The switching bias regulator is taking a bit longer than I thought it would.
There is a fundamental problem in applying a standard switcher chip in this application in that they are all wired so that they will supply max duty cycle until throttled back by the error amplifier. Since I'm essentially going for a switching shunt regulator, what I need is a circuit where the duty cycle is originally throttled back, with the duty cycle opening up in response to the signal from the error amp. This means I have to roll my own controller.
What I have in the works is a hysteretic controller ginned up using a TL431 as error amp along with a high-speed comparator (5-pin SOT-23!). I've just finished the layout and will be building and bringing up the circuit in the next week or so. Even if the concept ends up souinding like crap, it's been an interesting journey. If this approach is unsatisfactory (it may regulate OK but still sound awful), I'll fall back to any one of a number of linear shunt regulators I've designed that are fast and clean, though they will require some fairly serious heat sinking.
There is a fundamental problem in applying a standard switcher chip in this application in that they are all wired so that they will supply max duty cycle until throttled back by the error amplifier. Since I'm essentially going for a switching shunt regulator, what I need is a circuit where the duty cycle is originally throttled back, with the duty cycle opening up in response to the signal from the error amp. This means I have to roll my own controller.
What I have in the works is a hysteretic controller ginned up using a TL431 as error amp along with a high-speed comparator (5-pin SOT-23!). I've just finished the layout and will be building and bringing up the circuit in the next week or so. Even if the concept ends up souinding like crap, it's been an interesting journey. If this approach is unsatisfactory (it may regulate OK but still sound awful), I'll fall back to any one of a number of linear shunt regulators I've designed that are fast and clean, though they will require some fairly serious heat sinking.
I finally got my switching cathode bias network working using 1/2 of a 6080, which was in my junk box, is kinda like 1/4 of a 6S41S, and since it's octal, it doesn't suck up an expensive odd socket for the breadboard. I'm going to tweak the design for more bias current, and if time permits, cart it to this year's BA so that people can scratch their heads and be consternated. It'll just be a tube sticking up out of a board, run with a couple of bench supplies - no noise... My other amps will make up for it.
The design will take some more work to be ready for a real amplifier circuit, but the basic principle has finally been demonstrated to my satisfaction. I backed off and used a lower switching frequency with parts that I can actually see (like an LM311 in a dip-8 package). The real deal will probably use some very fast LTC comparators I grabbed recently. They're in an SO-8 pacakge, but at least I can see/probe them, unlike the SOT23-5 comparator I was trying to use before I backed off on the design a bit.
The design will take some more work to be ready for a real amplifier circuit, but the basic principle has finally been demonstrated to my satisfaction. I backed off and used a lower switching frequency with parts that I can actually see (like an LM311 in a dip-8 package). The real deal will probably use some very fast LTC comparators I grabbed recently. They're in an SO-8 pacakge, but at least I can see/probe them, unlike the SOT23-5 comparator I was trying to use before I backed off on the design a bit.
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