A common Eurorack standard for LFOs is 10 volts peak to peak, also written as +/-5v. This voltage, referred to as Bipolar, swings above ground into positive and then below ground to negative.
Sometimes, however, you’ll come across a module that expects 0-5v (Unipolar, with no negative voltage in its range) for its control voltage input – for example, 4ms’s Spectral Waveform Navigator, or the Bastl Grandpa expander, Spa. Or, you’ll want to use Eurorack LFOs outside of a Eurorack environment where unipolar voltages are expected.
Main board
That’s where this simple 2hp utility module, with its four bipolar to unipolar converters, comes in.
This being Eurorack, 0-5v CV inputs will most probably be diode clamped or whatever, and that being the case, you won’t do any harm plugging your standard Eurorack LFO into one – all that should happen is you lose the negative portion of the LFO wave, and it has no effect. However, if they’re not protected, the negative voltage has potential to cause damage.
My first version of this module was four copies of this simple circuit, based around the four op-amps in a single TL074 package.
D3 and D4 here are schottky diode clamps, limiting the input voltage to +/-5V.
R3 and R4 sum +5v and +/-5V together, and as they’re both the same value (10K), halve the result. So…
At the bottom of the range, a -5v input gives you (-5v+5v) / 2 = 0V.
If the input is 0v, you get (0v+5v) / 2 = 2.5v.
If the input is 5v, you get (5v+5v) / 2 = 5v.
This works, but only if you have a signal in the whole range 0-5v. If your signal is attenuated, say -2v to +2v, then your signal will have a DC offset and not pivot around zero volts like we want it to, because the bottom range of the swing would be (-2v+5v)/2= 1.5v.
ENIG finish panel
What we really need is a full wave rectifier, not a gain/offset combination.
Whenever the voltage swings below ground, it gets turned into a positive voltage.
On this scope, the blue trace is the original Bipolar voltage, and the yellow is the Unipolar rectified signal.
To achieve this we have four copies of this circuit:
Now the bipolar signal can be attenuated without issue. That’s about all that can be said about this one, it does the one thing it is designed to do.
Update #2 August 2023
All done, tested and working, the build files are here.
