ELA M251 C3 - anode to OT-coupling cap?

[vmanj]

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.

Yes, the size is small, but very high DCR.
Inductance T14/1 more than 160Н.
But regardless, I'm wondering how 12Hz can give a "hump" around 100Hz?

Maybe I incorrectly calculated the ratio of inductance 160Н and capacitance 1uF and it will not be 12Hz?

 

[MagnetoSound]

No, your calculation was correct. I believe it is your assumption of 160H to be wrong.

I think we’re talking at cross-purposes anyway. I didn‘t say that 1uF corresponds with a 100Hz bump. I said if you tinker the cap value you could create one.

But the C12 uses a much larger grid resistance anyway, and the LF response is already there. I think this would be a better way to get the full range out of the mic, if that’s your objective.

 

[vmanj]

I have T14/1 from MOBY and it has very close characteristics to the original transformer.
So this is not a guess and it has about 160H at such a small size.
It has completely different DCR windings and a much smaller physical size than AMI.

I'm talking about 251, not 12 mics.

I'm just wondering how a transient "hump" can occur much higher than 12Hz.
I do not have a goal to expand the range of the microphone, it's just a curiosity.

Maybe I understand something wrong.

 

[MagnetoSound]

Yes, I’ve just been searching and it appears it is the Haufe datasheet that is in error. My mistake.

Given 160H primary inductance you would need a VERY small cap to create a bump at 100Hz. Like 0.015uF.

But it could be done

You would still have the upward slope in low-mid response, so the mic might sound somewhat scooped in the mids. If you really want a flatter low end, then as I say, a bigger grid resistor as in the C12 will more easily get you there.

 

[vmanj]

Well, yes, I already thought so too, now it remains to try

 

[rock soderstrom]

I'm just wondering how a transient "hump" can occur much higher than 12Hz.
I do not have a goal to expand the range of the microphone, it's just a curiosity.

es, I’ve just been searching and it appears it is the Haufe datasheet that is in error.

Given 160H primary inductance you would need a VERY small cap to create a bump at 100Hz. Like 0.015uF.

But it could be done

Gentlemen, very exciting. Would it be too much to ask you to explain with one or two sentences the basic background of this topic (resonance between C and L)? A small example calculation would be fantastic. I am still learning.

 

[Khron]

Gentlemen, very exciting. Would it be too much to ask you to explain with one or two sentences the basic background of this topic (resonance between C and L)? A small example calculation would be fantastic. I am still learning.

Some information should be glean-able from here?

https://sound-au.com/articles/audio-xfmrs.htm

 

[vmanj]

Gentlemen, very exciting. Would it be too much to ask you to explain with one or two sentences the basic background of this topic (resonance between C and L)? A small example calculation would be fantastic. I am still learning.

For me, it's just a hobby, I'm not strong in such knowledge and I want to learn it myself

 

[rock soderstrom]

Some information should be glean-able from here?

https://sound-au.com/articles/audio-xfmrs.htm

Yep, thats a good link! Right on target. I guess I have something to read tonight! Thanks!

 

[rock soderstrom]

For me, it's just a hobby, I'm not strong in such knowledge and I want to learn it myself

You are lazy Joking aside, Khron's link has everything we need on this topic.
There are also nice example calculations!

 

[MagnetoSound]

That kind of arithmetic makes my head hurt. I just use an online calculator for stuff like this

https://www.omnicalculator.com/physics/resonant-frequency-lc

 

[rock soderstrom]

That kind of arithmetic makes my head hurt. I just use an online calculator for stuff like this.

Yes, very good for playing around. You can immediately see how the relationships are.

 

[vmanj]

Well, yes, I am also very lazy and use online calculators and the like.
And Khron's link is interesting.

 

[Khron]

Worth keeping in mind, though, that in that link, those transformers are driven from a very LOW impedance - "driving the transformer from a 50Ω source".

Very much not the case in these plate-output mic circuits. So the higher source impedance is another wrench thrown into the works. But, and i could be very wrong about this, that mainly affects low-end performance, due to the RL high-pass filter effect, in conjunction with the primary inductance..?

 

[rock soderstrom]

Worth keeping in mind, though, that in that link, those transformers are driven from a very LOW impedance - "driving the transformer from a 50Ω source".

Very much not the case in these plate-output mic circuits. So the higher source impedance is another wrench thrown into the works. But, and i could be very wrong about this, that mainly affects low-end performance, due to the RL high-pass filter effect, in conjunction with the primary inductance..?

Okay, point taken. I'm not sure yet what the implications are. The transformer is adapted to the high impedance of the source (tube) by its large step down ratio, so it should behave the same as in the ESP article. Right?

 

[Khron]

Large step-down ratio AND large inductance.

I seem to recall reading somewhere that you need large inductance figures to be able to pass any meaningful low-end through a transformer, when you're feeding it from a high impedance, and it kinda makes sense.

 

[MagnetoSound]

Large step-down ratio AND large inductance.

I seem to recall reading somewhere that you need large inductance figures to be able to pass any meaningful low-end through a transformer, when you're feeding it from a high impedance, and it kinda makes sense.

Exactly what I lost sight of a little earlier in the thread. In my mind, I've always equated high inductance with larger core size, but of course you can also just use high permeability core material and very fine wire allowing a greater number of windings on a much smaller core. Especially if the signal is not very big.

When you think about it, that's exactly what mic transformers are designed to do. But don't expect me to be terribly knowledgeable about transformers - we have experts here for that.