Hafler PRO-2400
A 27-pound MOSFET amp rated at 120 watts per channel, built to survive the kind of abuse that kills lesser amps.
Overview
The PRO-2400 is a two-channel medium-power amplifier from Hafler, engineered to meet high performance and reliability standards while maintaining affordability. That's Hafler's own framing from the owner's manual, and it fits. This is a no-nonsense rack-mount design built around MOSFET output devices rather than the bipolar transistors used in most competing designs of the period.
The manual describes the MOSFETs as providing exceptional reliability as a result of their inherent ruggedness and resistance to abusive operating conditions. In practice, the PRO-2400 can deliver high current into low impedance loads, and according to Hafler's documentation, the amplifier generally will not be damaged by an accidental short circuit at the output. Anyone who has seen a conventional amp go up in smoke after a speaker cable slipped will understand why that claim matters.
The power ratings are clearly specified. Hafler rates the PRO-2400 at 120 watts per channel into 8 Ohms at less than 0.05% THD from 20Hz to 20kHz with both channels driven. Into 4 Ohms it doubles to 200 watts per channel under the same distortion and bandwidth conditions. In bridged mono operation, it delivers 400 watts into 8 Ohms, again at less than 0.05% THD across the full 20Hz to 20kHz range.
The manual describes the drive circuitry as a simple and effective arrangement using high quality components to ensure longevity and, above all, sonic excellence. Simple is the word. This amp has no pile of features. It amplifies.
Intermodulation distortion (IHF) is specified at less than 0.005% from 1 to 120 watts per channel into 8 Ohms. Typical THD at 120 watts into 8 Ohms measures 0.003% at 20 Hz, 0.002% at 1 kHz, and 0.02% at 20 kHz. Those are low numbers. The 20 kHz figure is higher than the midband measurement, but still well within respectable territory for a MOSFET design of this era.
Frequency response is wide too. Into 8 Ohms at 1 watt, the response is -3dB from 4Hz to 100kHz. At full rated power of 120 watts, it's +0dB, -0.5dB from 10Hz to 40kHz. That kind of bandwidth at full power is not typical of budget amplifiers.
The signal-to-noise ratio exceeds 100dB, A-weighted, referred to 120 watts into 8 Ohms. Input impedance is 47,000 Ohms unbalanced, and 47,000 Ohms per phase balanced. Input sensitivity for 120 watts into 8 Ohms is 2.25 volts RMS unbalanced, or 1.125 volts RMS per phase balanced.
Damping factor is specified at 300 to 1kHz into 8 Ohms, dropping to 200 to 10kHz into 8 Ohms. That's a high damping factor, the kind that keeps tight control over woofer movement. Hafler's manual warns that speaker wires should be of sufficient size to preserve the PRO2400's high damping factor. It recommends standard #18 gauge lamp cord for runs up to 15 feet with 8 Ohm speakers, and heavier #16 or larger wire for 4 Ohm loads or longer distances.
The amp is 5 1/4 inches high plus 1/2 inch feet, 19 inches wide, and 10 1/2 inches deep. Net weight is 27 pounds. The front panel fits a standard 19-inch rack at 5.25-inch intervals, or the unit can simply be stacked. Feet can be removed for rack mounting.
The back panel has a chassis/float ground switch for ground isolation, which is handy for chasing hum in complex systems. The black output terminals are connected together internally, for use with external devices that share a common ground connection, such as headphone junction boxes. The red and black outputs accept standard banana plug connectors, including double banana plugs with 3/4-inch spacing. They'll also clamp spade lugs or bare wire through the center post.
The PRO-2400 is normally wired for 120V AC operation, but alternate connections inside the amplifier accommodate 100, 120, and 230 VAC. Units wired for 230 VAC may be operated over a range of 220-240 VAC with no compromise in safety or performance. Power consumption is listed at 9 Amps at 120 watts into 8 Ohms.
The power switch can be left on, with the amplifier switched remotely by connecting its line cord to a preamplifier or other control center that provides a switched AC outlet. The control device must be able to supply 10 Amperes to the PRO-2400 in addition to any other switched units.
The PRO-2400 constantly monitors the temperature of the output devices and takes corrective action to prevent damage from excessive overheating. Under most conditions, the heatsinks dissipate heat adequately and the amp plays continuously. But if it is driven at high volumes into speakers with impedance below 8 Ohms, or with inadequate ventilation, the heatsinks can reach a level that triggers the protection circuitry to shut off the amplifier for a short time, normally less than 15 seconds. The amp then returns to normal operation, and if the overheating condition persists, the cycle repeats. Hafler's manual advises checking airflow around the heatsinks and through the vent holes, and verifying the load impedance isn't far below 8 Ohms.
The protection prevents damage, but it also means the PRO-2400 will cut out during demanding passages if it's pushed hard into a difficult load. The manual is explicit that bridged mono operation requires speakers of 8 Ohms or higher, never less. The increased current in bridged mode could cause the amplifier to overheat.
The manual notes that adequate air flow is important to continuing reliability, and that cool air must circulate freely through the perforations in the cover, under the chassis, and around the heatsinks. The amplifier should rest on its feet on a hard surface so airflow underneath isn't obstructed. It's normal for the cover and heatsinks to become warm during use.
The PRO-2400 carries a three-year warranty from Hafler covering parts, labor, and return shipping costs within the continental United States, Alaska, and Hawaii. It applies only to the original owner and is non-transferable. That's a notably long warranty for the period.
Mix (October 1990, p.229) noted the PRO-2400 was introduced in July 1990, listing the product contact as Dan McLeod, sales manager, and describing the amp as employing a sonically proven circuit topology — language that echoes Hafler's own manual. The same issue listed the manufacturer address as 613 S. Rockford Dr., Tempe, AZ 85281.
Sound and Communication (April 1992, p.74) documented a system aboard the yacht Imperator that used two Hafler PRO 2400 power amplifiers alongside a Hafler SE-100 preamp. That's a professional installation in a marine environment, which is not exactly a gentle application.
The PRO-2400 is not one of Hafler's celebrated DH-series amps, and it doesn't have the cult following of the DH-200 or DH-220. It does have MOSFET outputs, balanced inputs, bridged mono capability, and rack-mount construction. The 27-pound weight and 5.25-inch rack height make it easy to fit into a studio or hi-fi rack.
The main caveats are the protection circuit behavior and the ventilation requirements. This is not an amp to bury in a closed cabinet or to drive into 2-Ohm loads in bridged mode.
Hafler's professional line, which includes this amp, was aimed at installations and studio use where reliability mattered as much as specifications. The three-year non-transferable warranty and the detailed protection circuitry fit that aim.
The PRO-2400 is often described as having MOSFET sound that is slightly warmer and smoother than bipolar designs, with low distortion figures, balanced inputs, and enough power for most reasonable speaker loads. It's not the most powerful amp Hafler made, and it's not the most collectible.
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