AOUT_LC calibration for V/Oct

[rutgerv]

Hi everyone,

as some of you already know I’m working on a 8xSID synth PCB and I’m hoping to include 8 Moog filters on it with CV control for Frequency and Resonance. One major issue in the whole process seems to be the maximum board size for a 19" module. To keep down the costs I want to design everything to fit on a single PCB that fits on the bottom of a 19" 1HE rack.

I was hoping that someone could assist me in understanding the AOUT_LC module. I’m using an 8-bit configuration to control the Resonance. The attached file shows a part of the schematic. On the top the 74HC595. The output stage of the AOUT_LC is connected to the input stage of the resonance control. This uses half a TL074. The transistor on the bottom right of the schematic controls a LM13700 OTA. This OTA controls the amount of resonance.

This is what I would like to know:

1. What is the voltage (max/11111111active and min/00000000) that comes out of the resistor network (i.e. pin 3 of the TL072 in the schematic).

2. How does R452 (tied to ground) affect the voltage going to the first 1/4 TL074?

3. Would it be possible to leave out the TL072 and use the 1/4 TL074 next in line to get the same kind of control over the range and the offset of the CV. What would happen with the resistor (R452) tied to ground?

Greetings,

Rutger

[rutgerv]

Ah I think I know understand what R451 and R452 are for. I guess this is a voltage divider that keeps the 10,67 specified voltage just below 10V at the input of the first 1/4 TL074.

That raises another question:

• Can I get rid of R451 and R452 if I calibrate the TL072 circuit to keep the voltage below 10V? I believe that should be possible… In that case I could also take out the first 1/4 TL074 and use it in place of the TL072, right?

[rutgerv]

I’ve just made a new schematic that leaves out the TL072 and moves all trimming stuff to the first 1/4 TL074. Would this work?