> actually patents the "electrical calculating device(s) and particularly to a device for obtaining the sum of a plurality of electrical voltages".
Or generalized: the Operational Amplifier.
> The actual patent for a negative feedback amplifier was...
Yes. But Black's problem was thousands of amplifiers in service, sometimes dozens in cascade. If each amp could hold 1dB tolerance (which is not easy), you still have +/-10dB or more differences between "identical" long lines. NFB stabilized the gain so amps and lines could be used interchangably without re-trimming.
But Swartzel was doing something else. You can always add voltages by putting them in series. But that is not easy when most source voltages want to have one side common. You can "sum" with simple resistors, but you have to re-compute the loss every time you change a factor or add or remove terms. The Summing Amp sums without much thought (and also stabilizes the gain of the amp used to recover the summing loss).
> studying a captured German wire(?) recorder after the war.
You could buy wire-recorders on Canal Street before WWII. I had an English friend who followed the Allied advance and set up both disk and wire recorders for a broadcast correspondent.
I suppose the Germans had them; don't know what's to study.
And literal "tape" recorders are older than vacuum tubes. Vladimir Poulsen?
Wire is not bad stuff. Main un fixable issue is: when it twists it sounds muffled. Bass penetrates the whole wire, treble only the surface. The wire has a curl, and if you don't get a twist it tends to stay same side to the head, but with some wandering.
Problem with all the pre-WWII mag recorders is the double-S shape of iron magnetism. Record around zero and the distortion is awful. You add a DC field and record halfway up one side of the "S", it is tolerable. DC-bias mag recording persisted into the 1970s on $13 cassette recorders.
Perhaps you know this, but THE big post-WWII excitement in mag-recording was barn-oxide and supersonic bias. These two tricks offered wide dynamic range and wide frequency response at low distortion. Better than phonodisc or optical film. Good enough to puzzle Allied radio listeners: they knew the speech was recorded but it did not have the shellac or silver-grain flaws of all known recording techniques. The Germans moved tape decks into France to broadcast official programs without actually putting talent or leaders in occupied territory; when the Germans left, they did not always take the decks back home. So Col. Ranger and others in the advancing Allied forces kidnapped Magnetophones and smuggled them back to the US.
The Germans had paper tape. Aside from bulk and fragility, it was not smooth (though calendared and used full-track, it was not bad).
They did have very good oxide: common barn paint is an acceptable iron oxide ground up very-very fine and available in ton lots cheap. (They may have been fooling with oxide because pure irons were diverted to military uses, but oxide is plenty good enough for the system.)
The final bit came when Ranger, who only had a few precious reels, found someone at 3M who could put barn oxide on acetate film and slit it precisely.
The product stayed like that for a decade. I recently pulled a red acetate master from the closet and played it. Aside from the damaged outside turns, it was a lush recording. We have not made much progress in 50 years.
