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

Showing 10 responses by 4krowme

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.
One more thing. In the past, I did own a Threshold T50. Do you have an idea of what differences might be between that and the Forte' 1A? I know that the Forte' line was more affordable, but it just makes wonder. And yes, Why Did I Sell that T50 for anyway?
 They really were a product worth owning. In fact, I did own a set of mono bloks at one time (before I knew squat about electronics), and sold them because there was a very small hum in each channel. Maybe they were the model 7? Too long ago to recall. Also seems that Nelson Pass was only responsible for the first couple of models as far as design, but I could wrong.
 Lastly, wasn't there a model that used IGBT transistors? In fact, it might have been the model 3.
Thank you for your input. A bit of audio history there.  Gee, those T50's would be an excellent pair. Please remember me if you would ever sell them.... Fat chance, right?
http://systems.audiogon.com/users/4krowme 

 Well, aren't we modern? I have not so good photos of the amp before and after (at least the external chassis). Point here is to show how the chassis was extended to house the much larger power supply capacitors. Probably wouldn't have gone this far with any other amp. 
Hard for me to say about that. Those sound to be pretty low dips in impedance. Perhaps someone here could better answer that question. Low impedances are tough for many amps, so I am not eager to answer without some experience in such a case.
HA! Had one of those myself. Just because you can, it doesn't hurt to clean the AC cord prongs and maybe even use a little Caig protectant. Believe me, I don't want to open a big can of worms here about cables, connections and the rest, but just to be as reasonable as possible with little effort of your own brings piece of mind. Good luck.
 The main fuse is 8 amps, fast blow......  As far as the current capability of the Amp Output, I doubt that it is 60 amps. Lucky if it were as much as 30-40 amps. Each channel does have 10 or 12 Toshiba transistors though. Leads me to think that the current capability is high. Sorry that I can't provide better info than this.

I can't argue with the look of the Nakamichi. Beautiful. The Stasis design is very high biased, but not class A. When I did own a PA-5 at 100wpc, I found that it ran out of gas right after it's 100 wpc rating. 

 It was years later that I owned a Forte' model 3. I know that it can be rewired pretty simply to run in class A at 50wpc. I did NOT do this, and my model 3 is not anything like the original that I started with. It now runs in class A at 40 wpc, but the circuitry is entirely different. Only the chassis is the same, well, basically, as I had to widen it to accommodate the much larger power supply caps that I used. FWIW, they were a used set from a PA7! Go figure.

Overall, I was never disappointed by the Forte' amps that I had owned. And yes, a 15 amp circuit is fine. Make sure that you have a good grade AC outlet at least though. Just today, as I was working in the museum, I attempted to use an old outlet that had no grip to it. There is nothing worse, and nothing easier to fix. just my experience talking.