4krowme I restored a Model-1A, years ago, 50w Class-A into 8ohms to all it’s glory, no opamp chip in it though, but from memory Nelson used a match pair of input fets (Q1) transistors that were in one package (these canlook like a 6 leg opamp chip), great sounding amp for the money, it was the the most sought after one for it sonic ability, from what I’ve been able to find out.
The 3 was the same circuit, but with higher rails to give it 200w into 8ohms buit only about 5 watts of Class-A.
Here is my circuit diagrams of the 1A they don’t last forever on the image hosting site so copy it and save it if you want it.
The Forte Audio (Threshold) Model 3 is a solid-state, dual-mono 2-channel amplifier that produces 200 wpc into 8 ohms or 400 wpc into 4 ohms. ... The Model 3 is very similar to the Model 1A, which is a 50 wpc Pure Class A design.
Circuit diagram
https://ibb.co/YdTTNgg 1A power supply
https://ibb.co/8rQwZ1m 3A power supply
https://ibb.co/4MQWQktA 1A on the mids and highs and a 3 on the bass, the ideal horizontal bi-amp setup!
Cheers George