AKG Ela M251 with AC701 Neuman M49 circuits

[gary o]

Ha quite funny abou the Soundfield Mk4.......

Im going to build a quick n dirty mic either C12 or M49 circuit ...... I only want cardoid I can see thanks to Dan how to wire the capsule for a M49, but cant seem to work out best way to connect capsule to C12.... I did some research was reading somewhere that certain ways of wireing a capsule can be better for SN ratio in cardiod mode .... & can also loose the capacitor into the grid , thats gotta be good right ? we dont our signal to go thru caps do we ?

I also read a while back how C12 & Ela M251 sound quite different, I notice the Ela doesnt need a grid cap , I also noticed the 30M grid resistor sounded well crap in my Ela circuit, 1 gig sounds lovely to me...I also read about the 100pf cap in a Ela reffered to as the EQ cap, so want to experiment with that

 

[bockaudio]

that 100pF cap is mostly for RF.

 

[gary o]

Thanks bockaudio Im just messing with two circuits now adding and taking the 100pf & it did seem to make no difference to the sound .....tho I did read of people trying different values to EQ the mic .... maybe Im messing with the wrong cap

thanks

 

[gary o]

Just build a M49 circuit with 5840 sounds full and nice has a lot less noise than my MK47 ran 5840 at 6.3 V will take it down to 5.7V also had a 22U cathode bypass cap in there...... I read 10U should be better for 5840 tube so will try these two things ...... using T14 from original AKG C28A for now ......

Been look at MGM church mic schematic..... might throw that together at weekend

 

[gary o]

Woo Hoo MGM Church mic circuit working 249V HT on moment will ramp it up & I think output polarity wrong way round so will swap & report back

 

[gary o]

Church mic experiment ......swapped output polarity raised B+ to 257VDC thats all I can get schematic calls for 275V...... I use my MK47 as my standard I prefer the church mic sound not  so far ...... Im going to try the M49ish same capsule & transformer now

 

[MagnetoSound]

Church mic experiment .... raised B+ to 257VDC thats all I can get schematic calls for 275V ...

10% off B+ is not enough to worry about.

 

[bockaudio]

Thanks bockaudio Im just messing with two circuits now adding and taking the 100pf & it did seem to make no difference to the sound .....tho I did read of people trying different values to EQ the mic .... maybe Im messing with the wrong cap

thanks

Shunt after plate up to 1000pF if you want to rolloff top...OR really do the calculations for an RC network and get exactly the HF attenuation you want.

 

[bockaudio]

Implying they may have started out with fixed bias, then changed it prior to release?

Perhaps - a few resistors would likely have cost less than the redesign needed to make it 'normal' cathode bias ...

It still is fixed grid bias even if the cathode is 1000Ω above ground.
I doubt the second sentence is true, "redesigning" for a cap & resistor is a no-brainer, and the circuit they used is more complicated. But I would venture that cost & space of electrolytic caps at that time, + form factor of that particular mic led to the final decision. FWIW the first 200 C12's had a much larger xfmr than all the following units, so they weren't adverse to changes.

 

[MagnetoSound]

It still is fixed grid bias even if the cathode is 1000Ω above ground.

Hi David, I'm not sure what you mean here ... as ricardo points out, the drop across R8 is proportional to the plate current (plus the much smaller capsule bias divider current), and so the tube is effectively self biased. They have simply moved the cathode resistor to another point in the circuit.

I doubt the second sentence is true, "redesigning" for a cap & resistor is a no-brainer, and the circuit they used is more complicated. But I would venture that cost & space of electrolytic caps at that time, + form factor of that particular mic led to the final decision. FWIW the first 200 C12's had a much larger xfmr than all the following units, so they weren't adverse to changes.

I think we are agreed here. The redesign I meant would have probably involved a larger housing.  8)

 

[MagnetoSound]

I'd forgotten we did cover this biasing question before, actually ...

http://www.groupdiy.com/index.php?topic=50204.0

 

[gary o]

Interesting read about the different biasing schemes a little over my head sadly tho trying to understand, I want to understand coz I want to get best sound for me  .......Bockaudio I didnt want to roll of the top at all Im looking for full sound but thanks, I read the Ela had a smoother top than C12, was trying to see what caused this ... I would like to knock a C12 circuit to maybe hear the difference between the other circuits Im currently trying right now...

I have a TB1 circuit looks very much like a Ela with 6072, 1 G grid res T14/1 tran
A Ela circuit but with 5840 in instead of AC701, 1 G grid res T14/1
M49 but with 5840 again 1G grid res t14/1
MGM Church mic 1 G grid res, T14/1...........B+260 now, thanks Dan, within 10%
Maxes MK47 kit mic

Using Tim Cambel capsule in each & although I so far have found as others point out the sound is mostly the capsule BUT the MGM Church seems to sound.... well better!, not sure why but I like it & its a lot quieter the MK47 in fact they all quieter than MK47, dont know if that because of the grid bias from the heater, or my power supply adding noise its not bad but others better I may even decomission it.

I would like to knock up a C12 circuit to try againts the others, not sure I have enough parts tho , been looking at how to connect just one side of my capsule but cant work out how I should do it...... can I lose the 1000pf cap to grid & lose some more parts can any one help me a little