The Mix-Pro (mixer-processor) from Low-Gain Electronics continues my 4U Serge project. The Mix-Pro is a straightforward modular mixer that features three DC-coupled inputs, one of which is invertible. There’s also a pot for adding a positive or negative offset, between -4.3V and +4.3V.
Construction
The main PC board holds all active components. Pots mount on the top side, and their nuts hold the panel. Banana jacks on the panel board stick through the back and solder to lands on the main board, like all of the 4U Serge modules. The panel is made out of a thick PCB material.
Top side of PC board has all active components plus pots
The components are a mix of through-hole and surface-mount parts. There are only five SMD parts, four 100nf bypass caps and a dual op amp. I used a TL082IDT for the chip. The PCB version I built is 1.2, which corrected the power reversal emphasized in the BOM for v1.1. Be sure to check which PCB version you have. On v1.2, the power header and electrolytic capacitors are installed as marked on the board.
Appearance
As you can see, I used yellow jacks for the inputs and a green jack for the output. Davies 1900 clone knobs: Black for the offset, Dark Red for the attenuverted input, Dark Blue for the two other input attenuators.
Thanks for the build overview Richard!! 🙂 You can adjust R4 (should be around 220K) to get +/-5V of offset. If you’re getting +/-4.3V w/ 220K, try lowering the resistor value a little! Looks fantastic! Love that red offset knob!
Thanks, Mr. Low-Gain! I used the 270K from the BOM for R4. I know I can lower it, if I want more offset.
BTW, I’ve got two more Low-Gain modules in the pipeline.
Oh good catch! I’ll update the BOM for the 220K. 270K is for +/-15v operation.
Actually, 240K puts it right on +/-5V. I’m going to tack a 2M2 on the back of the board across R4, just the right value for 240K equivalence.