Test Data and Specifications (PDF)
We are pleased to announce our newest vacuum tube microphone preamplifier the FetCode. This preamplifier uses one Tube/JFET high voltage cascode differential stage, followed by three high voltage vacuum tube stages to provide a transformerless vacuum tube preamplifier with extremely low input noise. All of the FetCode audio circuits operated Class A. The FetCode power supplies are all fully regulated. The high voltage power supply is double regulated, with dual mono secondary regulators (1 for each channel).
Forssell Technologies is well known for our cutting edge vacuum tube and JFET circuit designs. Thousands of channels of Forssell designed tube and JFET circuits are used worldwide. The Forssell designed M-2 mike preamps have achieved wide acceptance for its sonic quality. However M-2 and similar designs have one major drawback: noise. The noise performance of the M-2 prevents it from being used with ribbon and dynamic mikes in low level recording applications.
At Forssell Technologies, we love the sound of our M-2 designs, but for our own recording purposes we often need a preamp with lower noise performance. The commonly available low noise preamps often left us wishing we could combine the sonic qualities of an M-2 with the low noise performance of other preamp designs. So we headed back into the lab and got to work on a completely new preamp design using a single JFET and vacuum tube front-end, followed by multiple stages of vacuum tubes. The result of this work is the FetCode microphone preamplifier. The FetCode has a noise floor that is 12- 14 dB quieter than our old M-2 design.
As mentioned above, the front-end topology of the FetCode uses a cascode connected differential amplifier. This diff-amp uses a dual JFET as the lower element of the cascode and a dual triode vacuum tube as the upper element. Both of these elements provide gain. This stage is differentially coupled to an all vacuum tube operational amplifier which provides a small amount of gain and acts as a line driver.
The FetCode gain is continuously variable from 15 dB to 60 dB (higher or lower gain figures are optional). EIN figures of -127 dBu (22 Hz to 22 KHz unweighted) are easily achieved using this topology. The maximum output level is +36 dBu into 10k ohms, or +32 dBu into 3k ohms. Due to the 15 dB minimum gain, a maximum input level of +21 dBu will therefore cause the output stage to reach its maximum level of + 36 dBu. The output stage will always clip before the input stage.
The gain controls are 24 position precision Elma stepped switches. The other front-panel audio switches control high quality relays located on the printed circuit board, resulting in the shortest possible signal path. When phantom power is not switched in, the input stage of the FetCode preamp has its DC blocking capacitors switched out of the signal path. There is a dual color LED for each channel located on the front panel. These LEDs flash green when the output signal reaches 0 dBu, and change to red at +24 dBu (a full 12 dB before clipping). This, coupled with the individual channel mute switches, allows the FetCode to be easily configured when it is used as a remote mike preamp. A remote gain control version of the FetCode will be available soon.
We are really excited about this preamp. We feel that we have finely designed a transformerless vacuum tube preamp that has the best of both worlds- the sound of tubes and the low noise performance of JFETs without the sonic impact of input transformers.
