CM251

The Advanced Audio CM251 is a large-format condenser tube microphone based on the famous ELA M251. Like the original ELA M251 it is an outstanding choice for vocals. Historically, the original was used on countless hit records and recordings, including Joni Mitchell and Sade'.   Legendary producer Jack Douglas loved the CM251 so much he bought a 2nd one from us.  Advanced Audio’s CM251 emulates the smooth and present character of the original, while staying true to our own circuit designs by not simply designing a clone but improving the original circuit specifications.

The CM251 features Advanced Audio’s own AK12 capsule, an edge-fed, 34mm dual diaphragm capsule skinned with 6 micron German mylar that has a frequency response matching within ±1.5dB the response of the AKG CK12 capsule. The capsules are hand-picked and tested in our shop to insure both sides are within 2pfd of each other.

The CM251’s circuit has been designed around the 6072a dual triode, hand selected, tube. The unused half of the 6072a is incorporated as a direct coupled, cathode follower, resulting in a much lower output impedance than that of the original circuit. This lower output impedance improves the damping factor, low frequency distortion and transient response plus allows the use of a transformer with half the turns-ratio of the original ELA M251. The result is a circuit with more headroom than the original plate fed circuit with a better low frequency and signal to noise ratio. The open top end sound of the mic is smoothed out by rolling off the top end above 10khz about 3db like the original ELA M251 circuit.

The CM251 utilizes a very HiFi BV18 dual-bobbin output transformer, it is easily driven by the optimized low-impedance 6027a circuit, yielding 3db more output level than the original transformers used in the AKG C12 and ELAM 251. This allows the head amp gain to be further reduced, increasing the total headroom (output level before distortion) by 6db and increasing the signal to noise ratio which reduces the microphone's (self noise).

The BV18 transformer is driven through a 2.2-ufd mylar capacitor to take advantage of the ability of both capsule and transformer to respond to frequencies reaching down below 20Hz. The transformer can deliver > +14dBu with < 1% distortion.