Difference between Forte 4 and 4A amp ?

I had a model 4 and it was my first step into the high end world. I didnt even know it was made by Threshhold and I have no idea of the difference between models. I can tell you that its a pretty sweet amp and regardless of the difference between models you will be very pleased with it.

I owned a Model 4a - both face plate and back sticker were marked "4a".

I am not sure if this is correct, but I believe that it is close to correct. Forte originally offered a model 1A and a model three amplifier. The model 1A was a fifty watt per channel class A amplifier and the model 3 was a 150 watt per channel class AB amplifier.

Threshold and Forte began using IGBT, insulated gate biploar transistors, and the model 4, as well as the model 6 if I am not mistaken, were born. The Model 4 being the 50 watt per channel class A amplifier and the Model 6 being the 150 watt class AB amplifier. I am familiar because I owned a Forte Model 4 amplifier sometime around 1993.

I am not sure if there is a difference between the Model 4 and 4A. I believe that one model was preceded by the other and that one model, the latter model, offered balanced and rca inputs and the other model offered rca inputs only. I could very well be wrong. I know that the amplifier changed very little, if any, after its initial product run.

What I remember most is that the Forte Model 4 drove a pair of Celestion SL600's in a wonderful way. It was a great amplifier, a little dark and a pleasure to listen to.

Enjoy,
Drew

I've owned at LEAST a dozen of these amps. I cycle them thru my system and sell'em to friends. A truly great little amp with the ability to drive low/complex impedance loads. You should hear a pair of them driving my electronicly bi-amped Apogee Stages & Duetta Signatures.

Anyway, the basic difference, besides minor face plate cosmetic differences, is that the 4a has a set of balanced inputs while the 4 does not. Pretty much end of story. There were some minor parts differences as the production line aged but I've found little if any sonic differences even between samples with large gaps in the serial numbers.

There is one tweak I can recommend though and that is to get the 4a and bypass the input selector switch & balanced inputs. Run it with just the single ended inputs. I have noticed a slightly more open sound (even over the 4) running it this way. The tweak is easy & reversible.

In general, avoid the balanced inputs as they have an input impedance that is just south of 1K ohm, which is a very difficult load for most pre-amps (certainly tubed ones).

Voltage gain is 26 DB and they actually work quite well into an Adcom Ace 515 line conditioner, most other conditioning flattens the amp dynamically (and this amp DOES boogie). A Wire World Aurora power cord is a nice upgrade as well.

Good Luck, you have a fine amp!!