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An old Forte' model 3 made new

 This is an odd topic just because what I will be discussing is an old Forte' model 3 power amp that has been completely rebuilt and designed differently than the original. Years ago, it started with a loud popping noise in one channel. That was enough to take a look inside. I found out that this model uses a chip for the input and the chip had a reputation for this problem. When looking inside the unit, I decided it was time to change out the power caps, replace the bias setting pot, and hope for the best. Shortly thereafter, I knew that I was in over my head, and looked for a tech out there that would be of assistance. Almost by accident, I ran into John Dee, a former engineer at Sudgen? audio located in Europe. Point being John was just the right person for the job. It took a long time to complete the project due to personal reasons, but now the amp is up and running in my system. John converted it to class A bias, and also reduced the number of output transistors. Probably close to all the transistors were changed out due to catastrophic failure (you can thank me for that). A better diode bridge was installed, faster caps replace throughout, and numerous other parts I am sure were dealt with as well. Since my speakers are quite efficient at about 94 db, I had John set the bias for a mere 20 watts class A. It sports a damping factor of 600, has a much faster rise time, and electrolytic caps were all bypassed with Wima caps IIRC. The sound? Well, as expected the bass is to die for, the midrange has power that makes the music 'right there', and truthfully, I do have the hearing to give an honest opinion of the treble, but I feel that it is a bit light. I suppose that I could rig up an adapter and use headphones, giving me the chance at hearing more accurately what the signature of the amp is. Probably sometime soon, I will do that. I just felt that this sort of project is more common amongst some of us and would like to hear your impressions of said projects.
128x1284krowme
georgehifi's avatar
georgehifi
10,312 posts
 
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/4MQWQkt

A 1A on the mids and highs and a 3 on the bass, the ideal horizontal bi-amp setup!


Cheers George
4krowme's avatar
4krowme OP
542 posts
 
George,

Yes, I remember that the 1A was a sort of revision of the model 3. Where the model 3 had 76 volts on each rail, the 1A only used 36 IIRC. The difference, and improvement with the 1A, is that it is class A, BUT also that the secondary power taps on the transformer were now used as one per channel, rather than using them in tandem for a single 76 volt rail. To be clear, this means that the 1A has one secondary for each channel instead of the model 3 which shares the two secondaries between the channels.
It also seems that the model 3 could be made into a 1A simply by swapping the secondary power leads and readjusting the channel bias.
Thank you for the clarification of the Q1 transistor package. That was the one thing that I didn’t like seeing in this amp.
 Can you tell me how many amplification stages there are in the model 3?
Lastly, I thank you for the links! You know how rare the power schematic for the model 3 has become! Bless you.
georgehifi's avatar
georgehifi
10,312 posts
 
It also seems that the model 3 could be made into a 1A simply by swapping the secondary power leads and readjusting the channel bias.

Correct, I believe many 3 owners had that done if they owned efficient speakers.

Thank you for the clarification of the Q1 transistor package. That was the one thing that I didn’t like seeing in this amp.
I did have the parts list but lost it, those matched fets were still available from memory

Cheers George
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