All filters are with variable inductors (actually tapped transformers in the original) and variable capacitors.
The fundamental genius of the EQP-1A schematic is its "no-loss" passive design.
To move the signal from the filter to the gain stage. pultec eqp-1a schematic
The high-frequency section is equally fascinating but often misunderstood. Unlike the low end, the highs use an LCR (Inductor-Capacitor-Resistor) network.
The schematic shows a 600-ohm input transformer (often a Triad A-11J or UTC A-20). This is not just for balancing; it provides the proper source impedance for the passive filters. The EQ curves are calculated based on a 600-ohm source . Feed it from a modern 50-ohm output, and the frequencies will shift. All filters are with variable inductors (actually tapped
The Low Frequency (LF) section of the schematic is deceptively simple yet famous for its unique behavior. It offers two controls: a boost and a cut.
Unlike modern EQs, the Pultec has no Q adjustment. The Q is fixed by the inductor's internal resistance and the capacitor values. If your clone schematic has a "Q pot," you are no longer building a Pultec; you are building a derivative. The high-frequency section is equally fascinating but often
The EQP-1A’s magic lies in its interaction between a passive filter network and a tube-based gain make-up stage. Passive Filter Section:
If you build a Pultec EQP-1A from a schematic, you will fail if you ignore impedance.
The power supply section usually features a 6X4 tube rectifier, which contributes to the "sag" and warmth of the unit's character. 3. Key Components & Signal Path
The high-frequency silver-mica capacitor (usually 500pF for the 16 kHz setting) must be 1% tolerance. A 10% cap will shift your highest frequency to 14 kHz, and you will wonder why your clone lacks "sparkle."