Pultec EQM-1A Mastering Equalizer

Pultec EQM-1A Mastering Equalizer is the EQP-1A in a 3U chassis with the boost and attenuation potentiometers replaced by 21-position ELMA switches and bandwidth handled by an 11-position switch. Every step is loaded with precision metal film resistors chosen to give each control its own custom taper. The passive section and the tube line amp on 12AU7, 12AX7 and 6X4 are unchanged, so the curves match the EQP-1A exactly.

A setting you can rebuild a week later

A Pultec potentiometer will let you set any value but not repeat it: coming back to material after a few days, or revising after checking a reference, you are reconstructing a knob position from memory and a photo of the panel. A stepped switch turns that into a number you write in the session notes and dial back in one move. The full-range progressive taper used here gives 0.5 dB per step through the first half of rotation, after which steps grow to 1 or 2 dB. The practical result is mastering-grade precision without giving up the full 18-20 dB range needed when working on individual tracks.

A pair that holds the stereo image

Equalizing a stereo bus requires two units to respond identically, because half a decibel of difference between channels shifts the image and reads as a wandering centre. Potentiometer tolerance does not guarantee that, and the smaller the boost, the larger the relative error. Switches loaded with metal film resistors deliver repeatable values from unit to unit, so a pair of EQM-1As set to the same step genuinely does the same thing in both channels. It is the same reasoning that has put mastering equalizers on switches for decades, applied here to Pultec curves with their simultaneous low boost and cut.

KEY FEATURES

• EQP-1A curves and signal path in a 3U chassis
• 21-position ELMA switches in place of boost and attenuation potentiometers
• 11-position bandwidth switch
• Precision metal film resistors giving each control its own taper
• Full-range progressive taper: 0.5 dB per step across the 0-5 dB range
• Repeatable settings and unit-to-unit matching for stereo work
• Full 18-20 dB range still available despite the fine steps early in rotation
• Passive equalization with a tube line amp restoring level

TECHNICAL SPECIFICATIONS

• Type: passive program equalizer with tube line amplifier, stepped controls
• Low shelf boost: 20, 30, 60, 100 Hz, 0-13.5 dB
• Low shelf attenuation: 20, 30, 60, 100 Hz, 0-17.5 dB
• High peak boost: 3, 4, 5, 8, 10, 12, 16 kHz, 0-18 dB
• High shelf attenuation: 5, 10, 20 kHz, 0-16 dB
• Controls: 21-position switches, 11-position bandwidth switch
• Input impedance: approx. 600 Ohms
• Output impedance: approx. 50 Ohms
• Noise: 92 dB below +10 dBm
• Distortion: 0.15% max at +10 dBm into 600 Ohms
• Amplifier response: 20 Hz to 20 kHz, +/- 0.5 dB
• Tubes: 12AU7, 12AX7, 6X4 (one each)
• Height: 3U (5.25 inches)