◊ PCB Documentation

Micro-Mixer ("MixChiclet")

This utility circuit provides basic mixer (6 channel), isolation, level shifting, and/or level boosting capabilities in a tiny panel-mounted package.

Not limited to mixing, the circuit is based on a standard configuration of 2 inverting op-amps in series (so, non-inverting in sum), which provides many configuration options. Default values give unity gain, but can be modified to provide signal boost, or level shifting.

Circuit configuration based on Ken Stone's DC Mixer; this version is physically shrunk and adds the SMT options & onboard pot mounting scheme.

PCBs are two for $15.00 — ORDER HERE

Bill of Materials

(Because of the flexible nature of the design, values and quantities may vary)

  • Resistors (1/8w or SMD1206 recommended; 1/4w or SMD0805 will usually fit)
    • 100k x 8
    • 1k x 1
    • input attenuator pots: 100k x (up to) 6
    • master level: EITHER 100k pot (variable), OR 100k resistor(fixed)
  • Capacitors*
    • 0.1 uf (100 nf) x 2
  • Semiconductors
    • TL072 quad opamp x 1
  • Misc
    • Jacks, 0.1" headers, as desired

* there are pads for two 47pf bypass caps (SMD0805) on back of board if necessary to supress self-oscillation, optional in almost all instances.

Build Notes

The PCB is intended to be supported by a panel-mounted pot, chicklet style. Two mounting options are provided, using either the master level control pot on one end of the board, or the first input pots on the other end.

  • If the master level pot is board mounted, mount the pot on the BACK of the board (otherwise the operation will be reversed from normal, ie CW rotation will decrease levels, CCW will increase). If using a right-angled pot, you should also add a bit of tape to keep the pot housing from shorting against the back of the board. If using a master level pot (on- or off-board), DO NOT INSTALL THE RESISTOR MARKED WITH AN *, under the master pot postion (that's for fixed master gain).
  • If the input pot is board mounted, mount it on the FRONT of the board. Check for clearance if using a right-angle pot (PCB parallel to panel); the IC may need to be soldered directly to the board (not socketed) for clearance. The signal input for the board-mounted pot goes to "Input 1", and additional inputs are wired to the resistors starting with the second resistor position.
  • Board mounted pot can be 9mm PCB mount (Alpha RD901F-20-15R1 or similar), 16mm vertical PCB mount (Alpha RV16AF-20-15R1 or similar), or 16mm right angle PCB mount ( Alpha RV16AF-41-15R1 or similar).

All off-board input pots will need to be wired to the input resistors as voltage dividers: signal in to one side of the pot, ground to the other side, and the wiper to the input resistor(s) on the PCB.

You can omit the master pot entirely if a fixed overall gain is desired, simply omit the master pot and install a 100k resistor in the * position for unity gain. Higher values for * will result in greater-than-unity gain — for instance, 120k will amplify signals by 1.2, if you want to boost 1v/oct CVs to work with Buchla (you could even use a 200k trimmer in the variable pot slot, so you could precisely adjust to a preset boost).

[Mounting pot on channel end, fixed master gain, through-hole components]

Options are also provided for either through-hole or SMD parts (except for the IC, which is TH only). If you’ve been leery of hand-soldering surface-mount parts, this is an excellent intro project to try your hand at soldering them — it's easier than you think.

  • Though-hole resistors are on 0.3" spacing, a comfortable fit for 1/8 watt; 1/4 watt resistors will fit, if you bend the leads tightly at the body ends.
  • Resistor SMD pads are for 1206 size resistors, but 0805 will fit as well. 0.1uf power filtering capacitors pads are 0805 (or through-hole).
  • SMD 0805 pads are provided on the back of the board, for optional 47pf caps. Rarely, you may encounter oscillations, especially for configurations with high gain. If this occurs, these caps may help.

[Mounting pot (right angle) on master end, SMD components]

Both the signal inputs and power connections can be fitted as 0.1" headers. However, check the fit of both the headers and their connectors, to make sure there's enough clearance with all your parts and mounting scheme.