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Tube Microphone preamp.
The least expensive preamp that we have available (and probably ever will offer). We honestly couldn't beleive the performance this little preamp gave us... While no Avalon 737, this little VTB-1 is a great little solution for someone looking to get into a step above their sound card or mixers preamps, or someone who simply needs a decent phantom power preamp solution for a condenser mic.
The new Studio Projects VTB-1 is the ultimate in low cost mic preamplifier technology. Incorporating true class A/B switching, the VTB-1 is a totally discrete circuit. Unique to the VTB-1 is the "Tube Drive". The Tube Drive is a variable drive to a 12AX7, that allows the user to blend as little, or as much of the Tube Drive they want. This VTB-1 allows the user the most flexibility of any mic preamplifier in it's price class.
The VTB-1 allows you to go from a pristine solid state sound, to any combination of Tube Drive you choose...even hard distortion for guitars and bass! The VTB-1 is must have device as the perfect mic preamplifier for any microphone!
Applications
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).