Studio Projects VTB1 Manual - Manual (Page 3)

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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 addi-
tional 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
internallevels. 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.
FEATURE CONTROL DESCRIPTION
This outline will familiarize you with all of the features and what they do.
VTB1 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