Euro LFO CV Voltage Output Range

I’m considering adding an LFO mode to a Euro module I’m designing, as a learning project. According to the Doepfer Eurorack spec, modulation CV should be in the -5V to 5V range. Would an LFO that produced CV in the 0 to 5V range still be useful? Do most LFO modules out there produce/-5V signals?

a|x

16Vpp /±8V here. Its easy, you just need one TL072 Cell, just create a small mixer feeding your 0-5V signal and -2,5V via 100K into an Amplifier with Gain 2.

@fcd72 Wow, quick reply! Is ±8V common, then? The Euro spec on the Doepfer page states that LFOs should be in the range -2.5 to 2.5V (5 Vss).

Actually, I think it might be a bit more complicated.

I’m using a 12-bit DAC, with a max output voltage of 4.096V, so if I wanted a ±2.5V, I’d need to sum the DAC output with -2.048V, then apply a gain of… 1.220703125… does that sound right?

Would I be able to get the negative bias voltage from the -12V Euro power pin?

a|x

I’m now designing for 0 to 8V for unipolar modulations and/- 5V for bipolar (so you can use a LFO where an audio oscillator is expected and so on…). There are Doepfer modules which are not +/- 2.5V!

I don’t recommend getting the negative bias voltage from the -12V euro power bin. It’s not guaranteed it’ll be exactly -12V, and it could be noisy/whiny. LM4040 and co are your friends.

no, don’t rely on power pins. Better get a simple voltage reference (analog devices has a lot of those, but others will have as well). You can get them in all shapes and sizes.
Edit: oops, too late

Thanks for the tips, pichenettes and TheSlowGrowth.

a|x

Something like this ?

Would I run the -12V Euro power through the above, to get my bias voltage?

a|x

Check the Tides schematics, there’s a mild reconstruction 2-pole filter to remove some of the DAC stepping, then some bias / amplification with a constant negative offset from a Vref. Everything you need!

Great! Does tides use a 12-bit DAC, too?

a|x

No, 16-bit (DAC8552)

Could I make things easier for myself if I used a DAC with an external vref, like this , rather than the MCP4822 I was thinking of using? I guess I could make my DAC output in the range 0-5V that way, which might simplify downstream parts of the circuit.

a|x

In which way would it make things easier to use a DAC with external Vref?

You’ll need to provide this reference voltage anyway, so this would also require a reference chip!

If you had the idea to shift voltage to ±5V with the reference Voltage i suggest you buy some more DACs :slight_smile:

I thought you could use a 5V vref for the DAC, so it would output in the 0-5V range

Someone on the Arduino forum suggested something like this might work.

Then I have to generate the negative 5V power for the opamp, though.

Also, I was assuming a 0-5V DAC output when I wrote ‘2xr’ for the feedback resistor, but I need a gain of slightly more than 2 in order to get - 2.048V to- 5V. I understand the ratio between the input and feedback resistors determines the opamp gain, but do the actual values of the resistors matter?

a|x

@pichenettes I notice you use 2 opamp cells for the Tides outputs. A few people have told me I should be able to do what I want with a single opamp…

Sorry, I know this is reallllllllllly basic stuff.

a|x

> but do the actual values of the resistors matter?

Yes. Large resistors may create noise, low resistor will overload your DAC. 10k to 100k is a sane choice.

> Then I have to generate the negative 5V power for the opamp, though.

If your op-amp is powered by /- 5V, then you need a rail to rail output op-amp. If it is powered by/- 12V (and this is going to be the case in a modular system, right??), then any TL072 can do.

> A few people have told me I should be able to do what I want with a single opamp…

Have you told them you need a reconstruction filter? The first op-amp is there for this sole purpose.

@pichenettes ah, so I can power the opamp directly from the + and - 12V Euro power? That’s cool.

So the first opamp is a low-pass filter. I see. I didn’t mention needing one of those in my Arduino forum post.

I’m thinking of using 4 DAC outputs in as narrow as possible a panel width, and making it all through-hole, with ATMega328, optocoupler for MIDI input, so space might be a bit tight.

a|x

Would the bias voltage to the + opamp input as in the sketch do the job?

Could I use a single 2.048V voltage ref. for all 4 cells of a TL074 quad opamp?

Sorry for all the questions.

a|x