Dispre II - complementary-differential preamp
Dispre 2 preamp is a successor to previous Dispre, simple differential, openloop concept. The previous Dispre had wide bandwidth, high slew-rate, robust output diamond buffer, and no global feedback. The tax for that concept was higher distortion of low order harmonics and quite high DC output voltage, which resulted in use of an output 3.3u foil capacitor.
Dispre2 follows positive experience obtained during tests and utilization of the original concept. It has wide bandwidth and high slew rate again, complementary-differential input stage and same output stage. It uses small amount of global feedback to stabilize parameters and DC servo to avoid output capacitor. Complementary-symmetry brought much lower value of harmonic distortion. The output stage works in class A for all output amplitudes into 600 ohm load or higher.
The output is single-ended. The purpose of XLR is to offer possibility of 3-wire connection to eventual true balanced input of power amplifier; the 3-wire connection reduces interference voltages across signal ground wire by subtraction.
Circuit diagram
Technical parameters
The circuit is fast and is not slew rate limited. Step reponse preserves same shape for small signal and large signal. Regarding THD, it is very low and almost does not differ with frequency. It depends on BJT input stage and VAS matching, we were able to get less than 0.0003% at 1Vrms, for both 1kHz and 10kHz. Without special matching one may expect THD spectrum like this.
Assembled PCB
Schematics and component assembly drawing of previous V3 version now released for personal, non-commercial purposes!
Dispre II V3 project termination
The Dispre II V3 project was terminated in April 2009, PCBs are no longer available.
Power supply
Especially for the Dispre II, a new power supply was designed.
Hereby the datasheet
Dispre SNR with the new power supply, measured at -80dBVrms
Pavel Macura 6/2008, last modification May 16, 2009
