Most of the preamplifier’s circuitry is contained on one circuit board for ease of construction. The circuit board is double-sided and extensively covered with a copper ground plane. The selection of the op-amp was one of the most important decisions for the design. After some investigation, I selected the Texas Instruments (TI) precision field-effect transistor (FET) op-amp, the OPA627. It is a single opamp with a Difet input circuitry, which offers high accuracy, extremely low input bias current, and very low noise performance.
The selection of the input signal of the preamplifier is performed by three high-quality relays per channel. Each input has one dedicated double-pole relay that completely connects or disconnects both the positive and the negative part of the signal from the preamplifier circuitry. I used high-quality relays from Panasonic, type TXS2-12V-1 with AgPd contactssuitable for use in low-level load analog circuits.
For the PR-3 preamplifier’s supply, I used a low-noise toroidal transformer with a nominal output voltage of 2x 15VRMS at 15VA. The transformer actually gave a larger voltage 2x 17.4 VRMS, since the PR-3 preamplifier’s required power was much
lower than 15 VA.
Read the complete article as published in audioXpress magazine. Now available here.
George Ntanavaras still has stock of PCBs to build the PR-3 preamplifier. If you are interested, send him an email here.
This article was originally published in audioXpress, January 2019
