubes.

  3. Analog Level ShifterThis subcircuit converts the measurement from control grid voltages to a level compatible with the inputs of Analog to Digital Converters.

  4. Current Level ShifterThis subcircuit converts the current measurement from cathodes to a level compatible with the inputs of Analog to Digital Converters..

  5. Light Emitting Diode driver This circuit drives the 8 lights, 2 for each tubes, located in the bias indicator..

  1. Analog to Digital ConverterThis circuit converts an alaog voltage to a numerical value compatible with the microprocessor algorithms.

  2. MicroprocessorThe microprocessor contains all the intelligence through a program.

  3. IsolationThis subcircuit is responsible of electrically insulating the board from other electronic board, thus preventing any noise and ground loops.

  4. EEPROM memoryThis is the memory in which the board saves calibration parameters. This calibration is done on every amplifier and gives the best precision in current and voltage measurement.

  5. Audio Tone generatorThe use is advised through a audio signal in the condition of failure or if the bias is required to be changed.

How it work

A simplified diagram of Smart Bias sequence can be seen in the following illustration. The sequence consists of starting, bias changing and stopping the amplifier.

SmartBias Courbes legend
  1. High-Voltage startingAfter 30 second waiting for the heaters, the control board orders the high-voltage board to starts.

  2. Starting tube currentWhen the high-voltage are started, a small current appears on the tubes because the grid voltage is at his lower level.

  3. Bias tuningThe Smart Bias tunes the biases by rising slowly the grid voltage, which increases the cathode current.

  4. Bias tuned and fixedThe bias has reach his targeted value and after a setting time, is fixed.

  1. Bias switchingThe bias setting has been changed, the Smart Bias tune the bias to his new value.

  2. Bias stabilizedThe bias has reach his normal value and after a setting time, is fixed.

  3. High-Voltage stoppingThe High-Voltage power supply has received an order to stop and has cut off the high-voltage.

  4. Bias droppingThe bias follow the high-voltage dropping and is reaching a null value.

Embedded software

The Smart Bias technology uses a high-performance microprocessors. The programs are written in our engineering department in C programming language and totalize above 45,000 lines of code.

These complex piece of software are written according to high quality coding standards like QA.C - MISRA and has been intensively checked and tested.

History of the Smart Bias

The Smart Bias is the result of an intense research effort and has been developped since 2008.

Therefore, it is a mature system and has been proven to be efficient and reliable.

Analyses and first concept

The physics of tubes and their behaviors are analyzed, giving a new concept to enhance vacuum tubes amplifiers : a technology that will control the bias, and tune it automatically.

First biasing control board

The first board driving the tube's gates to regulate bias current, the control board, has been made and is working. The systems consists of two biasing control boards, each of them controlling two tubes. The high voltage supply is controlled via a relay.

New architecture

Following many use cases tests, a digital communication bus is added to the boards. The high voltage power supply became intelligent and communicates with the other boards. A control board is created to synchronize the whole system and communicate with the user via inputs, lights and sound signals. The exchange between the boards permits a more precise control over the starting and stopping process.

Merging the two biasing boards

The biasing boards are merged : a single biasing board will control four tubes. Moreover, the board is now equiped with SMD instead of through hole components. This gives drastically reduces the board size.

On the fly biasing

The biasing algorithms are reworked to add a special feature : on the fly biasing. It permits to change the bias while the amplifier is running, keeping balance and precision.

Integration on Promesse 34

The biasing system has been integrated to the Promesse 34, giving unprecedented results.