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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
Threshold,Spectral,Levinson,Citation,Sumo,etc

ferrari

Showing 9 responses by ferrari

ferrari OP
617 posts
 
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.
ferrari OP
617 posts
 
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.
ferrari OP
617 posts
 
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.
ferrari OP
617 posts
 
Arnold, Contact Jon Soderberg at http://www.vintageamprepair.net/

He is very knowledgeable in regards to Forte and Threshold.
ferrari OP
617 posts
 
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.
ferrari OP
617 posts
 
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.
ferrari OP
617 posts
 
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.
ferrari OP
617 posts
 
Yes it is a model 4a. Last generation of this amp from Forte, before they closed down. Just different logo, thats all, same unit.
ferrari OP
617 posts
 
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.
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