As described on the EQP-1A product page, the 3U EQP-1A was/is an enhanced version of the original (1953) Pultec product, the EQP-1. The EQM-1A (mastering version) takes the EQP-1/EQP-1A evolution a significant step farther by replacing the BOOST/ATTEN potentiometers with 21-position (and BW with 11-position) ELMA switches, loaded with precision metal film resistors to achieve custom tapers for each control. The current design implements a "full-range, progressive-taper" design, which provides 0.5 dB per step precision for the first 50% of rotation and then progressively larger (1 or 2 dB) steps as the control is rotated from 50% to 100% of rotation. This allows precise control for subtle equalization for mix bus or similar applications or the full 18-20 dB BOOST/ATTEN of the potted version when tracking or mixing of individual tracks. Furthermore, the stepped controls make recall a breeze and the precise tracking between units is essential for stereo image retention when using a pair on the stereo mix bus. 20, 30, 60, 100 CPS (Hz) shelf boost, 0 - 13.5 dB (FRPT* stepped control, 0.5 dB steps 0-5 dB) 20, 30, 60, 100 CPS shelf attenuate, 0 - 17.5 dB (FRPT* stepped control, 0.5 dB steps 0-5 dB) 3, 4, 5, 8, 10, 12, 16 KCS (kHz) peak boost, 0 - 18 dB (FRPT* stepped control, 0.5 dB steps 0-5 dB) 5, 10, 20 KCS shelf attenuate, 0 - 16 dB (FRPT* stepped control, 0.5 dB steps 0-5 dB) Input Impedance: ~600 Ohms Output Impedance: ~50 Ohms Noise: 92 dB below +10 dBm Distortion: ≤ 0.15% at +10 dBm into 600 Ohms Amplifier: Flat, 20 CPS to 20 KCS, +/- 0.5 dB Tubes: 1 ea 12AU7, 12AX7, 6X4 * FRPT = full-range, progressive-taper - Step size increases when control is rotated beyond 50% rotation
