Offset drift with servo circuit.
A servo circuits respond to error voltages. Therefore you must have and error before the circuit can respond. Normally this error voltage will be on the order of a few millivolts and will dance positive or negative over a short periods of time. The actual number of millivolts will be determined by servo loop gain and integration time of the RC network used. The shorter the integration time the worse the low frequency response of the amplifier will become due to modulation the main signal by the DC servo loop..
Normally the output-offset voltage of an audio amplifier is measured with the input of the amplifier shorted to ground or terminated by a specific resistance. This is presents a unnatural condition for the amplifier since it is not used in this manor during normal use. The audio signal normally present at the output of a audio amplifier is none-symmetrical in nature. Thus most servo circuits cannot maintain an amplifiers zero output DC offset voltage once a audio signal is present. Measuring the DC offset of a audio amplifier when there is no audio signal present may not accurately reflect what happens once a audio signal is applied. An improperly designed servo circuit may be worse than no servo circuit at all.
One of my tests is to use a pulse generator to generate only positive or negative going pulses. In the best DC coupled amplifiers utilizing a properly designed DC servo circuit these pulses will be accurately reproduced with no DC offset on the output of the amplifier. While a amplifier with a normal or poorly designed DC servo circuit will have the pulses reproduced along with a offset voltage. The offset voltage being produced is a product of the servo circuit as it tries to compensate for the none-symmetrical input signal that the pulse generator provides. Many amplifiers can sound better without the marginal DC servo circuits.
In one of my past notes I have described a servo circuit that can be adapted to most amplifiers and will give superior results since it is designed to ignore the actual audio signal to be amplified and cares only about the difference between the input and output DC signal when properly adjusted. Thus the circuit will compensate only for amplifier DC drift and is unaffected by the audio present as long as the amplifier is flat from input to output and is not driven into saturation.
I believe that the notes, however brief they may be on this design are still available on my web site,
www.audioamps.com ,which be the way my wife wants me to shut down. I have also developed out a suitable circuit board layout for this circuit if anyone is interested in improving their DC servo circuit. All component values must be determined based on the actual application the circuit is the be used in and thus must be determined by the user.
Remember in audio very simple circuits are not always the best. Make them as complicated as needed to get the job done properly.
Best regards,
John Fassotte