That means we are on the right track but haven't quite got there yet.Changed 510R to 1k, replaced 470k with 249k. Now, the oscillation apperars on power-on and fades out after 2 or 3 seconds (most of the times ).
My apologies. I checked everything except that because I didn't think it would have an effect since 470k is so much higher than the 10k (near the 3.6k). I tried to fix it but then I realized those gyrators are not working like you would expect. The 470k appears across the inductor not across the resonant circuit. That has a large impact on the notch. The resonant circuit looks like the minimum impedance is 1020 ohms however the parallel resistor messes things up and it's only going down to 2.7k. When we change the 470k to 220k it misbehaves even more, the minimum impedance only gets down to 4.5k.Maybe I should try the same mod for the next gyrator - lower the 470k value to 220k and increase 4n7 to 10n.
I have this circuit in lstpice now. From what I can see modification of he gyrator values takes around 1dB from the notch.
The high Q of the resonator is what is causing the problem. I can't see an easy solution which will stop the oscillation and not affect the circuit.
One idea is to change the type of gyrator. The idea is to change from the gyrator on the left hand side to the one on the right.
Ignore the Rs part.
The mods would be, for the existing cab sim circuit
- short the top 510 ohm resistor
- keep the 10n cap
- Replace the existing gyrator 510 ohm, 33n, 470k with the gyrator on the right hand side in the link.
- Use part values:
R1 = 1.5k, R2 = 1.2k, C1 = 4.35µ
Unfortunately we have to use a larger cap with a crazy value. 2 x 2.2uF in parallel would do.
From what I can see there should be no change in the notch depth and that circuit should be a little less susceptible to oscillation. It's better but I still don't know if it is good enough. The only option I can see is to reduce the Q of the resonant circuit. That will increase the 10nF cap which then helps the stability.
Not so easy to solve!