Krell MDA-300
Three hundred watts of mono differential power per chassis. Krell built it to run its own reference system.
Overview
The MDA-300 is a monaural power amplifier, one chassis per channel. Krell calls it a "Mono Differential Amplifier," and per the company's own literature the name only begins to describe what's going on inside. The topology was described as unique from any previous Krell amplifier. The front-end circuits operate as pure gain stages rather than designs employing global feedback. That absence of feedback from input to output, combined with direct coupling, is the design's central claim: an inherent stability that feedback-dependent circuits can't match. The MDA-300 completes the third series of Krell's convertible amplifiers. The idea behind that lineup was that a customer's investment should be protectable. The current line, as Krell framed it, was designed so a unit could later be combined with a second new unit rather than replaced outright.
The rated output is 300 watts per channel into 8 ohms, per the manufacturer's own published specification. The more interesting claim is what happens at the speaker interface. The MDA is arranged so the speaker is not referenced to signal ground. That eliminates current draw from signal ground by the speaker, which in principle suppresses interaction with other components in the system. Whether that is audible is a matter of taste, but it's a real design decision. Front-end gain stages without global feedback, direct coupling and a floating speaker reference all point the same way: Krell was chasing stability and isolation from the rest of the chain.
Contemporaries took the amplifier seriously enough to build reference systems around it. Audio (October 1990, p.183) grouped the MDA-300 into Krell's then-current line, and Audio (March 1991, p.83) described the MDA-300 as representing a new generation of monaural amplifier design. The MDA-300 was treated as a known quantity, stable enough to serve as the measuring instrument for evaluating something else. Audio (October 1990, p.183) also listed it alongside the KSL preamplifier, KST-100 stereo amplifier, KSA-150 stereo balanced amplifier, and KSP-7B stereo preamplifier. That is the "convertible" family the MDA-300 capped off.
The MDA-300 was not a small purchase. For context, in the same listings a KSA-150 sat at $3,550 and a KSA-250 at $4,900, so the MDA-300 traded at roughly double the stereo KSA-150 on the secondary market. Read that number carefully: it reflects what a specialist dealer was asking for a used pair-or-single in the early 1990s, not today's market. The fact sheet carries no current pricing data.
One caveat about the record: the surviving period material is thin on measured performance. The available review excerpts describe the design philosophy and the listening systems. Independent lab measurements of the MDA-300's actual output, distortion, and bandwidth aren't in the source material assembled for this entry. Anyone who wants hard numbers will need the original review reprints or a service manual. The period sources do establish that the amplifier was positioned at the top of Krell's convertible line, priced accordingly, and used as reference-grade equipment by reviewers who could have chosen anything.
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