N72 Microphone Preamplifier

The N72 preamplifier is a copy of the Neve 1272 Class-A line amplifier converted into a preamp design. The transformers are Carnhill equivalents of the transformers used by the original Neve designs. The gain is distributed evenly across the two amplifier stages, a point that is often criticized if missed. The tone is similar to the Neve 1073 preamplifier in all settings except at the highest gain settings where an additional gain stage would be switched-in on the original Neve design.

J99 Microphone Preamplifier

The J99 preamplifier is based on the Jensen Dual Servo design. It uses two Hardy/Jensen 990C discrete opamps each DC offset corrected by a servo circuit resulting in a capacitor-free signal path. The input transformer is a Lundahl LL1538XL and the output transformer is a Jensen JT-11-DMPC. The gain control is a stepped design distributing the gain equally across the two opamps.

A12 Microphone Preamplifier

The A12 preamplifier is originally designed to be a copy of the API 312 preamp and Authentic Audio has four of these circuit cards.

The first two of these preamps are built based on the original design. The input transformer for both of these circuits is a Jensen 110K-PC. One preamplifier uses an API 2523 output transformer taken from an original API oscillator module. This is not the standard 312 design but an alternate created for a 300 ohm load rather than a 600 ohm load. In this amplifier an original vintage 2520 opamp is used. The second amplifier uses a Melcor output transformer similar to the API design and a MAP5004 opamp design.

The second set of A12 amplifiers built by Authentic Audio uses the original A12 PCB design but the gain staging and impedance values are adjusted for a 1:2 input transformer. The input transformer used is a Tamura design taken from a Yamaha PM1000 mixing console. This required a redesign of the gain setting resistors and an optimization for a lower impedance transformer. The output transformers are steel core Jensen JE123 designs. The opamp used is a self designed duplicate of the Forssell JFET 990 series opamps created from Forssell's schematics and converted into a PCB layout. The overall sound is very similar to the API design but has subtle coloring differences.

General System Overview

The Seventh Circle Audio preamps are designed to provide high performance preamplifiers at a lower cost by taking advantage of the kit philosophy. The chassis can hold eight channels and all slots are filled (two each N72's, two each J99's, and four each A12 cards as described above). The system includes a fully regulated DC power supply that provides +48V and +/-27V. At the individual cards the power supply voltage rails are regulated to the voltage levels needed. The torroidal power supply magnetic field is further reduced by a mu metal shield.