Ive been thinking about a versatile but very simple tube mic/pre amp combo
A low noise cathode follower supplied from high step up ratio transformer chosen to suit the dynamic or ribbons mics impedence .
Lundahl does a 1:110 ratio ribbon mic step up , thats 40db of gain ,even though the circuit has no gain at all the cathode follower is very capable of driving a long cable , the Bruel and Kjaer cathode follower mic is the perfect example of this approach , the technique seems applicable to ribbon or dynamic mic without the need for capsule bias as in the case of LDC's .
Dual primaries in series or parrallel allows a change in step up ratio which amounts to -6db change in gain at the front end ,
Theres no gain control yet , what I would do is add a HPF , Baxandalls ribbon mic proximity EQ filter fits the bill perfectly , its passive and lossless .
The curve isnt steep but who cares , proximity effect is a very broad curve in the LF/MF region so a broad curve cut is perfect to counteract it .
Output stage is a bit like the BBC unit above ,
its basically a LTP phase invertor driving a transformer , but instead I'll use choke load /parrallel feed ,
I want to apply negative feedback to vary the gain either to second grid or via the cathodes
an ECC81(CV455) gives a gain of around 25db used in an R/C LTP , with choke/transformer loading
it could have even more ,
The small number of component required means you can build the CF into any mic body easily,
or use it direct as a very high z DI ,
or even make the unit into an XLR barrel jack for standard dynamic mics ,
Thanks to JB at Glassaudio for the use of the illustrations .
https://www.tubecad.com/2008/08/blog0145.htmhttps://tubecad.com/2014/06/blog0294.htm
A low noise cathode follower supplied from high step up ratio transformer chosen to suit the dynamic or ribbons mics impedence .
Lundahl does a 1:110 ratio ribbon mic step up , thats 40db of gain ,even though the circuit has no gain at all the cathode follower is very capable of driving a long cable , the Bruel and Kjaer cathode follower mic is the perfect example of this approach , the technique seems applicable to ribbon or dynamic mic without the need for capsule bias as in the case of LDC's .
Dual primaries in series or parrallel allows a change in step up ratio which amounts to -6db change in gain at the front end ,
Theres no gain control yet , what I would do is add a HPF , Baxandalls ribbon mic proximity EQ filter fits the bill perfectly , its passive and lossless .
The curve isnt steep but who cares , proximity effect is a very broad curve in the LF/MF region so a broad curve cut is perfect to counteract it .
Output stage is a bit like the BBC unit above ,
its basically a LTP phase invertor driving a transformer , but instead I'll use choke load /parrallel feed ,
I want to apply negative feedback to vary the gain either to second grid or via the cathodes
an ECC81(CV455) gives a gain of around 25db used in an R/C LTP , with choke/transformer loading
it could have even more ,
The small number of component required means you can build the CF into any mic body easily,
or use it direct as a very high z DI ,
or even make the unit into an XLR barrel jack for standard dynamic mics ,
Thanks to JB at Glassaudio for the use of the illustrations .
https://www.tubecad.com/2008/08/blog0145.htmhttps://tubecad.com/2014/06/blog0294.htm
