Building the CV Mixer

The CV Mixer crossfades between two CV, gate/trigger/clock, or audio signals. There is also an inverted output of the mixed signal. The circuit is built around four Opamp buffers, with one used to drive a bipolar LED to indicate signal polarity and level.


BOM:

Qty
Value
Device
Package
Description
Source 1
Source 2
4 3.5mm Audio SocketTHONKICONN3.5mm socket for Eurorack modular synths "Thonkiconn" style.http://shop.erthenvar.com/collections/accessories/products/3-5mm-inline-jackshttps://www.thonk.co.uk/shop/3-5mm-jacks/
1 Tile Power Tail"Futaba J" type power lead.http://pulplogic.com/product/tile-tail/EBay
2100kResistor0805
4100nCapacitor 0603! 0603 Package !
210kResistor0805
21kResistor0805
21uCapacitor 0805
14.7kResistor0805
1b10kPotentiometer9MM SNAP-IN9mm vertical snap-in pot, Alpha / Panasonic style with 6mm built-in knob.http://www.taydaelectronics.com/potentiometer-variable-resistors/rotary-potentiometer/linear/10k-ohm-linear-taper-potentiometer-round-knurled-plastic-shaft-pcb-9mm.htmlhttps://shop.erthenvar.com/collections/9mm-potentiometers/products/9mm-vertical-panel-trimmer
2M7DiodeDO214Polarity protection diode "1n4007"https://octopart.com/search?q=rectifier&start=0&category_ids=4226&specs2.2.values=SMA&specs2.330.numbers=(50__200)&specs2.313.numbers=1&sort=median_price_1000&sort-dir=asc
2TL072OpampSO08Almost any dual Opamp can be used.https://octopart.com/search?q=TL072&start=0&specs2.2.values=SOIC&specs2.2.values=SO
1LED3MMBicolour two-pin LED (red green)

Schematic:


Build Instructions:

 

Clean PCB with isopropyl alcohol to remove any manufacturing residues.

Solder the surface mount devices, then the power tail.

 


 

Add the LED and 3.5mm jacks.

The LED sits tightly between the two right-most jacks. The outer ground pin of the jack should be pushed against the body to make room for the LED before soldering.

 


Testing:

Check soldered connections for shorts, especially between the Opamp legs.

With a multimeter, check that there’s no shorts between the red, white, and black power leads.

Apply power, patch something (oscilloscope, multimeter) to the non-inverted output, patch a bipolar (+5 to -5) LFO to the left input, and turn the knob full left. The LED should alternate colours and intensity with the LFO. Turn the knob full right and the LED should decrease in intensity as the level drops.

Repeat on second input.

Add another LFO and rotate the knob to check that it equally crossfades between the two channels. Check that the inverted output is inverted.

 


Done!