Forte amplifiers: design and specifications FYI

Before sending my Forte 4a on to its next owner, I've copied the interesting bits of the manual as a public service and in the hope of building at some point a replica, a DIY version, of the 4a. The difference between models 4 and 4a, incidentally, is that the "a" version has XLR connectors.

(If the manual contained circuit schematics I would have copied them, but it doesn't.)


Design and Construction

Forte amplifiers are non-inverting, complementary symmetry power amplifiers and are the first to have a power output stage using isulated gate bi-polar power transistors. These are a new semiconductor design that exhibits the most advantageous characteristics of both bi-polar and MOSFFET devices and performs well with such inherent linear accuracy that Forte amplifiers are completely free from overall feedback correction. This means that the dynamic anomalies which characterize wide-band correction loop systems simply cannot occur and that the amplified signal will retain maximum purity.

The design of the Forte amplifiers places simplicity of gain path foremost and employs classic techniques to achieve linear response with the use of only nominal amounts of localized negative feedback for error correction. This approach seeks to place the signal circuitry in a high-current, or biased, state in order to minimize the distortion caused by variations in output current. In this pursuit Forte models 5 and 6 utilize pure class A operation in all gain stages prior to their high-biased class AB output stage and Forte models 4 and 7 include pure class A operation through their power output stages as well.

Forte amplifiers have a direct-coupled output stage for each channel consisting of ultra low-noise matched FET differential devices biased by regulated constant-current sources. These drive gain transistors are loaded with regulated constant-current sources. The superior linearity of this topology assures, again, that distortion can be controlled with only minimal local feedback application.

Your Forte amplifier is equipped with a custom manufactured toroidal power transformer that is capable of providing twice its continuous rating for extended periods. This ensures that the transformer is able to source a constant voltage under any load demand encountered by the amplifier. The output of the transformer passes through twin high-current bridges and is smoothed into a DC voltage by high capacity computer-grade capacitors.

All Forte amplifiers are built to the highest standards of craftsmanship. Circuit components are selected from the finest available. Circuit boards are military-grade glass-epoxy; all internal connections are hand-soldered. The conducting surfaces of input and output connectors are gold-plated to total immunity to oxidation. Metal film and wirewound resistors are used as are film and silver-mica capacitors. Assembly, testing, and adjustment of each Forte amplifier is done on an individual basis.


model 4:
nominal output: 50W/channel into 8 Ohms, 20-20KHz at no more than 0.1% THD

model 6:
nominal output: 150W/channel into 8 Ohms, 20-20KHz at no more than 0.1% THD

model 7 (monoblock):
nominal output: 75W into 8 Ohms, 20-20KHz at no more than 0.1% THD

models 4, 6, and 7:

bandwidth: -3dB points at 3Hz and 100KHz

slew rate: 50V per microsecond

input impedance: 47,000 Ohms

output impedance: 0.02 Ohms 3hz-20KHz, equiv. damping factor = 400

current capability: 50 amperes max, 16 continuous into 0.1 Ohm

output noise: < 300 microvolts
Yeah.. I still like mine. bought it new before the 'goN existed. nice.
The information provided for the model 6 is incorrect, even though it was what Forte' printed in the first run of amps and their very early literature. This amp was rated at 200 wpc @ 8 ohms and 350 wpc @ 4 ohms. The rating was corrected in later literature. The Four was a replacement for the earlier Forte' 1 / 1A as a 50 wpc Class A amp and the Six was a replacement for the Forte' 3, which was also rated at 200 / 350. Combined with high quality components, these amps can offer astounding levels of performance for what they sell for on the used market. With some performance upgrades / modifications, they truly are "giant killers". Sean
I agree, the Model 1 I had for a while a year or 2 ago, was a very nice sounding amp, and given its age and cost, rather amazing.
I was under the impression that the Model 4 was A/B and the 4A was Class A. Wish Threshold has some literature on their old line. Can anyone recommend another Class A amp still in recent production under $1,000?
Pass Aleph-3 <$1000.00
Doc777: The difference between the 4 & 6 and the 4A & 6A is that the A series added balanced inputs. The 4/4A are Class A amps and the 6/6A amps are high bias AB amps. Both idle at what most would consider quite warm temperatures IF running properly.

As far as literature on the Forte amps go, what are you looking for? Sean
Thanks guys! You read my mind and hit the nail on the head. I was thinking only the Forte Model 4A was Class A. Now my options opened up just a little more with Model 4 as well.
Oh, almost forgot. I wanted to know what years Forte manufactured the Model 4, and 4A? The Audiogon bluebook
says 91 but I wasn't sure what the oldest date is.
According to the resources i have at hand, the 4 was released in 1991. In 1990, the only amps that were available were the Class A model 1A and the AB model 3. Sean