I have hardly seen anyone put these up as a suggestion when talking about ribbons.
The design is generally cited as the problem but the arguably best, most heralded ribbon mics ever has almost the exact design. So is it the execution of this particular mic that is it's downfall or are the other ribbons in this GB just that much better?
Indeed, the ACM3 is a copy of one of the best microphone designes, and its origin goes back to B&O BM3/4. Three main features of the design--1) use of the body itself as a part of magnetic return circuit, 2) use of magnets with no pole pieces, and 3) use of the grill for frequency response shaping, were quite revolutionary for the time. Consequent refinment, esp. use of modern Neodymium magnets pushed the design to a new level, with high output and nice, open sound with quite extended highs.
So indeed, the execution is a limiting factor, but definitely, the mic has a great potential.
The original transformer... well it is "designed" against every possible rule and physics law, resulting in high noise level and sucky sound. Actually, the Edcor mentioned here although has better lamination, but still is not much better than that.
The Lundahl is the only currently commercially available transformer I could recommend. Hold on a second... from inside sources I know that a new premium ribbon transformer is on the way to the market

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Here is a couple tracks recorded simultaneously. The first is a highly modified ACM3 and second is original
Stellar RM3 (the same, rebranded mic) . Pay attention to the noise difference:
Best, M