Vox H. Release Notes: Rev 1 PCB

The Rev1 PCB is a blue ROHS board with “Rev 1” on the back. There is a single required fix with potentiometer R8, two links to be installed, and numerous component positions that should not be installed. There is one capacitor with a missing label: C12 (far right below, above R12.)

Correction: Jan 2023

R23a1 was incorrectly specified here as not to be installed, contradicting the correct instructions in the BoM. It should be installed.

Self-oscillation

The F1 section may self-oscillate, if there is no input. However, there should be no self-oscillation when an input signal is introduced. If you are experiencing this, then add the LED (blue) D1 and the 1k resistor R2, to limit the oscillation.

Required Fix

There is a single known problem with the PCB, that the F1 pot R8 does not align correctly with some panels.

FIX: Use a drill or dremel or circular rasp to extend the hole vertically enough for neat alignment.

ALTERNATE FIX: Do not solder R8 into the PCB. Instead, solder three wires on the underside of the PCB to the three holes for pot R8, making sure they are well clipped on the topside of the PCB. Run these wires round to the top side, soldering them to free R8. When attaching the panel, mount R8 rotated 90 degrees with legs facing towards the up direcion of the panel. (The solder lugs should give enough clearance for this.)

Unused positions and Links

The PCB was designed to allow experimentation:

• The LEDs D1and D2, and resistors R2 and R28 are provided to reduce self-oscillation. They are not needed when following the standard build of the BoM.
• The resistor R10 should not be installed.
• The resistors R15A1, R16A1, R23A1 should not be installed. They prove parallel resistances to R15, R16 and R23 to allow fine-tuning and unusual values. They are not needed when following the standard build of the BoM.
• Resistor R18 should not be installed. It can be used to reduce the value of the R1 potentiometer, for experimentation.
• However resistor positions R26 and R14A1 must be replaced by links. (By “link” is meant a 0V resistor or short. The offcut legs of resistors make good wires for this.)

Pimp my Vox H.

• The simplest way to customize Vox H is simply to alter the capacitor values for each section.
• Use an online second-order Sallen Key HPF calculator to get different resistor and capacitor values, factoring in the 10k (or whatever you use) pots.