The original 421 is one of the best dynamic mics EVER MADE. IMO the design was light years ahead of anything else. The only problem with this mic is its no where near as sensitive as the mics we use now. It was designed for close miking. This dynamic mic like most is subject to proximity effect; the closer you are to the source the better the bass response will be. Now in a very high decibel level rock show with the right placement the low end might be very good, you would need a very loud sound pressure to get these mics to really sing.
The 421 used a phase plug in the center of the mic capsule it had a winding of wire this acted as a humbucking coil to eliminate or reduce EMI. It also helped form a wave guide that acted as a boost for ultra high end thus giving it a very crisp top end unlike most dynamic mics of its era. The humbucking coil they developed for use with dynamic mic coils is still in use today and was a huge factor in lowering the mics self noise.
They also used a Helmholtz resonator, this tuned cavity in the body of the 421 feed by a small tube from the back of the mic capsule helped lower the resonate frequency of the mic capsule it self thus improving its low end response. This mic also used an alnico 5 magnet structure. The 421 of today is nowhere near as good as they once were in fact they use neodymium magnets and have a weight inside the mic so it does not feel cheap. The size of the diaphragm on this mic was 1.5 inches! It’s a very big capsule. Frequency response was 28 Hz to 18 kHz and this mic handles very hi SPL of 148db!
There was also the Sennhieser 409 a very good mic. I would also check out the EV RE20/27 and Shure SM7 they are also very good dynamic mics. Ev was a real pioneer of dynamic mic designs. I often wonder why more people do not try dynamic mics for live recording of loud concerts. Back in the "day" there was little choice most condenser mics required huge power supply's and were not portable so they used mics like 421's.
I'm listening right now to a master FOB cassette of the Dead on 7/2/89 at Foxboro Stadium, which someone asked me to digitize along with some of his other masters. It was taped using Sennheiser 421s and sounds better than the Schoeps source on archive.org. There's a lot of richness and warmth, but also definition and separation in the recording. Is this due more to the mike placement (it sounds like it was right in the Phil zone) or to the mikes themselves? What are the pros and cons of Senn 421's? I'm seeing some being sold on eBay and am tempted to click and buy right now. Thanks.