U87 / U87AI frankenstein sensitivity horrors

Theoretical exercise here.

Glancing at the specs walked through at this link, concerning comparative sensitivity of the 87A versus the 87AI:

http://repforums.prosoundweb.com/index.php/topic,36194.msg531542.html#msg531542

It looks like U87AI single backplate is 8.4dB more sensitive than the U87 dual backplate.

47V polarization versus 60V is a -2.1dB sensitivity difference.  Therefore a U87AI run at 47V versus 60V would be 2.1dB less sensitive.

Following from that, 23.5V polarization would be a 6dB sensitivity reduction, which with a single backplate  is still just barely higher than the original U87, so it comes out in the wash.  What else changes with response and distortion?  What does such low polarization sound like?  How does one best create a voltage divider to attain both 47V and 23.5V? 

For those not adding all this up yet, it's a possible (bad) path to getting all three patterns out of a single backplate capsule in a U87 build.  Better would be to come up with either a 94V  or -47V bias for figure 8.

47V and 220pF for deemphasis network is pretty good point to start, it should give you much more similar level to U87i.
Another thing is change of input topology.
For LDC less than 40V isn't best option in case of noise and response from the capsule.
Divider isn'tbest option rather i would go for switching zener diode value.
Don't understand idea with fig.8 voltages.

ln76d said:

47V and 220pF for deemphasis network is pretty good point to start, it should give you much more similar level to U87i.
Another thing is change of input topology.
For LDC less than 40V isn't best option in case of noise and response from the capsule.

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the biggest question.  What exactly happens at lower voltage?

Divider isn'tbest option rather i would go for switching zener diode value.

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Divider is what's done in the U67.  Pair of 2M2 from which comes the 120V and the 59V. 

Don't understand idea with fig.8 voltages.

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Idea is you can't use single backplate in U87 circuit and achieve figure 8, unless you modify the polarization voltage scheme.  47V and 94V would mimic U67.  As would 23.5V and 47V (absolute easiest path).  Or adopt the AI approach and derive +/- voltages from a DC multiplier.  A lot of possible recipes, so I'm wondering about the various outcomes.    Target is a DIY 87 build with a Neumann single backplate head, obviously easy to have cardioid and omni, but not f8. 

emrr said:

the biggest question.  What exactly happens at lower voltage?


Divider is what's done in the U67.  Pair of 2M2 from which comes the 120V and the 59V. 

Idea is you can't use single backplate in U87 circuit and achieve figure 8, unless you modify the polarization voltage scheme.  47V and 94V would mimic U67.  As would 23.5V and 47V (absolute easiest path).  Or adopt the AI approach and derive +/- voltages from a DC multiplier.  A lot of possible recipes, so I'm wondering about the various outcomes.    Target is a DIY 87 build with a Neumann single backplate head, obviously easy to have cardioid and omni, but not f8.

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Ok this is some misunderstanding

I was thinking that you have already U87Ai and want to convert it a little bit...

No you can't have figure of eight from single backplate without dc converter and extra negative voltage.
What happend?? Same as you are using PAD in original U87 - it is nasty designed - just lowering voltage. It change response.
If you go to low, your microphone will be also noisier, due to lower signal feeding input and response will be worse than PAD in U87.
U67 ivider is done from high voltage to typical, proper voltage, going around 23V is nonsense and in case of Ai DC converter is just matter of diode change.
Devil is in the details, there are few options to change of input configuration of Ai circuit to get similar level and response to old U87i, but this need to be done with DC converter in the circuit.

ln76d said:

this need to be done with DC converter in the circuit.

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That has been my assumption.

ln76d said:

I was thinking that you have already U87Ai and want to convert it a little bit...

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TLM67 head and metalwork.  And I'm happy enough with cardioid and omni, but exploring all options.  I already have the dual TLM electronics for every possible pattern. 

ln76d said:

going around 23V is nonsense

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Probably, just seeking to understand all the tradeoffs.  With single backplate being much more sensitive than dual, 23V simply returns sensitivity (at least by one metric, see first post and link again) to the same zone the dual backplate occupies.  There is clearly more to it than that, as you say, other metrics that swamp the one. 

read how the pad works in the following link
https://www.neumann.com/?lang=en&id=current_microphones&cid=tlm50_description

emrr said:

Probably, just seeking to understand all the tradeoffs.  With single backplate being much more sensitive than dual, 23V simply returns sensitivity (at least by one metric, see first post and link again) to the same zone the dual backplate occupies.  There is clearly more to it than that, as you say, other metrics that swamp the one.

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No  - the "single" backplate  capsule shouldn't be more sensitive than "dual" - it's matter of internal connection. Rather diaphragm tuning can change sensitivity for several frequencies, but still not in the whole audible spectrum.
Space between backplates rather don't give you huge difference.
This is also why lower voltage drop some frequencies. Diaphragm need correct tension to work properly with full bandwith.
Sorry, but not a fan of repforum, sometimes only downlading schematics if someone put link here, but even don't want to read anything there
Rather i would focus on U87i circuit and investigate where is the drop of sensitivity

Sorry, all that math comes from a Neumann engineer, look him up.  I'm inclined to take his word for the specifics of sensitivity.    What that sensitivity means in the real world is another question. 

It's late and i need go to sleep
Ok i read only Uwe post with calcuations - i didn't found anything about difference between "single" and "dual" backplate sensitivity difference, but rather parasitic capacitances calculation which came from the difference in the input topology configurations.
If i missed something please paste it here, i will read at the day

Gus said:

read how the pad works in the following link
https://www.neumann.com/?lang=en&id=current_microphones&cid=tlm50_description

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The -10 dB function is achieved by reducing the capsule polarizing voltage from 60 V to 23 V. It helps to avoid overloading the following units during very high sound pressure levels. The switch does not extend the dynamic range of the microphone amplifier, but shifts it upward by 10 dB.

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Thanks Gus.    The link strongly suggests, at least with the TLM50 capsule, that it is regarded by some to function fine at 23V. 

It appears the C414B-ULS achieves -10 with a change from 62V to 20V.  Then -20 adds the capacitive method to it. 

emrr said:

Thanks Gus.    The link strongly suggests, at least with the TLM50 capsule, that it is regarded by some to function fine at 23V. 

It appears the C414B-ULS achieves -10 with a change from 62V to 20V.  Then -20 adds the capacitive method to it.

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Focus also on the last sentence

The switch does not extend the dynamic range of the microphone amplifier, but shifts it upward by 10 dB.

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Now i quote myself

ln76d said:

If you go to low, your microphone will be also noisier, due to lower signal feeding input and response will be worse than PAD in U87.

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If you will get occasion compare response of U87 with PAD and without.

All manuals will tell you that PAD is uber fine

Note one thing - TLM150 is SDC.
SDC capsules due to diaphragm diameter can work better with lower voltges than LDC.
Many SDC works great with polarisation voltage under 40V and the diaphragm stiffness is corret.

Most similarity you will get with voltage conversion to 47V, deemphasis network specific values and manage input topology in similar way to get specific drop in sensitivity.

Now go to sleep

Good night!

ln76d said:

Note one thing - TLM150 is SDC.
SDC capsules due to diaphragm diameter can work better with lower voltges than LDC.
Many SDC works great with polarisation voltage under 40V and the diaphragm stiffness is corret.

Good night!

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Good point.

What about 414? 

Good night!

emrr said:

Good point.

What about 414? 

Good night!

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B-uls? The beginning of the end of C414
It's totally different circuit than U87, here you have whole bush of transistors vs. single FET.
Input topology also is much different.
In case of noise, B-ULS will handle really low input signal without any problem.
Transducer is also completely different design (pretty good btw.)  you will rather not notice huge difference in frequency shaping with lower voltage, These capsules are tuned in very speific way. That's why even with 60V they sound pretty neutral, dark whatever you would like to call it. The difference in sound you will hear with higher polarisation voltage for example 80V and this capsule will work properly in case of tension, but K87 probably will stuck diaphragm to backplate. 

Many microphones have PAD which lower voltage to really low levels, but this is only the PAD - something which you need to use when your chain can't handle high SPL. It's just a compromise and of course best is always to use microphone without PAD in case of response character.  Other thing is setup such low level for main work of microphone without PAD.
You will not get U87i response that way.
Focus rather on DC converter and change of input topology with deemphasis network - this is the way where you will be able to create something similar to the old U87. 

ln76d said:

Focus rather on DC converter

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OK.  How do we best do that to attain figure 8 with a single backplate K67 in a U87 circuit?    It appears adding 94V would be the least change to the U87 circuit, and probably the simplest method overall.  -47 would also work.    Both paths maintain a 47V difference between each diaphragm and backplate, one is more the path of the U67, the other more the path of the U77/U87AI.