User manual

VETTA Theory of Operation………………Line 6 confidential ……………………………. Page 3 of 18
In addition the Z_SELECT signal controls the voltage feedback circuit and allows to select
one of two sets for DC output values:
On connector J6 to On connector J2 to
the Power Amp the Main PCB
+ LO - LO –HI +20V -20V PS_Valid +8V
If Z_SELECT is set at +15V +32 -32 -36 +18 -18 +20 +8
If Z_SELECT is set at 0V (or floating) +45 -45 -50 +18 -18 +30 +8
This feature is used to adjust the voltage rail of the power amps (+LO and – LO) in function of
the speaker impedance that they currently drive. For 8 Ohm load the Z-select line will be left
floating and the supply will provide ±45 Volts. For 4 Ohm load the Z-select line will be pulled
to 15V and the supply will provide only ±32 Volts. This will result in the same maximum
audio power on each load setup.
The PS_VALID validate the other supplies. A high level indicates that all the supplies are
within a valid range. This line will also go low before any of the supply start to drop
significantly.
Note that the ground references for the voltages on J6 (to power amp) and J2 (to main PCB)
are not connected together on the supply. The same is true of the AGND and DGND signal on
J2. They will be connector together only through the Main PCB and Power Amp PCB.
On the Main PCB:
The ±18V (labeled ±20V on the connector J49) are further stepped down and regulated ±15V
by the linear regulators U7 and U8. These ±15V supplies directly drive a number of Op amps
on the main board and on the power amp PCB. The ±15V are both further filtered by an RC
network (R165 and R166) to create the ±15V_IN supplies, which are used by the op amps of
the guitar input circuitry. This extra filtering removed any possibility of supply induced audio
feedback between the audio output and input stages. Finally, the ±15V are also delayed and
slowly ramped up over a two seconds period after the main power is turned on (detected by the
state of PS_VALID) by the circuitry around Q2 and Q1. The resulting ±15VDEL supplies are
used by the op amp U14 which drives the Direct output connectors. This setup limits the audio
thump, which would otherwise appear on the direct outputs when the main power is turned ON
or OFF.
The +8V is stepped down and regulated to +5VA (A= Analog) by U25, +5V by U6 and +3.3V
by U8.
The +5VA is used only for the Audio converters U19, U20, U18, and U16.
The +5V supplies the logic on the Main PCB and the U.I. PCBs.
The 3.3V supplies the logic on the main PCB including the microprocessor (Coldfire) U27 and
the two Sharc DSPs U35 and U36.
The +8V also directly supplies the FBX (Foot Controller Option) where it will be regulated
down to 5V.