How to build an adapter to match a Shure SM57 / SM58 to modern mic preamps

This past weekend I mixed FOH audio for “Live and Let Die – A Symphonic Tribute To the Music of Paul McCartney & The Beatles” with the Fort Worth Symphony Orchestra. I made the mistake of using SM57s on the trumpets and trombones. I say “mistake” because it was very difficult to get what I needed using those mics. The rest of the orchestra was mic-ed with condensor mics with the exception of the brass (including horns).

The Shure SM57 & SM58 were both presenting to the audio world back in the 1960s when tube mixers were still in wide use. Both mics use the same capsule and have the same transformer. The real difference between then being the metal windscreen screwed onto the SM58 to make it more robust for vocal purposes.

I came across this article which explains how preamp designs have changed over the years but in doing so have left the SM57 / SM58 mics at an impendance disadvantage. – Shure SM57 Impedance Modification

The gist is modern preamps have standardized on an input impendance between 1500 & 2000 ohms which isn’t a good impedance match for dynamic mics which operate more efficiently driving in input impedance of around 500ohm.

Fortunately, using some inexpensive parts, one can build an XLR adapter than resolves the impendance mismatch.