When I started reviewing the diode clippers, the goal was to end up modeling a triode simple preamp. Thanks to Ivan Cohen from musical entropy, I’ve finally managed to drive the proper equation system to model this specific type of preamp.
Let’s have a look at the circuit:
There are several things to notice:
- We need to have equations of the triode based on the voltage on its bounds
- There is a non-null steady state, meaning there is current in the circuit when there is no input
For the equations, I’ve used once again Ivan Cohen’s work available in his papers (the modified Koren’s equations), available in Audio Toolkit.
So, for the second point, we need to compute the steady state of the circuit. This can be achieved by putting the input to the ground voltage and remove the capacitors. Once this is done, we can have the final equations of the system in (for voltage of the plate, the grid, and finally the cathode):
With this system, we can run a Newton Raphson optimizer to find the proper stable state of the system. It may require lots of iterations, but this is not a problem: it’s done once at the beginning, and then we will use the next system for computing the new state when we input a signal.
As in the previous analog modeling posts, I’m using the SVF/DK-method to simplify the ODE (to remove the derivative dependency, turning the ODE in a non linear system). So there are two systems to solve. The first one is the ODE with traditional Newton Raphson optimizer (from , we want to compute ):
Which makes the Jacobian:
Once more, this system can be optimized with a classic NR optimizer, this time in a few iterations (3 to 5, depending on the oversampling, the input signal…)
The updates for ickeq and icoeq are:
Of course, we need a start value for the steady state. It is quite simple as the previous state is the same as the new one which makes the equations:
Let’s start with the behavior for a simple sinusoid signal:
You can spot that in the beginning of the sinusoid, the output signal is not stable, the average moves down with time. This is to be expected as the tube obviously compresses the negative side of the sinusoid, while it almost chops the positive side after 30 to 40V. The non symmetric behavior is what gives the warmth to the tube. The even harmonics are also clear with a sine sweep of the system:
Strangely enough, even if the signal seems quite distorted, really harmonics are not strong (compared to SD1 or TS9). This makes me believe that it is a reason for the love of the user for tube preamps.
There are a few new developments here compared to my previous posts (SD1 and TS9). The first is the fact that we have a more complex system, with several capacitors that need to be individually simulated. This leads to a vectorial implementation of the NR algorithm.
The second one is the requirement to compute a steady state that is not zero everywhere. Without this, the first iterations of the system would be more than chaotic, not realistic and hard on the CPU. Definitely not what you would want!
Based on these two developments, it is now possible to easily develop more complex circuit modeling. Even if the cost here is high (due to the complex triode equations), the conjunction of the NR algorithm with the DK method makes it doable to have real-time simulation.