Studio Projects VTB1 Tube Mic Pre-Amp

Features

• 48 Volt Phantom Power
• Phase Reverse
• Variable Input Gain Control On Detented Pot
• 1/4" Jack On Front Panel
• High Pass Filter
• Mic/Line Switch
• VariableTube Blend On Detented Pot
• Meter Read Select Between Input and Tube Distortion
• Switchable Impedance between 50/200 Ohm
• 5 position LED metering
• XLR mic input
• Balanced outputs on XLR and 1/4"
• External AC Power Supply For extreme Low Noise Operation
• Output Gain Control On Detented Pot

Applications

• A superb vocal mic pre with warm transparent character
• Great for use as a Tube DI Box
• Great for production, broadcast, and voice over work
• Perfect for the professional and home project studio
• A quality vocal mic preamplifier suitable for all vocal styles

Specifications

Maximum Gain:

Mic In to Bal Out: 72dB (60dB preamp gain, 12dB output gain) Line In to Bal Out: 42dB (30dB preamp gain, 12dB output gain)

Distortion/Noise (THD + Noise):

-20dBu input-Mic; 0dBu input-Line Both measured at +15dBu output Mic or Line In to Bal or Line Out: <0.0015% (Blend set full CCW- no tube) Mic EIN: -128dBu (150 ohm source, 60dB gain)

Frequency Response (20-20kHz, ref 1kHz):

-20dBu input, +15dBu output Mic In to Bal Out: +0 /-0.25 dB 0dBu input, +15dBu output Line In to Bal Out: +0 /-0.5 dB

Phase response (20-20kHz, either input): <+/- 15 degrees

Input Impedance:

Mic Input (Rear switch in 200 ohm position): 2000 ohms Mic Input (Rear switch in 50 ohm position): 300 ohms Line Input (Front panel TRS jack): 1.5 meg ohms

Output Impedance:

Male-XLR Balanced Output: 100 ohms (50 ohms each leg) TRS Line Out: 300 ohms (Impedance Balanced)

Insert Jack (TRS):

Nominal level approx 0dBu Located between SS preamp stage and tube/output stage Tip=Send: 50 ohms output impedance Ring=Return: 7500 ohms input impedance

Circuit Topology

Mic Preamp:

Discreet, current-source fed, paralleled-transistor balanced input stage feeding a bipolar opamp balanced-unbalanced converter.

Dual feedback design for low distortion performance. 2-Stage design for increased performance; transistor stage provides up to 45dB gain, 2nd stage contributes up to 15dB gain.

Line Input:

FET-input opamp buffer with hi input impedance to avoid pickup loading when used as an instrument preamp. 2nd stage provides adjustable gain from 10dB to 30dB, constant hi-impedance is maintained.

Hi-pass filter:

75Hz hi-pass filter for low-freq wind and rumble elimination (3-pole filter @-18dB/oct).

Tube Blend:

Constant-level blend control allows the user to smoothly change from an all solid-state path to a tube-based preamp. This allows the user to add the desired amount of tube sound to the signal without having to deal with complicated "tube drive" controls and such. Alternately, it allows the user to use only the solid-state portion of the preamp to get a very low distortion, un-colored sound when desired.

Tube Stage:

12AX7 vacuum tube with DC filament supply for minimum hum and noise. 1st half of tube is operated as a hi-gain amplifier stage, 2nd half operates as a cathode follower to minimize loading of the first stage. A low-voltage plate supply is used to create a "starved-tube" condition for enhanced sound.

Output Stage:

Bipolar, lo-noise, hi output-current opamp used for best performance. 1st stage is used to provide additional gain (6dB) and drives the Line out jack and Pin-2 of the balanced out. The 2nd stage provides signal inversion and drive for Pin-3 of the balanced output (effectively an additional 6dB gain).

For best Tube-Sound results:

The tube operates best when provided with a solid signal. The best way to accomplish this is to use the on-board metering to set the internall evels. Press the Meter Select switch IN, the meter will now display the signal level within the preamp, just before the tube stage. Adjust the GAIN control to get a meter reading within the 0 to +8 range. With the OUTPUT control at "0", start turning the BLEND control clockwise until the desired sound is heard. Depending on the source material, more or less BLEND will be needed. If the output level is too high or low for your following equipment, adjust the OUTPUT control to compensate. This won't change the sound of the blended signal, just the overall level.

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