Dual 3 channel mixer solderpads and hacks?



The dual 3 channel mixer has some solderpads that you can use to change some functions on it.

TIP-signals

On the top left side you can solder a 2×8 angled pinstrip.
The outline for this connector is marked with the text TIP.
This gives you access to all the signals that are present at the tip of the 8 jacks.
The signal-pins are the pins nearest the printed numbers.
The GND pins are nearest the edge, at the back, of the PCB.

NORMALIZE-signals

In the middle, near the electrolytic capacitors, you can solder a 2×8 angled pinstrip.
The outline for this connector is marked with the text NORM.
This gives you access to all the normalize-signals that are present at the jacks when no plug is inserted at the jack.
The signal-pins are the pins nearest the printed numbers and nearest the capacitors.
The GND pins are nearest the edge, at the back, of the PCB.

Disable the 1K output resistor

On the top side of the PCB there are 2 solder-switches with the names SJ1 and SJ2.
If you solder any of these solder-switches you will eliminate the 1K output resistor.
SJ1 eliminates the resistor for the upper output jack, and SJ2 for the lower output jack.
The OP used for this mixer is a TL074 that has built in short-circuit protection on the outputs.
Therefore, it is possible to do this hack if you need, but it is still better to leave the 1K resistors working as default for extra protection.

Convert it to a 6 channel mixer

This is a BONUS-hack!
It is possible to convert this mixer to a 6 channel mixer with both inverted and non-inverted outputs.
If you look at the underside of the PCB you will see 5 solder-selectors each with 3 solder-pads.
All 5 should all be soldered the same direction and only 2 of the 3 solder-pads should be connected at the same time.
When they are soldered in the direction to the backside, the mixer will be a dual 3 channel mixer (the default).
If you change the direction of these solder-switches in the direction to the jacks instead, the mixer will become a 6 channel mixer with inverted and non-inverted outputs.
Use a multimeter with continuity test to check that you only have connected 2 and not made any short-circuits to all 3 solder-pads for each solder-switch.
The PCB is marked with the numbers 3 and 6 at the side of the solder-switches for guidance.
The frontpanel has printed text on both sides, and the backside has the labels for the 6 channel mixer.
Loosen and remove the 8 nuts and swap side for the new layout. Tighten the 8 nuts again.
There may be a printed serial number on one of the sides of the frontpanel. You can use a black pen to hide it or gently remove it with a razor-blade if you wish.