Replica of tube equalizer Pultec EQP-1A

The Pultec EQP-1A is a tube program equalizer designed via Eugene Shenk in 1951 year. This equalizer is still unsurpassed device model for treatment frequency in recordings among audio engineers worldwide. All family of original Pultec EQs, was originally designed to bring back the life and musicality lost in a recording. EQP-1A has been and still is used in major recording studios because of the exceptionally natural equalisation for highest and lowest frequencies and unique sound, arising from the use in the signal path three audio transformers, coils in the filter high frequencies and the output amplifier based on two classic tubes. All this gives a very smooth, almost velvety sound. This is the perfect device for the processing of the individual traces but also the whole mix so it can be used successfully in the work of mastering.

Specification electrical:

  • INPUT: XLR, transformer-balanced.
  • OUTPUT: XLR, transformer-balanced.
  • 20, 30, 60, 100 Hz: Shelf bost, 0 to 13.5 dB.
  • 20, 30, 60, 100 Hz: Shelf attenuate, 0 to 17.5 dB.
  • 3, 4, 5 ,8 ,10, 12, 16 kHz: Peak boost, 0 to 18 dB.
  • 5, 10, 20 kHz: Shelf attenuate, 0 to 16 dB.
  • NOISE: Above 97 dB.
  • LOSS: None. Insertion loss restored by amplifier.
  • AMPLIFIER: Flat, 20 to 20,000 Hz +/-1dB.
  • POWER SUPPLY: 115 or 230 AVC, 50/60Hz.

Independent review about units EQP-1A from ProReplicas. This is one snapshot from whole article in Attack Magazine. More: (This pair of units, was one of the first units and had the front panels with original name brand, which are currently replaced by name “ProReplicas”.)

studio picture of UREI 1176 Rev.G

* construction details

This copy is built on the basis original scheme Pultec EQP-1A presented below. One little modification is in power supply, instead tube of rectifier 6x4 is used bridge rectifier on classic diodes. It's specially modified because this one tube does not affect the sound and these tubes are less robust and expensive than semiconductors.

pultec eqp-1a scheme

The original schematic Pultec EQP-1A

List of key components used in the replica:

  • ECC821 x ECC82, tube from JJ Electronic company, used in block of output line amp.
  • ECC83S1 x ECC83S, tube from JJ Electronic company, used in block of output line amp.
  • 36031 x 3603, input transformer from Sowter, is copy transformer HS-56.
  • 93301 x 9330, amp input transformer from Sowter, is copy original transformer HS-29.
  • 95301 x 9530, output transformer from Sowter, is copy original transformer S-217-D.
  • inductor1 x inductor for filter of high ferequency with very low DC resistance under 2 ohms, the less the DC resitance, the higher the Q of the coil.

Rest components used are high quality: resistors metal film 1%, very good foil capacitors (in audio track) Philips, BC Components, Roederstein, potentiometers and rotary switchs Alpha and Allen Bradley. Additionally it unit have employed anty-interference EMI power filter. Whole construction is executed firmly very.

* pictures of inside

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