Review: Forte Model 4a Amplifier

Category: Amplifiers

The Forte Model 4a is clearly a gotta have amplifier. It is 50 Watts RMS per side in Pure Class A configuration.

Forte was the sister line to Threshold and with that being said much of the Threshold technology found it's way into the Forte product line. Such as the Forte 4a with its IGBT technology,which Threshold used in its T series of Power Amps.

The Forte 4a may only be 50 Watts per side,but what a glorious 50 per side it is. That is into 8 0hms,into 4 ohms it is 100 per side. During it's heyday the Forte 4a was compared against the Levinson 23.5,the VTL 225w, Muse Model 100 and other high end contenders of the day. In particular Corey Greenbery of Stereophile was more than duly impressed(Vol 15 No.11) as was Robert Hartley(Vol.16 No.7)

What was so impressive about this amp.,was its ability to deliver the music and without the slightest hint of strain. Always sounded much more powerful than the 50 per side it stated.

This amp clearly delivers the music with a transparency and detail that is hard to imagine unless you have heard it. Being Pure Class A the Forte has a sonic signature,that is agile,sweet and delicate,without a trace of the usual solid state forwardness,grain and glare were totally absent. Handles complex musical passages with a delicacy not often heard. No doubt the Forte 4a has in my opinion a damn near perfect balance of liquidity and resolution.

I listen to mostly Contemporary Rock N Roll,Jazz and Classical. This amp.,in short is just a dream to listen to. It's ability to retrieve the audio signal and pass it through to the speakers is very special indeed.

Being the jaded audiophile I am it is very hard to impress me with anything.Been in this hobby since 1957 and now 59,have just about heard and seen it all. The Forte Model 4a duly impresses. I was totally surprised by it's performance.

When new this amp had a retail of $1,790.00.They can be had in the secondary for about $400.00 to $600.00. Trust me,these prices on the Forte Model 4a wont stay this low for long. Once the word gets out on this amp.,the used prices will start moving up quite quickly. So if you have the need for a Pure Class A amp.,at what I now consider a giveaway price. Get one soon. At this point in time the Forte Model 4a is an outright steal at todays secondary market prices.

Make no mistake this amp.,can compete with any Pure Class A amp out there today.Like any Class A amp.,it runs hot and will need plenty of ventiliation. In my opinion it just doesn't get much better than this.

The difference between the Model 4 and Model 4a, is that the 4a has balanced inputs. Sonic signature is the same.

Associated gear
Click to view my Virtual System

Similar products

I agree the Forte 4a is an audio classic. In as much as it sounded more powerful than it's 50 watt rating, I've always wondered what the 75 watt mono model 7's sounded like. Anybody care to share their thoughts on them?
Caveat: I believe that the unique IGBT transistors used in this amp are long gone and repairs that involve them will be problematic to say the least. The upgrades offered by Threshold for the regular line involved replacing these with conventional bipolars at great cost.
True IGBT are no longer made. With that being said IGBT seldom fail,their reliability is legendary. Should an IGBT fail Jon Soderberg of Vintage Amp,has enough IGBT's now in stock to last for several years. I now see no reason to avoid the Forte 4a,because of IGBT resourcing. The IGBT in the Forte 4a are tested at 150 percent before the build and operate at only 50 percent of rated capability in the amp.Each one was hand selected and tested before the build.Failure rate of the IGBT in the 4a is for the most part nil. Many thanks to Viridian for pointing this out.After talking to Jon Soderberg,felt it was a moot point to put in review. However many may not be aware that IGBT are available. Highly doubtful they will ever need replacing though.
It is great to know that there is a resource for the excellent IGBT transistors. I have found that the demise of output transistors in solid-state gear is frequently caused by failure in the previous stages so I would continue to have concern about purchase of what is an amazing sounding, but aging design. I might add that, even more than age, it is heat that diminishes the life of the building blocks of electronic circuits and this is a particularly hot running amp.
The above subject amp came from Jon Soderbergs private collection and as such was fully tested and set to factory specifications. If one should acquire a Forte Model 4a. I would suggest a call to Jon and discuss,what ever service it may need.Jon worked at Threshold/Forte and doubt if anyone knows more about this product than he does.
For further information refer to Stereophile November 1992 review by Corey Greenberg and July 1993 review by Robert Hartley. Both reviewers were more than suitably impressed with the performance of the Model 4.
Thanks for the review. I happen to have a Forte'6, which I have heard is identical to the 4, but runs class A/B, with the voltage rails "turned up". It was my first good amp and it is now running my 4-12" subs. I have on occasion tried a few others, but so far nothing I have tried has beat it. I figure it can be bettered, but at a much higher cost. I think it is fairly ideal for a sub amp the way it puts out 220 watts in it's A/B class. Since there are many who seem to know about the Forte's, can anyone share a little insight about the voltage rails and what that involves, benefits, downsides, etc?
The Forte Model 6 can be revised to a Model 4 with very little work or expense.The Model 4 has 39V rails while the Model 6 gets 76V rails. The IGBTs operate at 50 percent of SOA while the Model 6s are pushed to the limit. The biasing in the Model 4 is set to pure Class A, while the Model 6 is set for A/B. Also the Model 4 uses different secondary taps for the DC supply voltages. If you want to change a 6 to a 4 I would contact Jon Soberberg at Vintage Amp. The sonics of the Model 4 are superior to the Model 6, plus it will live a lot longer in Model 4 configuration.
I've used the Forte 4a since 1994 with no problems, just getting very nice music. As others have said, it's detailed, sweet and packs a pretty good wallop. The 4a is driving Spica TC-50's, and is fed by c-j PV5. I added one 10" Velodyne powered sub in a small room, rolling off the Spicas around 70 Hz. Very nice, but I don't crank it up as much as I'd like for Mahler or Rock music for fear of blowing the Spica tweeters.

I recently moved my system to a larger room (16x24x10) with less furniture. I am getting a pair of used Thiel CS 3.6's soon, and will be interested to know if the Forte 4a can drive them. I have serious doubts, and if this amp runs out of gas I'll need to replace it. I am open to suggestions for a used SS amp under $4000. Any ideas, such as the Sim Moon W-5?

I would also appreciate peaker cable suggestions -- a LONG run under the floor, but that was the price of getting the system into our living room!
I would definitely try the 4a first to see if you need additional power. I have used the 4a with Acoustat Model 2 speakers with great results. Probably one of the hardest speakers to drive properly because of its widely varying ohm load.

If you feel the need for additional power I would get the Threshold T 200, fits in your price range. The Sim you menetion lacks the damping factor to really handle the Thiels.

As you know the Forte 4a is 100 watts into 4 ohms and 150 into 2 ohms with high current capability.

Second on my list would be a solid state Audio Research in the 200 watt range. But you have grown comfortable with Class A ouput stages and anything less will dissappoint.

The Pass Labs x 350 would also be a terrific choice,albeit somewhat costly.
I am the proud original owner of a Forte' 4a.
Mmmm I like it. No need to search for a 'better amp' I already have it!! Whoo Hoo!!!
I just bought a Model 4a off eBay for just over $550. I've been wanting one of these since I sold my model 5's some time ago.

Was wondering what preamp would be a match for this amp?? I'm thinking of building a kit tube amp (Foreplay) ... would this work with this amp?? Thansk.
Anyone compare this amp to the Forte 1A? I had one of those; regret selling it. I've got a 4a on the way...Yippee!
There is little to compare. The 1a was bi polar output while the 4a uses the IGBT output devices.

You are going to be totally surprised by the sonics of the 4a. It totally blows away the 1a and the 1a was very good indeed.

The 4a just effortless reproduces music with astounding clarity and transparency. Sound stage is as deep as any I have heard in recent times. Just make sure you have a very good SS preamp to go with it. I use a Forte Model 40 with glorious results.

All I can say is enjoy the the 4a it is truly remarkable.
Thanks, Ferrari! I've an Audio by Van Alstine Transcendence T7 hybrid preamp. Tubed inputs/Class A FET outputs. Despite its funky looks, it is just an amazingly clean and dynamic sounding preamp, without an ostensibly SS sound. Ought to be a good match.
I have found the Threshold T3 (early to mid-'90s vintage---used around $900)to be a perfect companion to the 4a. This combination provides true "see through" clarity with high frequencies of exquisite delicacy.
Like most things in life you pays your money and takes your chances. After reading this review and the follow up posts as well as further research decided to take the plunge. Trying to find a Model 4a was not as easy as I had thought, wanted a very pristine example with box manual and all. After a few months was able to locate what I was looking for.

So how do I like it? I just don't have the vocabulary to express my pleasure in owning this amp. All I have read is true and then some. I have heard things in my recordings I just did not knew existed. Much to my wifes displeasure I have to keep prying myself out of my listening room. In the 4 months I have had it, I think I played every CD and vinyl LP I own. This replaces the Spectral DMA 50 that finally gave up the ghost. The Spectral was damn good, but the Forte 4a puts it to shame. In the interim while I was finding the 4a was using my friends Levinson ML 11. Very good amp and tried to talk him into letting me buy the Levinson, very glad he did not take me up on it. The Forte 4a is just liquid what sonics this produces. Now I have an excuse to have the Klyne SK5A pre amp updated.

I think to do better than the 4a I will have to spend a great deal for marginal increase in sonics.

If you need a very good Class A power amp, get one of these and like all else be sure of whom you get it from. It pays to be careful.
FYI the IGBT transistors used in these have substitutions of the same type of device and should work fine. IE the
2SC3281/2SA1302 are available still from Motorola in the
number MJL3291/MJL1302 are still in production are meant for audio. They are also available in Matched pairs. BTW
love my Forte 3 great sounding as well.
I note with interest that the principal designer, Michael Bladelius, is now with Primare. Has anyone heard his recent work?
I currently own three Forte' 3's and two Forte' 6's along with a couple dozen other amps. Have also had Forte' 1A's and Forte' 4's in the past. Ned aka Ferrari has a pair of the Forte' 7's, which i've always been interested in but never tried. Given that they are more powerful monoblock versions of the 4A, i'm sure that they sound quite similar with better dynamics, greater control and improved channel separation.

For best results, these amps work best if left powered up 24/7. Make sure that you have adequate airflow / ventilation around all sides of these amps as they run quite warm. If your Forte' amps are not idling quite warm to the touch, the amp is out of bias and needs repair. I've run across a few amps like this, so be on the look-out for such things if shopping for a Forte'. I've seen sellers of Forte' amps state "amp never gets hot", so that's one that you know is not working properly / in need of repair.

My experience is that the Forte' amps work best with lower impedance loads. They aren't the most ruggedly built amps in terms of current capacity though, so don't expect them to drive very low impedance loads to staggering sound levels while retaining great amounts of control. Having said that, they do work very well / sound good when driving some speakers that other amps may not do all that well with. This could be related to their higher bias levels and reduced crossover distortion that results.

To sum things up, i am a big fan of Forte' amps. When they are running properly i.e. set to factory spec, they are one of the greatest bargains on the used audio market. Combined with solid support components and high quality low impedance speaker cabling, they can form the foundation of a truly accurate yet musical system that is phenomenally cost effective. When tastefully modified beyond factory spec, they can easily compete with and many times beat much more expensive and prestigious amps into the ground. Sean
Recently picked up a 4A for my sister since she had tight budget and I had an 1A long time ago and liked it. Forte 4A has to be the biggest bang for the buck in audio world now. It's dynamic, smooth, silky with proper amount of texture, depth layering, and transparent. You name it, it has it. It will rival anything out there, and better the big Pass X-350 in most area which I used to own.

Forte 4A is driving my Sonus Faber Amati Homage now which is normally handled by BAT VK-75SE. If I was blindfolded, I would not guess it was solid state. It's that smooth.

It's so good I was debating if I want to give it up for my sister :)
I am not sure where the people who are posting that the
IBGT transistors are no longer available but this is
fictional. Possibly the same exact device say 2sc3281 is not in production, however there are present day versions
for a full replacement of this device made by both
Toshiba ( the orignal manufacturer )as well as Motorola
and others that are in many ways as good if not better.
IE a second generation of IGBT's that are even more
reliable than the previous device. The new Motorola IBGT's
are in particular aimed at and designed for low distortion
audio . Just wanted to clear up some confusion. I presently own a pair of 3's and love em. Open clear
with the advantages of mosfet by bi polar qualities as well
(IE tube like sound due to limited stages and very limited
negative feedback).. For any interested check out the group. The IGBT is far from dead.
I read this post years ago, and then I picke4d up the old stereoph8ile from November 1992 about the Model 4 and the 6. I bought the 6, it replaced an old Carver tfm-55. I liked the Carver more, the 6 sounded terrible to me. I had planned on buying the 6, and having it re-biased my Jon Soderberg who happens to live about ten miles from me. But, it sounded so bad, I begged Jon to take it ASAP, and he put in four conrad dublier (misspelled?) capacitors at 31,000 uf each, and Jon re-biased it. What a nice sounding amp, I am not sure what is sounds like in comparison to a 4a, but I am very happy with this amp. It is very detailed in the high end, and my BAT VK-D5 tamed the slightly metallic midrange......Now with a Van Alstine Transcendence 7ECR on the way, I will find out just how good it really sounds. I was using an audible ilusions Modulus and a FET-2 threshold before. I spent 1050 dollars for this amp with the upgrade, I am thinking about buying another to bi-amp...........
I own a model F4 with rca and balanced inputs. The lettering on the front looks much different than that of the amp pictured at the start of this thread. It has block lettering and doesn't have the Forte logo as on the one pictured. It was represented as 50wpc class A.

Do I have a model 4a?
Yes it is a model 4a. Last generation of this amp from Forte, before they closed down. Just different logo, thats all, same unit.
Thanks for the response!!
I just recieved a pair of Forte 7 mono's. Wow!!!!! They had the fully monty from Jon Soderberg. By far the sweetest amps I've heard. Very detailed, great bass and, like I said, silky smooth. I think they are even better than my Threshold SA/3, which just came back from Jon, as well. Very musical amp.
i just bought the forte model 6 (together with preamp forte f45). it sounds great but there's a permanent hum, which can get annoying in quiet passages. i've got the 220v-version and i'm using it in paris. has anyone got an idea what to do about it? thanks.
Arnold, Contact Jon Soderberg at

He is very knowledgeable in regards to Forte and Threshold.
I've been using a Forte Model 6a for about a year on a pair of ML Sequel IIs. The ~360w per side that the 6a delivers to the 4ohm Martin Logans seems to be a perfect match. I'd venture to guess that I'd have to spend almost $2,000 on a used amplifier to even get close to the midrange performance that I have :)
Would the 4a have enough grunt to drive the top half of some Infinity Kappa 8s?
Yes, should work fine. I've used a 4a on the top half of Infinity RS-IIb's with no problem whatsoever, and the sound is excellent.
I bought a Soderberg-modded Forte 4a and am using it on Quad ESL-57's, it sounds GREAT.
Just recently purchased the Forte 4A in its stock form. This particular one has the removable power cord. The person I purchased it from told me that the 4A he used to own received upgrades and when it was returned to him he felt the upgrades had removed the magic the amp previously had. I forget who done these upgrades.

I think what has been said above is very true and accurate. The 4A is a very fine solid state amp and I have just begun to realize its full potential. To move forward I am now thinking that a tube preamp could showcase the 4A's capabilities although it sounds darn good with solid state. Any suggestions on a tube preamp?

Also I would like to thank Ferrari for his recommendations via email and for this wonderful review.
Have heard this unit with the Musical Design SP 2B tube preamp from John Hillig. Very nice match with the Forte. Also the Audio Research SP8 sounded damn fine as well.