ELA M251 C3 - anode to OT-coupling cap?

[rock soderstrom]

Hi guys, I have two different info's regarding the output coupling capacitor in the Telefunken ELA M250/251. The schematic dated 2. January 1961 says 1uF, the schematic dated 14. February 1961 says 3.2uF. The related C12 with the same tube and OT (Haufe T14/1) has a only 0.5 or 1uF capacitor at this position.

What would you use? I am aware of the technical consequences of the different values, the lower corner frequency shifts accordingly in a range in which I do not hear any differences anyway.

The only thing that makes me think is the saturation of the OT.

 

[RuudNL]

How important is the value of the output coupling capacitor, in a microphone that attenuates almost everything below 100 Hz?

 

[LevinGuitar]

How important is the value of the output coupling capacitor, in a microphone that attenuates almost everything below 100 Hz?

What makes that attenuation? The T14 transformer?

 

[rock soderstrom]

Wow, is that the frequency response from an original ELA M251 or the American replica?

I actually heard an original M251 this week as I am researching this type of microphone. The low frequency response was excellent and not nearly as described in the frequency write-up above. The lower -3 dB point here is just below 100Hz! Unbelievable!

 

[rock soderstrom]

But maybe it's really like that and again a proof that we normally record relatively little meaningful below 100Hz.

To the original question, I'm still wondering why the designers at that time after a month tripled the original value of C3 to the first schematic? A drawing error?

I think 1uF is sufficient.

 

[RuudNL]

Check the internet for frequency responses!
I think the 30 M.ohm resistor at the grid plays an important role.
30 M.ohm and 60 pF (cardioid) gives a low -3 dB point at about 88 Hz.

 

[kingkorg]

How important is the value of the output coupling capacitor, in a microphone that attenuates almost everything below 100 Hz?

This is exactly the graph i wanted to post.

 

[rock soderstrom]

30 M.ohm and 60 pF (cardioid) gives a low -3 dB point at about 88 Hz.

Yes, some math helps. I think you are right. Ergo, the 3.2uF cap makes no sense. Thanks.

 

[kingkorg]

Hi guys, I have two different info's regarding the output coupling capacitor in the Telefunken ELA M250/251. The schematic dated 2. January 1961 says 1uF, the schematic dated 14. February 1961 says 3.2uF. The related C12 with the same tube and OT (Haufe T14/1) has a only 0.5 or 1uF capacitor at this position.

What would you use? I am aware of the technical consequences of the different values, the lower corner frequency shifts accordingly in a range in which I do not hear any differences anyway.

The only thing that makes me think is the saturation of the OT.

View attachment 111959View attachment 111960

Also when you go for lower cap values, depending on transformer characteristics, there could be a resonance at cut-off point causing a quite nice bump before the attenuation.

 

[rock soderstrom]

Also when you go for lower cap values, depending on transformer characteristics, there could be a resonance at cut-off point causing a quite nice bump before the attenuation.

Yes, this is something to consider, since the C interacts with the L. I will see when the transformer arrives how this behaves. Maybe some kind of built in Pultec trick.

 

[MagnetoSound]

How important is the value of the output coupling capacitor, in a microphone that attenuates almost everything below 100 Hz?

That response graph from RecordingHacks is ridiculous. A microphone with that response would barely be useful for vocal work, let alone anything else.

Here is the chart from the original M251 manual from Telefunken. (Which I found, ironically enough, on RecordingHacks.com!)

As you can see there is a moderate tilt in response between 100Hz and 500Hz; below 100Hz a gentle rolloff to -6dB @ 40Hz. From there we can extrapolate -12dB @ ~20Hz. Still a little steep among it’s peers, but broadly in line with what you might expect from a 30M grid resistor.

 

[RuudNL]

I think I would throw a microphone with a frequency response like in the graph straight into the trash!
But still I think the 30 M.ohm doesn't do the microphone much good for the lower frequencies.

 

[MagnetoSound]

As suggested above, if you tinker with a smaller output cap you might create a pleasing lift in the 100Hz range that brings it closer to a C12.

Have fun

 

[Gus]

this has been posted about at this forum.
Zebra 50 had a good post years ago about a Gefell microphone IIRC.
I tune the value by ear or measuring.

 

[rock soderstrom]

this has been posted about at this forum.
Zebra 50 had a good post years ago about a Gefell microphone IIRC.
I tune the value by ear or measuring.

Before I opened the thread, I searched the forum and also found this thread.
It used to be very interesting, but now all the graphics and photos are missing. What a pity.

https://groupdiy.com/threads/um57-um70-observations.37511/post-462749

 

[MagnetoSound]

This one too, no pics left …

https://groupdiy.com/threads/conversion-from-c12-to-ela-m251-build-photos.66993/

 

[rock soderstrom]

This one too, no pics left …

https://groupdiy.com/threads/conversion-from-c12-to-ela-m251-build-photos.66993/

That's a pity!

 

[vmanj]

As suggested above, if you tinker with a smaller output cap you might create a pleasing lift in the 100Hz range that brings it closer to a C12.

Maybe I'm wrong, but if you use a capacitor of 1 microfarad, then in relation to the inductance of 160H of the winding of the output transformer, 12 Hz is obtained.
If 3.2 microfarads, then this is 7 Hz.
How can there be a "hump" in the region of 100 Hz from such frequencies?

 

[Murdock]

It’s a perfectly good question, but where did you get 160H? According to Haufe, T14/1 has primary inductance of ~350mH.

I think he probably refers to the Bv08 from the U47.
Also that 350mH is probably the lower secondary and it’s at 1khz. So probably a lot more at 100Hz.

 

[MagnetoSound]

Apologies, I deleted my last post by accident. I have conflicting information on T14/1 from the Haufe data sheet and from AMI.
One of these must be wrong. I can’t imagine anything the small physical size of the T14/1 providing 160H, but I’m open to correction.