The first challenge was to find a capsule that would be in the same sonic area as the historic CK12 that was custom made for the 251. One big factor to the sound of an historic 251 is the split capsule design with its resonance chambers.  This type of capsule has historically been very difficult to replicate, but with the help of a well reputed capsule manufacturer in Asia we custom created the CK25 especially for the UM25, based on the original historic Telefunken blueprints.  With a new resonance concept that is now easier to manufacture with more consistent results, the capsule still produces the very distinct sound of the resonator plates found in the historic CK12 capsules.  The porting has been enlarged and the thickness of the backplate slightly enhanced to offset the slightly different resonator design.  The result is a split dual backplate design that is edge fed with a resonator that can be made for a more reasonable cost than the original CK12 design without straying too far from the classic 251 tone. In the Blackspade UM25, the polarity patterns are produced the same way as in the historic 251; a triple pole triple throw switch, that is remote controllable from the PSU.  This gives the UM25 a brilliant sound in the omni setting, a strong vocal set up in cardioid as well as a figure of eight pattern with a minimum of noise and maximum of sensitivity. 

To pair with the new CK25 capsule, we selected a Telefunken NOS tube capable of driving the tone into the area of the classic 251. The tube is nearly identical in tonal response to the historic 6072a that was used in export models of the 251, and a 6072a can be swapped in without any modification. The amplifier circuit is an exact replica of the one used in the historic 251, and is handmade in the same spirit on plexi-glass boards with point to point wiring here in the US. Our AMI T14 transformer ties the whole ensemble together into a microphone that delivers an accurate sound with a smooth high end.  While the UM25 shares the same body tube, bottom bell and power supply housing as the UM17, everything else about the mic is different. The headbasket is constructed like those of the historic 251 microphones, not only providing a different filtering response than the UM17 but also making it stand apart from its sibling.