The m251 Upgrade
The Original ELA M251 made by AKG for Telefunken Germany.
See the new Telefunken Electroakustik version here
The FAR 251 is based on the famous ELA M251 Microphone, sold by the original Telefunken company as shown above.
The M251 was built by AKG for Telefunken as a replacement for the u47 after Neumann stopped selling it to Telefunken.
The original 251 was the epitemy of German microphone engineering and was very expensive due to the internal mechanical design. The AKG signature sound still had that mid-range dip in response around two kiloherz and a gradual 5dB response lift in the high end. But to make it sound a little more like the u47, that it was to replace, the AKG engineers needed to remove some of that top end... so they added a special little part to the circuit.
Not enough "O"s in the word smooooth to describe this mic.
This is a VERY smooth sounding microphone with a wide range of applications. It is exceptional for live string instruments or orchestras, acoustic guitars, vocalists, choral groups and choirs. You can find out more about the original ELA M251 at this excellent web site.
FAR m251 V 1 Sound Examples
These examples were recorded simultaneously using four microphones. You can hear all the files on one page here.
The tracks below were recorded direct into our Presonus Firestudio project. The DAW was Logic Pro 8 and the sample rate was 44.1Khz. No compression or EQ was used.
A common complaint with some low price condenser mics is call the "eshy" sound. This link demonstrates the "S" in the FAR m251 upgrade.
A couple of announcer examples:
Male announcer at 24 inch distance
Male announcer at 5 inch distance
Female singer w/compression, NO EQ
This guitar was recorded about 20 inches in front of the guitar with the mic slightly above the top of the guitar body.
We give your studio an m251 style microphone starting with one like this
New Capsule creates the m251 sound
Starting with your APEX 460 microphone we replace your stock capsule with an edge terminated capsule similar to the one used in the ELA M251. We use the Peluso CEK12, Gold sputtered 34x14 mm, 6 micron, dual diaphragm capsule shown below.
Peluso CEK12 capsule
m251 Version II (Sept 3, 2010)
After a considerable amount of research and listening tests, we have made some changes in how we use the CEK12 capsule. Although the CEK-12 capsule sounds good un-corrected, we find that the microphone becomes far more like the original m251 when very delicate frequency response adjustments are used to correct the huge lift that is built into the capsule. Our approach is to continue to use our low distortion CCDA amplifier design, with the addition of a high frequency correction component.
The result is nothing short of exceptional! The mid-range "hollow out" complaint of this capsule is actually created by the high end lift changing the balance with the mid-range artificially. By correcting the high end the mic balances up perfectly.
Below is the CARDIOD response of the NEW FAR m251 Version II from 70 Hz to 20,000 Hz. You can see that it is very smooth and compares much more closely to the response of the original ELA m251. With the m251 V II we used a tiny EQ cap like the original m251 to shape the top end response. This gives flexibility to adjust to each capsule individually.
FAR m251V 2 response measured at 0.5 M from source
AKG m251 response graph from original manual
(0 dB line hi-lighted in red)
AKG original CK12 capsule
Before someone writes and complains, we know that the old AKG capsule has not really been perfectly re-created in the CEK12 but it is a good second choice and with a little electronic help from us it sounds PDG!
Listen to our Podcast recorded with the FAR m251 V II
Listen to the simul-recording using the FAR c12 VII
New Capacitors create Transparency
We replace the critical capacitors in the audio path to a polystyrene input cap and a polypropylene output cap. These materials are recognized as the superior dielectrics to use in audio amplfiers. This gives you a transparent, low distortion sound path.
We also replace the electrolytic power de-coupling caps with Nichicon.
And we add one special capacitor that gives the M251 that really smooooooth high end; just like the AKG design engineers did many years ago.
New Transformer Manadatory for this one
We replace the output transformer with Peluso BV11P which gives you 6dB more output, flatter frequency response and less high frequency distortion.
And one more thing...
In our version of the m251 we use the two stage amplifier used in the APEX 460. For those who don't know, this is an innovation of the the ALCTRON microphone factory that has merit. The technical name of the circuit used is "constant current draw amplifier". (CCDA)
The first stage tube is the normal "plate follower" circuit used in the m251 microphone. The second stage is a direct coupled "Cathode Follower" stage. There is no capacitor between the stages so the signal transfer is virtually perfect.
The low output impedance of the cathode follower second stage drives the low primary impedance of the Peluso transformer MUCH better than a single stage 6072 circuit. This allows our mic. circuit to preserve the sound quality of the first stage amplifier and lets the 2nd stage do the dirty work of the driving the transformer. We get improved high frequency and low frequency response, and lower distortion.
But we aim to please
Please note that if you disagree with this engineering philiosophy, we will happily build your microphone with a single stage 6072 triode preamp like the original m251.
For the original one stage amplifier design please request:
"The m251" Original
New Tube... 
We top it off with a selected 6072 vacuum tube to replace the noisy 12AX7 that came with your microphone when you bought it. We have adjusted the circuit to bias the tube properly to optimize the gain, head-room and signal-to-noise ratio to 6072 specs.
We usually have a small supply of Old Stock 6072 tubes which can sell for a competitive price.
We finish off by adusting your power supply to 124 volts; a voltage that is closer to the voltage used in the ELA M251.
New Bias voltage Needed
Changing to a 6072 tube means the we need to adjust the bias voltage on the tube. This is done by replacing the cathode resistor on the first stage tube. This optimizes the circuit to allow the 6072 to create the best sound it can.
We don't just say it's better...
...we measure it and send you the results!
After we are done we re-measure the electronics in your microphone and print the results before and after our work you can see how the changes improved the specifications.
Your ears will tell you the real difference the moment you lay down your first track.
APEX 460 |
Fox Audio "the m251" |
|
|---|---|---|
Type |
Condenser Pressure Gradient Microphone with 1-Inch Dual Diaphragm and Tube Preamp |
Condenser Pressure Gradient Microphone with 1-Inch Dual Diaphragm and Tube Preamp |
Polar Pattern |
Omni, Cardioid, Figure-8 with 6 Intermediate Stages |
Omni, Cardioid, Figure-8 with 6 Intermediate Stages |
Frequency Response |
30Hz - 20kHz +/- ?? |
30Hz - 20kHz +/- 3dB |
Sensitivity: |
14mv/Pa (-35dBu) |
19mv/Pa (-33dBu) |
Output Impedance |
<200 Ohms |
<200 Ohms |
Signal to noise |
76dB * |
79dB * |
Max SPL: (For 1% THD @ 1000Hz) |
125dB |
129dB |
Equivalent Noise (A-weighted) |
20 dB |
17dB |
Power Requirement |
B+ 145VDC, 6.5VDC filament |
B+ 124VDC, 6.5VDC filament |
Controls: |
Pattern Selector on supply |
Pattern Selector on supply |
Capsule |
Alctron original 32mm |
PELUSO CEK 12, 34mm |
Capsule Voltage |
72 volts |
62 volts |
Tube |
12AX7 |
selected 6072 |
Transformer |
Alctron original |
Peluso BV11P |
Input Cap. |
ceramic |
Polystyrene |
Output Cap |
Electrolytic |
Polypropylene |
Bias method |
Cathode resistor/Cap |
m251 Cathode Bias circuit |
To get a quote for The M251 Upgrade
send your request to:
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