The clipping diodes will reduce the gain of the op-amp gain stage to 1 (unity) above the diode threshold voltage, but since this is a large-signal parameter, this is not visible in your AC-simulation. A 6.3V clipping threshold should work fine in a pedal running on 18Vblackboarcult wrote: ↑28 Nov 2022, 15:13 I did some simulations of this circuit in LTspice and I was actually quite puzzled at the behaviour of the zener diodes in the feedback loop of the 2nd gain stage.
All plots taken just after the gain stage w/5.6V zeners:
- In blue, no diodes.
- In red, D815A diode model.
- In turquoise, BZX384B diode model,
- In green, GP5V6 diode model.
Unfortunately the sims with 1N5232 were giving me a hard time and disproportionate gain amounts.
All diodes present a varying degree of low pass filtering. Earlier in the thread Baja talked about the input signal levels needed for those zeners to conduct being absurdly high and questioning their utility in the circuit, and also mentioned trying them in the third gain stage, getting "muffled"/dark results.
Sorry if it might seem obvious to anybody, but could the zeners be there simply for tone shaping and reducing the highs? or are my sims just misbehaving?
Does anybody have a knowledgeable explanation for this, or for their presence?
What you see in the AC-simulation, are the variations in small-signal parameters between the different diode configurations. The diodes depletion region introduces capacitance to the PN-junction, and any proper semiconductor model will include some level of parallel resistance to account for leakage currents etc. Hence the differences in gain and bandwidth between the different configurations can be explained by these two factors.
