Hi Bleyrad, the transformer is 10.5:1 as I measured it for David at Cinemag to find an appropriate substitution which is the CM-2480.
The cathode follower circuit is brilliant it does lower the output impedance and increases the damping factor so the economical Chinese output transformer becomes quite useable. My RCA tube manual which I have owned since 1966 states, that the cathode follower does not increase gain but rather provides a lower output impedance with less distortion that typical plate coupled circuits this lower output impedance increase the damping factor and provide a better transient response. Because there is no plate resistor with the cathode follower it will provide a larger peak to peak output voltage than the plate coupled circuit. It is the output circuit used in very high end "HiFi" tube preamps like Macintosh and Marantz that were built in the 50's and 60's.
The stock transformer is still inferior to the Cinemag but quite useable on vocals and guitars with a 6072a tube substituted for the 12AX7 that is supplied with these microphone when they are stock. The output transformer is just hit too hard with a 12AX7 tube. I measure an increase of 6-8db of headroom with a 6072a. The stock Chinese transformer starts to distort a +12dbm while the Cinemag will handle +18dbm before distortion and with a 6072a the microphone will output +18dbm before distortion. However, the typical operating output of the microphone is usually around -20 to -30db with a 6072a.
The circuit was designed around the AKG C12 circuit which uses a 6072a and and the only difference is the second half of the tube in the 460 is uses as a cathode follower. However, ALCTRON the Chinese manufacturer of the M-16/460 and Nady TCM 1150 build these to a price point and substitute cheaper 12AX7b tubes even though the specification say selected 12AT7 (6072a). This is even wrong as a 6072a is a 12AY7 NOT a 12AT7. The 12AX7 has the most gain (100), the 12AT7(66) has less gain and the 12AY7(6072a) has even less gain(44).
I have not found any problems with the power supply other than the odd bad solder connection or the line voltage switch set to the wrong setting and one defective 6v filament regulator. I have had 3-power supply problems out of nearly 100 microphones that we have upgraded.
Removing the inner screen is not a good idea. the microphone will shut down earlier from moist air blasts on vocals, it will pop sooner and the 5 micron capsule will get contaminated with dirt particals a lot sooner.
With the cathode follower circuit the output acts more like the VF14 used in the U47 which has a lower output impedance. Also the 6072a will deliver more current than the 12AX7 and this also accounts for the increase in headroom.
If the tube is upgraded and you have a ALCTRON capsule that is reasonably smooth (they all have a lift at centered as 2.7khz but some are brighter than others) then the microphone is quite useable and I really like it on acoustic instruments and for miking the room.
With a Peluso capsule installed and Cinemag output transformer the microphone moves into a completely differrent realm. It will compete with microphones in the $3500 range and above. One client who I built an Advanced Audio CM-12 (460/Peluso CEK-12/Cinemag CM-2480) now uses the CM-12 50% of the time on vocals and his Neumann 149 the other 50% of the time.
Like any high quality large diaphragm condenser microphone the response smoothes out in OMNI and even with a stock capsule the microphone will work very well on most vocals in OMNI with a 6072a tube but you must have a "real" studio with good acoustics and good isolation.
In a typical project studio were I cannot use OMNI then I use the microphone one or two stops toward OMNI from Cardiod and it starts to smooth out the response. However, now I have several upgraded 460 microphones with Peluso CEK-89/CEK-12 and CEK-367 capsules and these give me a sound that is close enough to the original "Classics" for rock & roll". John Peluso recently told me that Muscle Shoals are now replacing worn out capsules in their original Telefunken U47 microphones with CEK-367 capsules and are very pleased with the results.
We will glady upgrade any 460/TCM-1150 or TCM-1050 for folks with Peluso capsules and Cinemag transformers.
Best regards, Dave Thomas (Advanced Audio Microphones)
bleyrad said:
The trafo is not 10.5:1. I believe it is 2:1 or something thereabouts.
The reason is that the cathode follower is used as impedance conversion. The tube lowers the impedance before the transformer.
This is a weird design. They did this to save costs on a transformer I guess, because you can get away with a cheap transformer without a huge hit on sound quality with low ratio, but to me it sounds weird.
If you want to stick with the original circuit, I would take the cathode follower out of the equation and use a higher-ratio trafo. This will probably change the sound a lot.
What problems did you notice in the PSU Gus? I don't remember any discussion of that on GroupDIY. It seems OK to me.
