User manual

VETTA Theory of Operation………………Line 6 confidential ……………………………. Page 6 of 18
Audio system:
On the Main PCB:
There are four independent audio circuits on the main PCB. The only audio connection
between them is through the DSP signal path.
Guitar input (Page 7 of 8 of Main PCB schematics):
The guitar input is first buffered and amplified on the Guitar Input PCB before it reaches the
Main PCB at connector J26 (15Vpp @ 5Vpp at the guitar jack). An analog switch made of U15
and U22 allows for selecting the input signal either from J26, or from an optional I/O PCB
connected to the main PCB at connector H2. This option board is currently not available, and
until then this switch will not be used and is permanently switched to the J26 input (= The
Guitar/Line_Select control signal should be at 3.3V). After the switch, the signal is spliced into
two branches. R96 and R23 divide by 3 the upper branch to +/-2.5 VPP (@ 5Vpp at guitar
input). C40 allows this signal to be DC biased at +2.5V by the ADC input. Q7 and Q8 clip the
signal to a maximum range of 0/5V. The signal then enters the right channel of the ADC U19.
In the lower branch, U22-B adds a gain of 5.3 to the signal. C39 allows this signal to be DC
biased at +2.5V by the ADC input. D14 and D15 limit the maximum signal swing to a -0.6V
/+5.6V range before entering the left channel of ADC U19. The net result is that the ADC left
channel is feed by a signal 16 times larger than the one on the right channel. Once these signals
are converted and moved into the DSP#1, the DSP code will monitor the amplitude of the
signals and use the one of the two versions most appropriate for the current input level. This
scheme allows for significantly improving the signal noise and low-level distortion
performance of the ADC.
Direct output (Page 5 of 8 of Main PCB schematics):
The direct output audio circuit supplies the audio signals to the Direct ¼”jacks and XLR
Outputs. The digital signal from DSP#2 feeds DAC U21. The left and right differential outputs
of the DAC are amplified and Low Pass filtered (FC = 31KHz) by U18_A and B. U18 outputs
directly feed the headphone amplifier made of U24 and U23. They also feed a differential
buffer made of U14 which drives the XLR balanced outputs, and, through a divider made of
R12/R6 R13/R7, the ¼” unbalanced outputs. Notice that U14 is supplied by ±15VDEL
supplies (see Main PCB power supply operation for the justification).
Power Amp output (Page 6 of 8 of Main PCB schematics):
The power amp output audio circuit supplies the audio signals to the power amp differential
inputs. The digital signal from DSP#2 feeds DAC U20. The left and right differential outputs
of the DAC are amplified and Low Pass filtered (FC = 31KHz) by U13_A and U4_A. Their
outputs directly feed the left and right positive inputs of the power amp through connector
H12. (U39 shown on schematic version C was removed and bypassed by an ECO prior to
FCS). U13_B and U4_B invert the signal and feed the left and right negative inputs of the
power amp.
Effect loop (Page 6 of 8 of Main PCB schematics):
The effect loop audio circuit provides a path to bring the analog FX Return signals into the
DSP#2 and to bring DSP#2 digital audio output to the analog FX Send.
FX Return: