Oh good, my phone lost my blog post.
Well, there's an LC notch filter in the Matamp's NFB loop, which seems clever except for inductors' sensitivity to noise, adding phase shifts in a marginally stable NFB loop (that anode bypass cap isn't an accident, nor is the speed up cap parallel to the feedback resistor) - plus we have an extra stage in the loop thanks to the cathodyne.
Also I should point out that when you inject NFB into the driver before a cathodyne, any presence/resonance controls serve to partially bypass the driver's cathode for a double dose of whatever range you're trying to boost. The blackface Princeton gets around this by fully bypassing the cathode and then applying NFB between the regular Rk//Ck and ground.
So the Matamp idea is an interesting one, but it's clear why it didn't last. I prefer building stable amps to GHz oscillators, plus shielded inductors are expensive, so I'll probably not bother with implementing this. Maybe an overall NFB control. Maybe.
You know what's easier to keep stable, and lets you implement a notch filter with adjustable frequency and Q? A local feedback loop.
It's not a horribly unique idea - check out the Electro-Harmonix Tube Zipper sometime. There's also a German patent on a similar circuit. As far as active mid boosts go, though, it could be super interesting. Either you could add a gentle Matamp/Orange mid boost, or sharpen the Q and get a "cocked wah" sound, or even defeat the loop for a gain boost... If you wanted to get really clever, you could even sweep the frequency at a constant Q, but at that point just buy a Tube Zipper. Puretube (who also designed the Flanger Hoax) is one of my heroes. Give him monies.
I'll fiddle with this idea first using a Kustom Defender 5H, turning it into basically a massive pedal. It might be a huge waste of time, who knows. Plus the James stack is perfectly capable of creating a "mid boost" by itself...
But seriously, puretube is a brilliant fellow.
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