Hafler P-4000
A three-rack-space MOSFET amp that built its "professional standard" reputation one studio rack at a time.
Overview
The P-4000 is a two-channel professional power amplifier in three rack spaces. It is 19 inches wide, 11 inches deep and 5-1/4 inches tall, and it weighs 34 pounds. You notice that when you slide it into a rack. The ratings are 200 watts per channel into 8 ohms, 275 into 4, and 550 watts in bridged mono into 8 ohms, all rated FTC across 20 Hz-20 kHz at 0.2% THD. Hafler's own documentation describes it as a moderately powered compact amp.
The P-4000 runs Hafler's trans•nova topology, which is TRANSconductance Nodal Voltage Amplifier, U.S. Patent 4,467,288. It is paired with the company's DIAMOND driver stage (Dynamically Invariant Amplification Optimized Nodal Drive). The manual describes that stage as combining the linearity of Class A operation with the current headroom of a Class B system. The output stage is MOSFET, which Hafler had been using in its amplifiers since the 1970s. The manual credits MOSFETs with being fault tolerant enough to drive reactive speaker loads without the sound degradation that elaborate Safe Operating Area protection schemes tend to impose.
Signal-to-noise is rated at 100 dB below rated output from 20 Hz-20 kHz, A-weighted. Frequency response is quoted at 20 Hz-20 kHz, ±0.1 dB, and overall bandwidth runs from 0.2 Hz to 200 kHz at +0/-3 dB. Slew rate is 100 V/µs. Damping factor is 500 at 1 kHz, 150 at 10 kHz and 18 at 100 kHz. Common mode rejection ratio exceeds 75 dB at 1 kHz. Input impedance is 47,000 ohms per phase balanced.
Connections are XLR and 1/4-inch combo inputs, 5-way binding posts on the output side, and an IEC standard line input for mains. Input sensitivity ranges from 710 mV to 4V at 8 ohms, or 592 mV to 4V at 4 ohms, per phase balanced. The front-panel gain control offers a 58 dB range, with in/out gain from +29 dB maximum to -29 dB minimum. Idle power consumption is 120W (1A at 120VAC), rising to 250W at 1/8 power into 8 ohms and 720W at maximum power. The front panel carries a power switch, the gain control, and a row of LEDs: Signal, Clip, Thermal, Short, and Line Power.
The design brief was dependability as much as performance. The manual says a primary focus during development was ensuring reliability, and a parallel effort went into making the amp technician-friendly. The modular construction allows swapping the entire operational portion of either channel quickly, without soldering or specialized equipment. Hafler backed the P-4000 with a five-year warranty. The coverage applied only to the original owner, was non-transferable, and was limited to units sold in the Continental United States.
Mix (1998, p.123) covered the amp at a list price of $999, highlighting the patented trans•nova circuitry and the DIAMOND driver stage. The same magazine later named it a winner of the 1998 Mix Magazine Tec Award, and subsequent coverage in Mix (1999, p.99) and Mix (1998, p.89) repeated the "professional standard" line. Hafler leaned hard on that award in its own marketing, and the ad copy in EQ-Magazine (November 1998, p.117) is essentially a victory lap.
The P-4000 is a 1990s professional amplifier with a documented design lineage, real patent numbers behind its circuit topology, and a service manual that explains how to repair it. The MOSFET output stage and modular channel construction make it more repairable than most of its contemporaries. Hafler's own documentation goes out of its way to describe the amp as "technician friendly."
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