Arduino Nano Midi to cv convertor interesting issue

Other Hardware General Electronics
1

Hello,

I created the circuit in this schematic on protoboard,

1179×657 74.2 KB

and the code,

I use a dual opamp and a single dac, because i only need note and velocity.
The opto i use is, H11L1

It works but has a strange issue that i can not figure out.

The author says the code supports midi notes from 21 to 108.

When I send midi notes to test the device,

(Arduino powered with +12, TL082 Opamp powered with +15 to GND )

The notes <= 36 gives +15V in the output of the opamp.
The note 37 gives +3.43V
The note 38 gives +1.7V
.
.
.then it increments as expected until note 108 which gives +7.52 V

Also to add I have +1.5V DC offset on the both outputs of the opamp.
(This, i can live with but i would love it not to happen. )

I would be very happy if you help me on this, thank you.

2

I changed the opamp to LM358 and all problems solved it works perfectly!

If you have enough motivation and time please explain me why, but i will research and learn anyway.

3

I believe the original used 9-12v input and changing to 15v you have to change the values of R3 and R8. Put a 10k potentiometer there and adjust it. Not sure what changed with the new op amps. I believe the resistors R3, R4 and R8 are adding bias to final output.