I thought the Nakamichi products were akin to the Threshold S-300, etc. They used the STASIS sliding bias under license from Threshold. The Forte amps used fixed bias.
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.
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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. |
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'. |
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. |
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. |
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. |
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. |
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. |
Will this drive a pair of Thiel 3.6 If your asking about the Forte’ model 3? I say yes fine, as they are much just a resistive load, and above 2ohm. The -30’ phase angle at 70hz isn’t combined with a low impedance at that frequency, so it’s not too bad. I had one of those for repair, good cheap powerfull amp, 20!! x complimentary push pull Toshiba output fets total . And a decent sized linear supply and low negative feed back. Cheers George |
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. |
Eric Lauchli, B.D. Dale, Lorin Peterson of Coda along w/Michael Bladalius formerly w/Classe, Primare and his current company Bladalius Audio out of Sweden were responsible for most of the Forte and Threshold T series. I owned a Forte 4A which was very good still regret selling it. I still have a Forte 6 powering my center and a pair of T-50's which I bought off a former Threshold Executive. While he was at the company he had Michael Bladalius modify both amps into 100 wpc class A mono blocks. I've has them for over 12 years. |
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. |
So it no longer a Forte designed product. Class A yes. Recently build the old Luxkit amp which is class A and it probably is better sounding than most amps anyone here on Audigon has ever had or heard. I also build hybrid power amps like the old Counterpoint were like with a complete design of our own and much better parts quality with all point-to-point wiring. You would be hard pressed to find something that sounds better in any system application. |
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 |
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. |
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 |