Fisher 55A & 90A mono power amplifiers
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Vintage test report


Taken from 'High Fidelity Magazine', April 1957

If weight and size were the sole criteria for evaluating power amplifiers, these would without further ado vie for first place in the field. Massiveness is not, of course, the sole criterion, but it does often help give some idea of how good an amplifier may be, simply because excellent output transformers usually weigh a good deal.

The 55A is rated at a little more than half the power of the 90A, yet there is only about a 5-lb. difference in their weights. (The 90A weighs in at about 55 lb.; the 55A at 50 lb.) The 55A's rated power output is 55 watts (1% IM distortion point is about 48 watts), otherwise it is essentially a less complex version of the 90-watt (at 1% IM) model.

Both amplifiers are equipped with Fisher's Z-Matic variable damping control, and both include an illuminated meter which indicates average power output. The 55A's meter also shows output tube plate current, and a small removable cap beside the meter exposes a screwdriver adjustment for the bias voltage.

The 90A's metering facilities are more versatile, providing means for adjusting screen current and output tube balance, as well as giving plate current and average power readings. There is also a separate balancing control for the driver stages, which can be set to give minimum distortion after the tubes have aged or when they are replaced.

The power indications on both amplifiers are quite accurate, but it should be remembered that these show average power rather than instantaneous transient peaks. They are very useful for running spot checks on steady-state power output (as an indication of tube condition), but the peak power levels in musical program material will often far exceed the indicated values. This is emphasized simply as a reminder to those who might be tempted to overestimate their safety margin when using underpowered speakers, a miscalculation that could (particularly in the case of the 90A) damage the speaker.

At the same power levels (below the 55A's overload point), the 90A has a distinctly more solid "feel" than the 55A. The difference is subtle, but the 90A's over-all sound is smoother and more transparent, and its low end is unmistakably superior in definition and body. The 55A is an excellent performer, but the 90A is better in practically every respect. It is one of the most highly listenable amplifiers I've heard for some time, reproducing musical timbres and intricate sonic detail with remarkable realism. This is unquestionably the finest amplifier Fisher has produced to date, and while its price is nothing to dismiss lightly, neither is its performance.

The Z-Matic variable damping incorporated in these amplifiers has been praised and damned by experts in the field ever since its development, but experiments with these amplifiers indicates that it is indeed useful in compensating for many loudspeaker deficiencies. It does, however, increase distortion and limit high-frequency power-handling ability when advanced to its minimum damping position. If your speaker is good enough to deserve one of these amplifiers, you shouldn't have to use the Z-Matic control.

Both of these amplifiers, then, are fine performers. Whether the difference in sound between the 55A and the 90A is worth $60 will depend upon how sonically critical the buyer is. It would be worth it to me. — J.G.H.