tk@halmi
Well-known member
Anyone knows the 90415 transformer's ratio? I am guessing at least 5:1 up to 7:1 because just a single little FET (presumably 2N3819) is driving it.
Thanks,
Tamas
Thanks,
Tamas
U87Ai Mic Transformer Ratio?
- Thread starter tk@halmi
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pucho812
Well-known member
here is a link to a U-87 schematic.
http://www.recordist.com/ampex/schematics/neumann/u87sch.jpg
http://www.recordist.com/ampex/schematics/neumann/u87sch.jpg
Bo Hansen
Well-known member
Hi tk@halmi,
The U87Ai transformer (90415) have aprox. 20 dB loss so the ratio can be around 10:1 (20 k to 200 ohm)
--Bo
The U87Ai transformer (90415) have aprox. 20 dB loss so the ratio can be around 10:1 (20 k to 200 ohm)
--Bo
tk@halmi
Well-known member
Thanks Bo & Kubi.
It is interesting how much of a transformer step down is designed into these microphones. You would think that adding a simple source or emitter follower stage after the FET would enable one to use a 5:1 transformer and get 6dB more gain. It is possible that as far as total noise goes it really does not matter and one transistor is always simpler than two transistors.
Tamas
It is interesting how much of a transformer step down is designed into these microphones. You would think that adding a simple source or emitter follower stage after the FET would enable one to use a 5:1 transformer and get 6dB more gain. It is possible that as far as total noise goes it really does not matter and one transistor is always simpler than two transistors.
Tamas
tamas,
i read in a neumann publication from the 60's that after having done research on xformer and xformerless o/p topologies, they decided to go with xformers for their main series because if allowed for the least current draw while taking up about an equal amount of space. but maybe noise was an issue, too.
they did use the xformerless topology in their km7x series, but aside from being a/b-powered, these obviously were made mainly for mobile recording. o/p level was higher than the xformer types, maybe even close to line level, and probably able to drive e.g. a tape deck (=> nagra) without any extra pre.
clearly, there obviously are/were a lot of design philosophies regarding o/p trannies. some mic's use an emitter follower after the fet's drain, others use a source follower before the emitter follower (e.g. the old akg c414's) for even lower Z, again others (e.g. new 414's and u89) use more amp stages that bring the o/p Z down so low that it seems likely they use a 1:1 ratio.
i read in a neumann publication from the 60's that after having done research on xformer and xformerless o/p topologies, they decided to go with xformers for their main series because if allowed for the least current draw while taking up about an equal amount of space. but maybe noise was an issue, too.
they did use the xformerless topology in their km7x series, but aside from being a/b-powered, these obviously were made mainly for mobile recording. o/p level was higher than the xformer types, maybe even close to line level, and probably able to drive e.g. a tape deck (=> nagra) without any extra pre.
clearly, there obviously are/were a lot of design philosophies regarding o/p trannies. some mic's use an emitter follower after the fet's drain, others use a source follower before the emitter follower (e.g. the old akg c414's) for even lower Z, again others (e.g. new 414's and u89) use more amp stages that bring the o/p Z down so low that it seems likely they use a 1:1 ratio.
