Moody Sounds BJF JFET Compressor kit

designed by Bjorn Juhl (BJFe)

Moody Sounds BJF JFET Compressor kit

  • Moody Sounds BJF JFET compressor

    To get that tone that really hangs through the solos, the compressor is a given thing! We offer BJF JFET compressor, with carefully selected components. The circuit creates a pulse when you send a guitar signal to it. The pulse affects the amplification of the circuit so that peaks are evened out and so that the tone remains when the volume from the guitar decreases.

    As the name suggests, the volume is controlled by a field effect transistor (JFET). There are several well-known compressors that are based on similar technology, such as the Dan Armstrong Orange Squeezer. Some of them can be difficult to get the right touch on for the builder. It may be hard finding matched transistors and to give the pedal a dynamic compression. The BJF JFET compressor has a detector that is as advanced as many studio compressors, and no matched pair is needed, but the working point is set with a trim pot. There is a very big difference between JFET transistors, even though they have the same designations, and even from the same batch, but the trim pot range covers some variations, and a suitable JFET is included in the kit.

    JFET Compressor has three control knobs, Volume, Treble and Compression. It has true bypass. Powered with 9v power supply or battery.

    This is not a kit that is recommended if you are testing pedal building for the first time. The components sit tightly on the board and they are relatively many to keep track of!

    The JFET Compressor kit is designed by Björn Juhl. Take part in the discussion about this circuit on our forum.

    How to set the trimmer

    Set the trimmer to minimum so that the compressor is quiet and turn slowly while playing until the compression starts. If you turn too far, the volume drops sharply when you play and you can then reverse slightly until no strong transient causes the volume to drop sharply. When the volume drops sharply with transients, the JFET is completely switched off. Then, with a transient, it is switched on with a volume drop as a result. Properly set up, the JFET is just ON and there is a basic attenuation at the input that increases the range. When the signal is applied, the detector takes out the average value of the signal and adjusts the attenuation dynamically. The detector both full wave rectifies and takes the absolute value of the signal, and therefore the integral of the signal controls the attenuation. It is thus the mean value of the signal that controls the attenuation. The fact that the signal is detected in this way means that attack and release times can be set more freely. However, there are limitations and the times are optimized for the JFET’s range, but anyone who wants can experiment.

    Since transients can always slip through, the most important quality of the subsequent amplifier is how it handles fast transients. Another important quality is low noise because the amplifier comes after the attenuator and therefore amplifies noise by the same factor as it amplifies the signal. The OP amplifier has been chosen to be an old workhorse from Phillips, which has a very low noise factor. The resistance before the JFET is set high to increase the range. The JFET also works with a linearization feedback that forces the JFET to be linear over a larger range. But this also means signal feedback and it can cause problems with transients. Also for this reason the resistance is given a high value.

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