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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
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.
pelv
38 posts
 
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?
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.
tarjin22
8 posts
 
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!
Thanks.
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