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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
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4krowme OP
542 posts
 
 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.
willieaspen
20 posts
 

Here’s a quick rundown of what I know of the Forte products.

While Nelson Pass ran Threshold:

Model 1/1A - 50 w/ch Class A (more like 15-20W class A)

Model 2 - Opamp-based preamp w/phono and separate power supply

Model 3 - 200 w/ch Class AB

After Nelson Pass left:

Model 4/4A - 50 w/ch Class A (Stereophile calculated 16W class A)

Model 5 - 100 w/ch Class AB

Model 6 - 200 w/ch Class AB

Model 7 - 75 w/ch Class A monoblocks (not sure how many watts actual class A)

Model 40 - preamp (2 versions (one with RCAs; one with RCAs/XLRs) w/separate power supply; could add phono board

Model 44 - preamp with remote (internal power supply)

Model 45 - preamp (don’t know much about it...can’t even find a picture online)

Model 50 - DAC (reviewed by Stereophile)

Model 55 - 100 w/ch class A/B (bipolar devices)

The post-Pass amps all used IGBTs (except the 55), which are no longer available. There’s still a Threshold company (operating out of Houston) and they offer to rebuild T-series amps (for $$$) with all new devices. Might be able to do the same for Forte IGBT amps.

As noted above, the Models 1/3 and 4/6 were basically the same amp running at different voltages and bias.

I had the second version of the Forte 40 and found it difficult to beat anywhere near its price. Look at the pics online to see significant differences in the circuit board between the 2 versions.

zimmer266
21 posts
 

I’m running a Forte 3 with a pair of Vandersteen 2ci. I sent it to Soderberg in California several years ago. He went through it, and also biased it more toward class A. The setup sounds warm overall, and is enjoyable for hours. Source choice might influence the overall system balance.

The Forte amps look to be more popular lately, judging by recent prices.

robkimsj
1 posts
 

This might be a bit off topic, but I have a model three and was looking at a Nakamichi PA-7. Can anyone comment on the similarities or differences between these two amps. I was thinking that I might have the model three modified more toward class A, and have the Nakamichi for higher power needs. Plus, I love the look of the Nak'.

4krowme's avatar
4krowme OP
542 posts
 

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.

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