Hafler XL-600
A MOSFET monster with a relay that won't let go until the amp is fully awake, and a price tag that made high power almost reasonable.
Overview
The XL-600 is the big one in Hafler's XL line. It took the excelinear concept introduced in the XL-280 and pushed it to sixteen lateral MOSFET outputs per chassis. Audio (October 1988, p.190) described the design as a host of refinements to the Hafler tradition. The input is a double differential J-FET push-pull cascoded stage, feeding a current-mirroring second stage, which drives that bank of MOSFETs. Near-zero phase shift was the marketing claim, and the circuit was laid out to back it up.
The relay surprises people. The XL-600 does not connect the amplifier to the speakers until the amp is completely turned on. According to the service documentation Musical Concepts produced for the PA-5 driver board, the relay sits between the output stage and the terminals on both the DH-500 and the XL-600. It also disconnects the load if significant DC appears at the output or if the heatsink's short-term temperature climbs too high. Under those conditions the fan keeps running, a detail Audio (February 1989, p.89) noted specifically. So the protection circuit does two jobs: it keeps DC off your woofers and keeps the amp from cooking itself.
The rear panel carries gold-plated input jacks and four multiway binding-post speaker terminals, as HiFi-Stereo (April 1989, p.50) recorded in its test report. That level of connector quality wasn't universal on American power amps of the period.
Power output is where the XL-600 earns its keep. Audio (October 1988, p.190) reported that the relay-protected output exceeds 900 watts per channel into 1 ohm. That is not a typo, and it is not a peak rating. It explains why Hafler used sixteen lateral MOSFETs instead of a more conventional output stage. Lateral MOSFETs have roughly ten times the bandwidth of bipolar power transistors, which minimizes the phase and bandwidth problems that plague the output stage of a conventional high-power design. They are also known for being rugged under adverse drive conditions, so they tolerate the kind of load a pair of big electrostatics or a low-impedance ribbon will throw at them.
One quirk to know before you buy: the XL series amps, including the XL-600, have a thermistor in the input ground circuit. Musical Concepts' documentation says to remove it and replace it with a piece of wire, because it sits between the chassis ground lug and the power supply star ground, and it does not belong there. On the XL-600 the device looks like a black-matte nickel or quarter with lead wires, connected to a terminal strip near the power supply caps. Very early units did not have the device at all. If you're buying one sight unseen, ask the seller whether it's still present. It's a five-minute fix.
The XL-600 also shares the DH-500's reputation for being a pain to service with the power module in place. The documentation is blunt about it. On the DH-500 and XL-600, if you're replacing the RCA jacks or running a ground wire from the secondary star ground to the ground lug between the inputs, you'll probably want to pull the module. There are wires routed beneath it to the fan switch and thermal breakers, and they need to be desoldered carefully, with your own records kept for reconnection. This is not a job for someone who hasn't done it before.
The PA-5 driver board is a three-stage design: input, voltage amplifier, output. The input stage is a current-sourced differential amplifier with a current mirror, which gives tight balance across the front-end operating parameters and produces minimal DC offset and thump-free startup. The voltage amplifier stage runs at significant current for linearity and uses wide-band video transistors with low Cob. The output stage is collector-coupled to the VAS, which is why there are only three stages. It's an unusually simple, straightforward design for an amplifier with this much output capability.
Musical Concepts offered a range of upgrades for these amps, and the power supply options matter most for the XL-600. The TP-500 is a pair of transformers, 500 watts each, used to replace the single transformer of the DH-500 and XL-600. When used with twin capacitor power supplies they're paralleled. They're used separately with the XL-600 or when installed with the PS-500 dual-mono power supply kit. The claimed improvements are better bass solidity, wider dynamics, and greater ambience. It's the usual transformer-swap story, but with a specific transformer designed for this chassis rather than a generic substitute.
Speaker fuses are another thing to know about. Stock fusing on the DH-500 and XL-600 is 5 amps, with 5 or 10 amp sizes optional if you're running 4-ohm speakers. Musical Concepts' position was that 5 and 10 amp sizes give better sound, and that the stock fuse rating offers little or no speaker protection anyway. They suggest bypassing the fuse holder entirely with a wire from the PA-5 to the output post, and then warn, in bold, that under rare circumstances this could lead to catastrophic speaker failure. If you bypass the fuses, you are relying on the relay and your own judgment, and the relay only trips on DC or excessive heatsink temperature, not on a transient.
The XL-600 was not a cheap amplifier, but it was an affordable one relative to what it could do. Audio (October 1988, p.190) framed it as establishing a new era in affordable high-power amplification. That sounds like ad copy until you look at the 1-ohm number, the connector quality, and the fact that it was made in America by a company that had already earned its reputation with the DH-200 and DH-220. The XL-600 is the end of that lineage in the XL line.
Maintenance-wise, the usual caveats apply. The electrolytic capacitors in the power supply are decades old at this point, and the Musical Concepts PS-500 dual-mono supply is a known path for revival. The relay contacts can oxidize, and the fan is a wear item. The thermistor in the input ground circuit should be removed if it's still there. None of this is exotic, but none of it is free either, and a neglected XL-600 can be a money pit if the output stage has ever been abused. The lateral MOSFETs are rugged, and the design is simple enough that a competent tech can work on it without a schematic for the whole amp, though you'll want the PA-5 documentation if you're going inside.
The XL-600 sits at the top of the Hafler power-amp hierarchy alongside the XL-280, and it trades on the same reputation: MOSFET output, high current capability, and a build quality that predates the cost-cutting that would come later. It is not a rare amplifier. People buy them because they have speakers that need current, or because they want the relay protection, the gold-plated jacks and the sixteen MOSFETs.
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