CV, mux'es and op amps.

 I am working hard to select the appropriate mux and op amp to use for my CV multiplexer.

As stated in previous posts, the OB6 uses a combination of TL062 and 74HCT4051, very common components. When using these (or actually TL072) on my breadboard I see tiny dips of around 30mV-50mV every time the cap is recharged.

I've tried various op amps for buffering the DAC, different buffer op amps and different mux'es. None perform significantly better than what I have now (on my Saleae scope that is). 

Here is a list of what I've tried:

Muxes:

DG4051

CD4051 (worse)

DAC buffer op amps:

TLV9352 

AD711     

OPA2137

TLC071

CV Buffer op amps:

LT1014

TL062

The only thing I have yet to try is the 8ch DAC used by the OB6. It supposedly does not require a buffer.

This is the config I'm going for:

DAC feeding into a 2x gain TL072 op amp using two 10k resistors that feeds a CD74HCT4051 through a 470R resistor. 

The CV buffer is a TL072 with a 1nF C0G cap.

Triple waveform panner using a single CV

I got my SSI2130 VCO, or rather, the DAB2130 (chip soldered onto a breakout board) from amazingsynth.com a few days ago. I'm very much looking forward to figuring out what through zero phase modulation is all about - but that's not today's topic.

Among other things, the 2130 comes with built-in waveshapers for tri/saw/pulse/sine, and multiple linear VCAs to let you do waveform mixing in a single chip.

Reading through the datasheet, I stumbled upon a nice little circuit that could be very useful for me - a single-CV input that will pan (linearly) through three waveforms. The SSI2130 VCAs are current driven, so the input CV is converted into three control currents. Also, the input is 0 to 1V.

I looked at the circuit, and it's basically two precision rectifiers plus a differential amplifier. The rectifiers create the CV for the first and last VCA while the differential amplifier uses the original CV plus 2 x the inverted version of the CV for the last VCA to generate the CV for the middle VCA - one that first rises and then falls. Pretty neat. Here is a simulation of the voltage-output version of the circuit:

 

Using a 5V CV to output 5V CVs is simply a case of replacing the -1V reference voltage with a -5V. 

In MY synth however, I intend to use the AS3364 (quad linear VCA based on the CEM3360 dual VCA). It has a CV range of 0 to 2V. But for the pulse wave, I use a little trick from the Juno to control amplitude without using a VCA. Unfortunately, this needs a 0 to 5V CV.

Luckily, tweaking the resistors in the circuit above lets us do all this without any additional components:

I need to do a little bit of testing, especially sincethe first op amp actually attenuates the original CV slightly (gain is 20/25 = 0.8). The last op amp (differential amplifier) also does something similar. Not sure if that is an issue, we'll just have to see.

Update: Tested with a TL074 (-5 ref generated with a 20k/10k resistor voltage divider between 0 and -15V and buffered using the spare op amp). Works like a charm.