FT A70 hybrid power amplifier

The FT family of amplifiers represents the culmination of an exhaustive design effort at Musical Technology. Developed with the same attention to detail and commitment to excellence that led to the design and introduction of the highly regarded Musical Technology C-1, these amplifiers are ideal complements for this reference line level preamplifier. From a technical standpoint they eschew complexity for the simplest circuits possible. They pay great attention to the signal path. They utilize low feedback and employ the finest components available. From a musical standpoint the FT amplifiers exhibit almost limitless dynamic range. They are capable of rendering nuance and revealing fine layers of detail. They are solid, exceedingly fast, and they reproduce music with an honesty and authority that must be heard to be appreciated.

THE DESIGN

As to the question of vacuum tubes versus transistors, the natural inclination might well have been to base this family of products on an all vacuum tube approach. The company’s C-1 preamplifier, an acknowledged reference quality line stage, makes a persuasive case for tubes. However, despite a well deserved reputation for providing an intimate and involving musical experience, tube amplifiers are unavoidably transformer coupled to their loads. They have low damping factors. They are bandwidth limited. And they exhibit phase shift. They simply do not possess the requisite speed and control to effectively drive speakers with low impedances or complicated networks.

After much debate and countless hours in the lab Musical Technology designers opted for an approach employing vacuum tubes and MOSFETs in a design offering the best of both worlds. Two 6DJ8 vacuum tubes configured in a cascode push-pull arrangement and fed by a dedicated power supply, form the driver stage in these amplifiers. This produces precisely the right gain and delivers the classic open sound and sweetness characteristic of good vacuum tube designs. Matched power MOSFETs are then utilized in the output stage (not in the driver stage as with typical hybrid designs) where they deliver the necessary damping and current to effortlessly drive any kind of load, and even impedances as low as one Ohm! The use of MOSFETs, selected because of their high thermal stability and desirable tube matching qualities, eliminates the need for protection and current limiting in the signal path. Extended bandwidth, high intrinsic linearity, low distortion, excellent damping, and total stability have all been achieved with an exceedingly small amount of feedback (6 dB).

A vacuum tube driver stage operates in pure class A in all FT models. The MOSFET output stage is then biased in either pure class A, as in the FTA70; or such that the amplifier works in class A at all normal listening levels, as in the FT200, FT140, and FT414. To achieve absolute channel separation and for a better, more solid soundstage, FT amplifiers are configured either as dual mono, or offered as true monoblock designs. Massive multiple power supplies anchored by toroidal transformers of prodigious proportions, with separate windings and multiple taps isolate, divide and provide huge amounts of reserve. Computer grade filter capacitors of huge capacity add to the already impressive power supply credentials.

Countless hours of exhaustive listening by a team of musicians and music lovers have helped Musical Technology craft products that deliver music with honesty and integrity. The result of all this is a family of amplifiers of timeless value with alluring qualities, products that combine the natural sound of vacuum tubes with the authority, speed, and detail of solid state.

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