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 |
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. |
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 |
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? |
Threshold T50. Do you have an idea of what differences might be between that and the Forte' 1A?
No, never came across a T50, had a Stasis 300 and 500 optical. Cheers George |
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. |
I have a Forte Model 6 power amp which I use for my 2nd system. I had it converted many years ago by a tech in CA to a Class A 50 wpc. Still works and sounds great. |
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. |
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.
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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? |
The person I purchased them from had to move back to California on sudden notice. So he was selling off a lot of his audio equipment. I got the T-50's for $600 ea. He also threw in a pair of Zoethecus amp stands to sweeten the deal. Probably my best deal.
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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. |
Will this drive 2ohms? Or a pair of thiel 3.6? 2.5 ohm average throughout the low mids to high frequency range |
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. |
There is a picture of the frequency impedance graph unfortunately can’t post here so you guys would have to look it up |
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 |
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Even if it ends up not being the best match for my 3.6 I can use it with the 3.5 |
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. |
I have an outlet that grips plugs maybe too much so I should be good. |
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. |
What the current rating of the amp? Apparently needs to be above 60 amps |
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. |
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.
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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.
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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'.
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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.
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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.
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