User manual DW FEARN VT2

[. . . ] FEARN VT-1 VT-2 Vacuum Tube Microphone Preamplifiers Operating Instructions How to Contact us: Telephone: 610-793-2526 Fax: 610-793-1479 Mail: P. O. Shipping Address: 182 Bragg Hill Road West Chester, PA 19382 U. S. A. e-mail: dwfearn@dwfearn. com www. dwfearn. com D. W. FEARN VT-1 & VT-2 Microphone Preamplifers 3 VT-1 / VT-2 Vacuum Tube Microphone Preamplifier Final Test Report Model _______________ Serial Number_______________Mains Voltage ______________ Date ___________________ Tested by ________ VU Calibrated to _______________ dBm Test Equipment ____________________________ Microphone ________________________ Channel _______ Frequency Response: 20 cps to 20 kc/s +/- ___________ dB THD+Noise: 20 cps ______________ % 200 cps _____________ % 2 kc/s _______________ % 20 kc/s ______________ % Noise: ______________ dB below +4 dBm out Equivalent Input Noise _______ dB Channel _______ Frequency Response: 20 cps to 20 kc/s +/- ___________ dB THD+Noise: 20 cps ______________ % 200 cps _____________ % 2 kc/s _______________ % 20 kc/s ______________ % Noise: ______________ dB below +4 dBm out Equivalent Input Noise _______ dB Operational Tests: -20 pad ________________ Lo-Z input ______________ Phase Reverse __________ +48V ___________________ Listening Test ____________ Operational Tests: -20 pad ________________ Lo-Z input ______________ Phase Reverse __________ +48V ___________________ Listening Test ____________ VT-1 & VT-2 Microphone Preamplifers D. W. [. . . ] This is a function of the interaction between the microphone transformer and the VT-2 input transformer. Input Lo-Z position The Lo-Z position should be used only with certain microphones (e. g. the Neumann TLMseries, some newer Schoeps) that exhibit an extremely low output impedance. These microphones typically feature a transformerless output. In the Lo-Z position, an impedance-matching network is inserted ahead of the input transformer and this optimizes the VT-2 performance with these microphones. The best position can be determined by listening, although the difference may be subtle on many mics. If the 20 dB pad is needed with a very low impedance microphone, simply select the -20 position of the INPUT switch. Keep in mind that many musical instruments and vocalists produce peaks that are as much as 20 dB above the average level. Often the "peak clipping" that occurs is of such short duration that it is not easily noticed and may be acceptable in many situations. This is particularly true with the VT-2 because there is no hard threshold of clipping but rather a comparatively gradual increase in harmonic distortion. Over-driving the VT-2 will increase second harmonic content, adding fullness and richness to the sound. Use your ears to determine the best position for the Input control. +48 Volt Switch (1) Solid-state condenser microphones in professional environments are usually phantom powered: the microphone electronics are DC powered through the audio cable. This is accomplished by feeding the positive side of a 48 volt power supply to both pin 2 and 3 of the input VT-1 & VT-2 Microphone Preamplifers D. W. FEARN 24 connector (through precision matched resistors), and the negative to pin 1 (ground). The DC voltage is recovered at the microphone with negligible effect on the audio signal. Vacuum tube condenser mics and dynamic microphones do not require this power and the +48 switch (1) should be turned off when using non-phantom powered mics. Although leaving the +48 on will not damage any properly wired balanced mic, some ribbon mics have reportedly been damaged when connected or disconnected from phantom powering. Also, some engineers feel that the performance of some dynamic mics may be subtly degraded with the phantom power on. The phantom powering circuit used in the VT-2 is suitable for use with all Neumann microphones, AKG 12 and 48 volt microphones, B&K phantom powered mics, all Schoeps mics, Shure SM81 and 85 mics, Crown PZM mics, and virtually all other phantom powered mics that require any voltage between 12 and 48. When turned off, the phantom-power resistors are completely disconnected from the circuit in the VT-2. Attenuation (3) The Attenuation control (3) is between the second and third amplification stages of the VT2. It should be adjusted to provide the desired output level to the console or recorder. In some situations, the Attenuation control can have an effect on VT-2 distortion, but at +4 dBm output the input stages will overload before the output. If you find that you need to run the Attenuation control near the bottom of its range, the input level may be too high and you might want to experiment with the microphone pad (if it has one), the VT-2 Input switch position, or both. [. . . ] Note that the LP-1 is not designed to be used as a "direct box. " For most musical instruments, the level and impedance are not correct for use with the LP-1. An exception would be if the instrument had a professional level, low-impedance, balanced output for recording. To use the LP-1, simply connect the +4 dBm, balanced signal source (such as the output of a mixer) to the LP-1 "Input. " Then connect the LP-1 "Output" directly to the input of the VT2 preamp. The VT-2 output would then feed the recorder. [. . . ]