Credit goes to Bill Seiler for the original version of this modification.
If you really want to lower the noise floor, perform the Vectrex Shielded Wire Buzz Reduction in tandem with this modification.
Purchase these parts as a kit
Pre-amp circuit schematic (dual OpAmp wiring)
- Parts List:
- R1: 100k resistor
- R2: 200k resistor
- R3: 33k resistor
- R4: 1k resistor
- IC1: TL082, LM358, or other Single Op-Amp or Dual Op-Amp
- C1: 0.1uF Ceramic Capacitor
- C2: 0.1uF Ceramic Capacitor
On the Vectrex CPU board, C323 and R327 form a junction right before the volume control. The idea of this modification is to desolder the common legs of those two components and insert the opamp circuit inbetween the junction & the volume control.
Connect the INPUT of the opamp circuit to the (now floating in the air) C323/R327 junction
Connect the OUTPUT of the opamp circuit to the negative PCB pad for C323 (or unused pad for R327)
Connect +5, -5, and GROUND. Don't sever, splice, or disturb J204 or it's wires to tap for power. There are pads for all 3 connections next to the connector.
On the back of the Power PCB, add the 1K resistor in parallel with R105.
Waveform showing reduction in amount of audio buzzing after performing the various modifications, recorded with a digital camera microphone at 4" from speaker.
Leftmost waveform is an unmodified Vectrex.
The middle waveform represents grounding & shielding mods
Rightmost waveform is after adding the pre-amp modification to the grounding & shielding mods.
Listen to sample audio of before & after pre-amp modification
CPU PCB Post-modification
CPU PCB Post-modification (photo)
Resistor added to power board
These vias near J204 can be used for +5, -5, and Ground. When passing a wire through a via like this, remember to solder on both the top & bottom side of the PCB. These two are tied to the joysticks, so if you lose joysticks after installing this circuit, you'll know why!
An example of the circuit on perfboard