> your circuit looks something like some of the PASS amps at passlabs DIY section.
We all stand on the backs of giants, or at least step in their old ideas. Nelson does it too, but he has vision and timing and the guts to explore relatively new territory. There is nothing in my plan that wasn't "state of the art" in 1972, except "the art" was heading in other directions then.
> You're building a discrete SSM2017 (the input stage)
There are not that many ways to build a good (low noise, good CMRR) transformerless mike input. You start with two BJTs, Bases to the input, Emitters to the feedback, and Collectors running off through some gain.
The output stage on the usual implementations is important. My plan has a glitch because I have omitted it. It is probably a workable glitch, but the kind of thing that would confound most buyers. That's another reason this is probably stuck in DIY-land. OTOH, omitting the output stage AND the added stage needed for balanced output does shorten the signal path.
> Hope I am not putting the cart before the horse here!
If that horse ever limps in, you will see that:
> a "gain block" which could made up on small boards and then maybe potted
Maybe in a lobster-pot. Or "pot it an old muppets lunch box". I'm headed to a VERY extravagant design, LOTS of heat.
And quite specialized FOR the low-Z mike input and Pro-output situation. Not general-purpose utility amp. A one-trick pony.
> would the usual input add ons like pads, rf filters, phantom supply etc have any effect on the noise level
Yes.
But one neat trick is that pads will rarely be needed, since it goes to unity-gain and can do low-gain with high S/N.
And phantom should not have a significant effect on noise (though I have seen that said). 13,600 ohms across 200 ohms is "nothing".
RF is a problem and I admit my (to be described) values are unproven and may not be suitable in high-RF areas. And if you just try to flesh-out the above sketch, it is likely to be an RF transmitter (in the 100MHz range) rather than an audio amp.
> all TO3 packages...
I had TO220 outputs in mind. But you can use TO3.
> the biggest op amp ever.
Top of my head, there are a dozen production op-amps bigger (more power) than this.
And this isn't the classic general-purpose op-amp. It is a differential amp, with the feedback (operation) tightly woven into the design.
> Will this evolve into a fet design?
No. To my mind, FETs are not optimal for low-Z mike inputs. Some wise designers disagree, which means they know more about FETs than I do. There are many ways to do it. This is mine.
> Can't wait for the thermal analysis!
Actually, laying-out the resistance paths is harder. There are paths where 0.25 ohms stray resistance will throw the gain way off, and similar small inductance will make it puke radio waves. And these nodes are on the ends of big hot resistors, not far from transistors that should not be hot.
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