Hafler Pro2400

The Hafler Pro2400 was introduced in 1990, about 11 yrs after the DH-200. It is rated at 120W into 8Ω and 200W into 4Ω. That would mean the power transformer is a 40-0-40VAC as what I used in my Bi120 amplifier kit.

The Pro2400 adopts the same mirror image typology as the DH-200. The main difference is the use of Jfets for the input differential pair (Red line). Further to that, it has a cascode to shield the Jfets from damage because the rail voltages exceed that of the Jfets. One other benefit of using cascode is the extended bandwidth.

The VAS has a darlington, same as the DH-200. From there, a single driver drives a pair of output mosfets, the 2SK1057 and 2SJ 161. These are the newer plastic flat paks that superseded the elderly TO-3 2SK134 and 2SJ49.

Hafler Pro2400 Power AmplifierFig 1 – Hafler Pro2400 Amplifier Schematic

For such a circuit to work without any DC Offset adjustments, all the transistors must be tightly matched. That will lead to higher production cost. This is something most manufacturers would want to avoid. Hafler stuck to their guns and refused to compromise. You can see their philosophy in their designs.

Summary

The Pro2400 is worthy of a collectors item. She can be restored easily, even upgraded. You will be rewarded with a slice of history and quality audio to boot. They just don’t make amps like this anymore.