Forte Model 4 Amplifier
Audiophiles can sometimes be divided into classes: One class chooses its amplifiers on the basis of sheer power-the ability to drive any speaker to any. conceivable volume; another class believes that all amplifiers sound alike or can only have their sound characterized by a committee. The Forte Model 4 is not for either of those groups. The Model 4 is a gem of an amplifier, but its quality depends on your willingness to accept an amplifier limited to 50 watts per channel, albeit with a continuous maximum current capability of 16 amperes and a peak current capability of 50 amps.
The other technical specifications of the Forte Model 4, incidentally, are relatively conventional. Input impedance is 47 kilohms, and bandwidth is 3 Hz to 100 kHz between -3 dB points. As is usually the case with modern transistor power amplifiers, distortion is vanishingly small. The damping factor, however, is a comparatively high 400, and output noise is less than 300 μV, which is better than many amps in this price range.
The Forte Model 4 is a Class-A power amplifier costing $1,795-unusually low for a Class-A design. At a given price point, true Class-A operation inevitably sacrifices power relative to Class-AB operation, and it is interesting to note that the Class-AB members of the Forte family have substantially more power output.
Many audiophiles argue, however, that Class-A operation offers substantial benefits in terms of more linear operation, sweeter musical harmonics, more musical detail, a better defined soundstage, and added depth and air.
The Model 4 was designed by a team led by Nelson Pass, designer of the more expensive Threshold power amplifiers, who has produced a long series of truly excellent Class-A designs.
The technology of the Forte Model 4 is also interesting because it is a noninverting, complementary symmetry design and is one of the first power amps to use insulated-gate bipolar transistors in its output stage. These are new transistors that Pass feels exhibit the most advantageous characteristics of both bipolar and MOS-FET devices, and perform with enough inherent accuracy to make overall feedback correction unnecessary. This is said to eliminate any of the dynamic anomalies that characterize wide-band corrective-loop systems.
The Model 4 has no special control features (just an LED to show that the amplifier is on), RCA input jacks, and five-way binding posts for the speakers. I should note, however, that it is a relatively compact design, exhibits no mechanical noise, and runs cool enough to place in any reasonably well-ventilated area. This is an amplifier that is very easy to live with.
I did not, however, begin this review by calling the Forte Model 4 a gem because of its technology. Rather, it is a gem because it is a truly musical amplifier. It has the kind of musical sweetness that I normally associate with tube designs, or with much more expensive high-end designs. From the mid-bass on up to the highest octave, a well-broken-in Model 4 produces an open, airy sound, does an excellent job of reproducing low-level transients, reveals the details of the soundstage in terms of both imaging and depth, and handles complex musical and vocal passages without ever seeming to lose definition or the fine harmonics of the music. (As is the case with most power amps, the Model 4 needs a couple days of breaking-in to sound its best.)
This is the kind of amplifier that is perfect for an average-sized apartment or home listening room where your goal is a musically believable, natural experience with the kinds of music that suit home listening. Chamber music, small jazz groups, rock recordings that emphasize subtlety rather than power, ballads, and small vocal groups really come alive. Demanding instruments like the piano, flute, and harpsichord are very well handled, without the touch of hardness common in most transistor amplifiers at competing prices.
Using the Forte Model 4 does involve some trade-offs. For one, it is not an amp for rock 'n' roll or top orchestral volume. It sounds louder than its output wattage would suggest (perhaps in part because it is capable of peak dynamic power levels well above 100 watts, or because exceptionally clean and detailed power amps always sound louder than their rated power), but it still cannot drive large, power-hungry speakers to their limits.
Another trade-off is that the bass, while very good down to quite low frequencies, is scarcely as extended and powerful as the deep bass you find in more expensive units. Overall, the low end is better defined and controlled than in many competitively priced transistor amplifiers, and much better defined and controlled than in any competitively priced tube amplifier I have heard, but there are limits to the bass and lower midrange dynamics.
In saying earlier that the Model 4 has an unusual sweetness, I do not mean to imply that its overall timbre and dynamic balance are similar to those of most tube amplifiers. There is no gentle roll-off in the upper octaves. There is a great deal of treble information and considerable upper-octave excitement, and the Model 4 is a very revealing amplifier indeed. It will get the best out of a fine cartridge or CD player with well-recorded material, but it will not forgive a mediocre preamp or poor recording.
Good as the upper midrange is, I do not want to imply that it equals or surpasses that of amplifiers costing two to three times as much. I did a great deal of comparative listening with the Forte and higher priced Class-A amplifiers. The upper midrange of the Model 4 is, beyond a doubt, good enough to raise the question of whether any further improvement in higher priced competitors has reached the point of diminishing returns. But with more costly units you can hear more soundstage detail, you can hear subtle improvements in truly demanding choral music and mass string passages, and solo piano and guitar notes do sound a bit more natural.
This is also an amplifier with a natural, rather than an exaggerated, soundstage. You will get more depth information when it is on the recording, but there is no tendency to add depth. The soundstage is not wider than usual, and the imaging is not striking. I would argue that you may well be surprised at how much soundstage information your collection has if you have been listening to less revealing amplifiers, but you will make that discovery in isolated moments over a long period of time, and not by hearing the Forte Model 4 impose a whole new set of characteristics on the soundstage.
In short, this is an amplifier for people who truly love music, not sheer sound. The Forte Audio Model 4 is ideal for driving loudspeakers that emphasize subtlety and transparency without presenting extremely complex loads and high power demands.
