Dispre II - new 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.
PCB versions and circuit modifications
The first PCB version was labeled 'dispre2' and released in June 2008. This version had a small mistake, one of the XLR output connectors was closer to the edge of the PCB (1.27mm). This mistake was corrected in the second version, labeled 'dispre2.1' and released in August 2008. I hoped everything was done.
But after some 5 months of evaluation by customers, I got a feedback that some of them had troubles with hum and higher noise in one channel. I took the modules back, and found that output diamond buffer had been oscillating at pretty high frequencies above 200MHz. It resulted in higher hum spectral components, as can be seen in the measurement of oscillating and stable status in the same circuit, same box, same channel.
Several modifications were made to fix possible oscillations. They are shown as red components in the schematics hereabove. For stability, it is necessary to cut two PCB tracks and add two 100R resistors and two 68pF capacitors. Their placement must be made exactly as shown in this picture. The last modification is VAS load by 22k resitors. It can be seen in the next photo, there are free positions R64 and 65 for these 22k resistors.
The new PCB with all the modifications was evaluated, here is the new PCB photo. The new PCB was labeled as 'dispre2 V3'. and was available since 2nd half of February 2009.
Schematics and component assembly drawing now released for personal, non-commercial purposes!
Dispre II V3 project termination and start of V4
The Dispre II V3 project is terminated since April 28, 2009. Version V4 is now released!
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
