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Fractal Audio DRIVE models: FET Boost (and a bit of KLON)


The contents of this thread, including updates, are available as a PDF Guide. Download it here.

FET Boost

Owner's Manual: "a gentle, smooth clipping booster with tone controls". It has not been disclosed whether this model is a custom model (virtual only) or based on a real pedal. If it’s based on a real pedal, it might be Fulltone’s Fat-Boost (discontinued), but that’s just guessing.

FET is abbreviation of Field-Effect-Transistor, a semi-conductor. FETs are used in pedals as well as amplifiers. For example the Dumble has a FET input, designed for low-gain instruments. In products for guitars you’ll encounter the MOSFET and JFET types the most.

“The field-effect transistor (FET) is a transistor that uses an electric field to control the shape and hence the electrical conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are also known as unipolar transistors since they involve single-carrier-type operation. The FET has several forms, but all have high input impedance. While the conductivity of a non-FET is regulated by the input current (the emitter to base current) and so has a low input impedance, a FET's conductivity is regulated by a voltage applied to a terminal (the gate) which is insulated from the device. The applied gate voltage imposes an electric field into the device, which in turn attracts or repels charge carriers to or from the region between a source terminal and a drain terminal. The density of charge carriers in turn influences the conductivity between the source and drain. (...) The most commonly used FET is the MOSFET.”

“FET’s can behave like Tubes in a properly designed circuit. They can “clip” (distort) in a pleasing way instead of being buzzy or spiky sounding (like a Transistor) and can also add a subtle Limiting to the signal which can even-out the volume making your Live and Recorded tones more pleasing to the ear.”

FET is implemented as a Clip Type in Fractal Audio’s Drive models. The FET Boost model showcases it. Note that the FET Boost is different from the "FET Preamp" model which will be discussed separately.

The FET Boost model has been a part of the firmware since the early days. It’s a favorite model of many players, boosting the signal going into the amp. Slamming the input of amps often results in better tone than using an overdrive because the increased amp distortion is natural and transparent and the tone gets fatter. It enhances the characteristics of the amp instead of changing it. Boosts are often used before vintage amps like Vox and Plexis. Alternative boost solutions are discussed in the FAS Boost thread.

Why do people love this particular model? Well, how do you describe sound, why does person A like a certain tone, and person B doesn’t? The FET Boost is fairly neutral but still adds something to the tone. It becomes dry, warm and fat, reminding of old records. Note that you won’t get (much) distortion from this model, that’s not its goal.

Personal note:
I would totally dedicate one (global) Drive block to the FET Boost ...... if we had more than two Drive blocks to our disposal. For now I use the poor man's alternative: a PEQ block, as described in the FAS Boost thread.


The FET Boost is also a favorite method to get KLON-like clean boosts from our Fractal Audio Devices. Yes, the illustrious KLON Centaur boost/overdrive pedal, now selling for close to thousands of dollars.
  • @Admin M@ / @simeon: FET Boost - Silicon - Gain about 6 - Mid +1 - High Cut about 6000 - Level to about 7.
  • @austinbuddy: FET Boost - Germanium - Gain 5 - Tone 4 to 5 - Mid +0.9 - Level 6.5 to 8.
  • @groovenut used IR capturing to simulate the KLON tone.
About CPU:
  • Fractal Audio's Drive models take up varying amounts of CPU. The FET Boost model requires a moderate amount.
  • When a Drive block is engaged, CPU percentage will rise during playing, because CPU usage is "amplitude dependent".

Link to the list of published threads
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Fractal Fanatic
You are a brave man Yek - Thanks again for this series, it is very informative, and it really inspires me to investigate all the different drive models. Especially this one, it sounds like it would tickle me in all the right places.


FET is implemented as a Clip Type in Fractal Audio’s Drive models. The FET Boost model showcases it. Note that the FET Boost is different from the "FET Preamp" model which will be discussed separately.
MOSFET transistors have internal diodes in them, for technical reasons. If you jumper two of the leads on the transistor together, you effectively expose the transistor for external use. This is what the Fulltone OCD drive uses for clipping diodes. So, when FET is used as a clipping type, I've always assumed this is what it means.

I had always thought that the FET Boost would indicate that it's based on some variation of Jack Orman's Mini Booster, which uses a pair of JFET transistors to provide the amplification. Alternatively, there's a very similiar MOSFET booster design that's used a lot. The MOSFET booster has a more transparent amplification than the Mini Booster, and I'm not sure why you'd model that in the Axe, since there's all kinds of other ways to provide a really transparent boost in the virtual signal chain.

But there's no particular reason that I can see that the FET Boost would naturally use MOSFET diodes for clipping. The two uses - amplification and clipping diodes - wouldn't be related in the circuit.
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