Application Notes
     I place in this section some outcomes of my developments which are contained in my main audio and measurement equipment. Just their implementation has made this equipment excellent in characteristics, versatile in functionality and convenient in use.
     The proposed items are universal and might find a wide area of their application. They were designed and built many years ago and have a long history of their immaculate service, so they have passed the main test - by time.
     Their thorough electronic simulation has been performed recently, it one more time confirms the claimed parameters and characteristics. The virtual testing of my electronic creations has become for me a mandatory procedure before publishing any materials on them.
Reducing the oscillator distortion down to 0.00001% (-140dB) is considered on the example of the state-variable oscillator being most perspective in this respect. The original approach of running interactive simulation allows to get this and even lower, down to -170dB, distortion figures with the accuracy (0.5dB) and reliability never achieved before. Discrete and op amplifier versions of the oscillator are offered, the methods of improving its amplitude stability are analysed too.
An advanced method of measuring in 3sec and displaying on the screen distortion down to 0.0000003% (-170dB) with 0.5dB accuracy in the whole audio range is used in this study. It's shown how a typical 100W-4ohm power amplifier can be step-by-step transformed into the device with record distortion characteristics (0.0000008% or -162dB at 1kHz and 0.0000085% or -141dB at 16kHz with 80V/usec slew rate). The article is supplied by numerous circuit diagrams, screenshots and simulation data.
This differential amplifier employs merely nine transistors but is capable of driving a 300-Ohm load with a 2V RMS signal featuring THD of less than 0,000003% (-150dB) in the whole 20Hz-20kHz range. This linearity is achieved by running its output stage in class-A, effective functioning of the applied feedback and at last by the amplifier circuit optimization. Its open-loop gain is 100dB within 10Hz-100kHz and 0dB at 35MHz, slew-rate of the output exceeds 30V/us. It's intended for use first of all in the precision measurement equipment where low-impedance circuitry should minimize noise without raising lowest distortion.
The offered circuit compares the amplifier input and the reduced proportionally to its gain output, the difference in their shape (more than 2% distortion of the clipped output) turning on the LED indication of constant 0,3sec duration even if the clipping occurs in a single period of the highest audio frequency. Circuit modifications for the inverting and non-inverting amplifier configurations are described.
This simple item is an indispensable addition to any power amplifier for those who care about safety of their acoustic systems. The circuit provides a 4sec delay in connecting the loudspeakers to a power amplifier while the latter settles after its turning on, it also immediately interrupts this connection when the amplifier turns off or a DC voltage of either polarity and more than 2V appears at its output. Over twenty years this circuit demonstrates reliable operation.
The discrete transparent class-A phono preamplifier with the rarely used shunt feedback demonstrating the rarely met natural sound reproduction. This correlation was discovered by John Linsley Hood forty years ago and is confidently confirmed subjectively. With the twofold overloading the preamplifier output features distortion of less than 0,001% over the whole 20-20000Hz range. Music sample recorded with the help of this device is downloadable for listening.
The converter adapts the linear variable voltage for use in my voltage-controlled oscillator where the octave and decade law of changing its frequency requires an exponentially variable control voltage. The converter input voltage range is 0-3V, output voltage range - 4mV-4V. The achieved 0,5% conversion accuracy absolutely doesn't depend on ambient temperature. Results of the circuit computer simulation are enclosed.