Gearslutz.com - View Single Post - links to a couple of interesting mic pre schematics.

manning1 · 9th September 2004

flatpicker. this is difficult to get hold of.
but a few years back i got hold of a raytheon op amp book,
and in there hidden away is an interesting circuit.
it shows a jensen transformer(if you got the bucks try a LLUNDAHL as an alternative) handling the diff in from the 3 pin mic. then single ended into the op37. the reason this is IMPORTANT is the transformer gives one free gain. the ADVANTAGE being then one can lower the value of the feedback resistor around the op37(or another op amp if you wish).
thus lowering THERMAL NOISE significantly.
very important in mic pre's. (note ; use low esr caps and low noise metal film resistors).
my suspicion is the venerable sm57 (from my tests) does not seem to like coming into an op amp direct but through a transformer. this is why some people dont find it performs as well
using cheapie consumer mixers with no transformer inputs.
anyway - i'm afraid i lost the contact but get hold of someone in raytheon if you can (i think i called them up). and get a hold of their op amp book. its a nice circuit.
also you can use low noise siliconix type fets as front ends for mic pre's. (advantage - high input impedence).
analog devices (analog.com) have a big fat book showing all sorts of circuits and will send it - if you ask them nicely.
hope this helps.

tiny - by ssr - do you mean single supply rails ?
ive yet to do a successfull single supply mic pre unless using
transistors. did you bias half supply voltage ?

9th September 2004	#14
manning1 Lives for gear Joined: May 2004 Location: canada Posts: 3,998 Thread Starter	flatpicker. this is difficult to get hold of. but a few years back i got hold of a raytheon op amp book, and in there hidden away is an interesting circuit. it shows a jensen transformer(if you got the bucks try a LLUNDAHL as an alternative) handling the diff in from the 3 pin mic. then single ended into the op37. the reason this is IMPORTANT is the transformer gives one free gain. the ADVANTAGE being then one can lower the value of the feedback resistor around the op37(or another op amp if you wish). thus lowering THERMAL NOISE significantly. very important in mic pre's. (note ; use low esr caps and low noise metal film resistors). my suspicion is the venerable sm57 (from my tests) does not seem to like coming into an op amp direct but through a transformer. this is why some people dont find it performs as well using cheapie consumer mixers with no transformer inputs. anyway - i'm afraid i lost the contact but get hold of someone in raytheon if you can (i think i called them up). and get a hold of their op amp book. its a nice circuit. also you can use low noise siliconix type fets as front ends for mic pre's. (advantage - high input impedence). analog devices (analog.com) have a big fat book showing all sorts of circuits and will send it - if you ask them nicely. hope this helps. tiny - by ssr - do you mean single supply rails ? ive yet to do a successfull single supply mic pre unless using transistors. did you bias half supply voltage ?