Q-tron Audio Automatic Bias Calibrator

A circuit that automatically controls bias and offset in OTL amplifiers and also automatically tune bias when changing power tubes

Q-tron Audio has developed a unique circuit for controlling the real quiescent current and output DC offset in our OTL amplifiers.

The circuit which is based on a microprocessor automatically measure and if necessary adjust quiescent current and output DC offset just before the amplifier is being switched off. The circuit works in the following manner:

When the power switch is pressed to switch off the amplifier the microprocessor circuit will start measuring the quiescent current and output DC offset and adjust those, if necessary, when these values are correct the amplifier will switch off completely. The whole procedure normally takes less than 1 minute even if the bias values need to be adjusted and the values for bias are then stored in a non-volatile memory. Normally the bias values don't need to be adjusted at all and then the amplifier will switch off immediately after measuring current and offset.

To ensure that the amplifier has been thoroughly warmed up and have reached its stable condition when the adjustment is performed, the circuit will only perform calibration if the amplifier has been switched on and operating for at least 30 minutes before it is switched off.

When the amplifier next time is switched on the stored values for bias will be applied.

The circuit will also give an alarm if it is not possible to adjust bias correctly, if so an orange "service" LED is lit, the amplifier can still be used but will not be optimally adjusted.

If any value is completely out of specification even after adjustments, (as in the case that a  tube catastrophically has failed) there will be a red "alarm" LED lit and the amplifier can then not be used for playing music, (the inputs and outputs will be shorted) until the fault has been corrected.

Automatic bias tuning of new power tubes

The bias auto calibrator also automatically set correct operating parameters for new power tubes.

When old power tubes are exchanged for new the owner should set the amplifier in test mode and switch the amplifier on. The amplifier will then heat the amplifier to stable conditions and automatically adjust the bias and offset to correct values, when the amplifier next time is switched on the stored values for bias are applied.

For more details see "Bias Calibrator Technical background”