The final stage of the LFO Eurorack module is the output. This can be tricky. I examine how you can best design an output stage that plays nicely with other modules. Besides that, I also share some of the helpful insights I got from the synth DIY community on the matter.
It took me quite some time to get a clear view on how to design the output stage for the LFO Eurorack module. First, it looked so simple. In almost every schematic I found a 100k resistor on the input and 1k resistor on the output. This is in line with the idea of high impedance in and low impedance out.
But why the 1k? I asked this question in the Synth DIY Facebook group and got a lot of interesting answers. Two of the most given reasons were
- Short protectionIf you short an output to ground or power rail, the amount of current flow could fry the op-amp.
- Current limitingEspecially needed for the bad practice of mixing multiple output signals with stackables, y-cables or passive mults. Just don’t. Bad things might happen.
Most of the reactions state that you don’t need the 1k resistor (anymore). Especially if you put an op-amp buffer before the output stage.
Most modern-day op-amps (like the TL074 I’m using in the circuit below) have an output short-circuit protection. Texas Instruments states in the datasheet that the output may be shorted to ground or to either supply for an unlimited amount of time.
Op-amp buffer as an output stage
So the circuit below shows the op-amp buffer for the triangle-ish signal coming from the relaxation oscillator. It also shows an op-amp outputting the inverse of that.
This should do the trick 😊
Putting a 1k resistor in the output stage will not harm the signal. It can, however, influence the 1V per octave tracking, since there’s a voltage drop. If you need the output to be more precise you can place the 1k resistor inside the feedback loop. This way the op-amp will adjust for that voltage drop.