CM251 Technical Question

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CM251 Technical Question

Postby Bradlygmiranda » Fri Mar 03, 2017 11:01 pm

Hello Dave,

I recently purchased the Advanced Audio CM251, and I am so impressed with it. Love it. This is more of curiosity, but I was wondering why the microphone picks up less room and ambience than for example the CM48? Most condenser microphones I try out in my room just picks everything up even in cardioid. I love that I can use it in my office, and it barely picks up any flutter echoes in my room. Is it based on the diaphragm and/or circuitry? I also learned that the Ela M 251 tube was actually closer to the diaphragm compared to the C12. Did you place the tube in the same location as the original Ela M 251? If not, why did you choose that? Looking forward to my next purchase. Great product. Great company.


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Joined: Sun Feb 05, 2017 10:44 pm

Re: CM251 Technical Question

Postby Dave Thomas » Sun Nov 19, 2017 4:10 pm

Hi Bradly, the CM48T has "passive" capsule polarization like the original U47 and U48. This means the rear diaphragm is not energized in Cardiod.

With the C12 9-pattern polarization method the rear diaphragm is energized and there will be a slight difference in voltage between the rear diapragm and back-plate voltage and this changes the response curve in the rear.

Also, the AK12 capsule is smoother through the 3-5khz (presence range) while the CM48 has the typical Neumann rise in this presence area where more of the room sound exists.

We are busy working on a Long body U47 type microphone (CM47LE). It will have the 9-pattern option but there will be a toggle switch on the power supply to passive polarize the AK47 capsule in Cardiod or from the 9 pattern switch.

The larger body will allow us to fit a larger BV8 transformer with identical specs to the BV8 used in the U47 and U48's made after 1958.

We are using a traditional single stage class "A" tube circuit based on the GE/JAN 5654W tube which has the same tube specifications as the VF14m but runs from the much more commonly available 6v filament supply.

I actually have a good working VF14m tube coming to me. We will be able to compare several parameters between the VF14M and the GE/JAN 5654W tube.

For, example I have the original VF14M specifications (in German) but we already established that the plate resistance (very important) and the transconductance are the same.

We only need to compensate the 5664W with a 5 pf capacitor to match the gain and high frequency cut-off frequency. This means the 5654w and VF14M will react the same with the BV8 transformer when used with the same value coupling capacitor.

We are hoping to have the CM47LE ready for release by early spring 2018. It will have our tried and true AK47 capsule which has the same brass surround as the original K47 from 1958 with 6 micron skinned diaphragms.

advanced-audio-ak47 SS.jpg
advanced-audio-ak47 SS.jpg (76.35 KiB) Viewed 1056 times

K47 1958.jpg
K47 1958.jpg (70.44 KiB) Viewed 1056 times

Cheers, Dave
Dave Thomas
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Joined: Mon Dec 30, 2013 2:46 pm

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