U67 de-emphasis network

[OPR]

How efficient is a condenser capsule at energy conversion, in either direction?

My understanding is that condenser capsules put out a very low power signal, lower than ribbons', which are lower than typical dynamics; that's why condensers need some kind of head amp (in the mic) to put out enough current to drive a preamp.

That suggests to me that they're a lot less efficient than dynamic mic capsules, however efficient those are, so not very efficient.

That in turns suggests to me that the feedback-into-the-mic-connection trick is doing a lot less to oppose the diaphragm's motion than to electrically cancel its LF electrical output at the input of the head amp.

let me try and explain it this way.
In order for the capsule to pass the NFB audio in the form of AC voltages the diaphragm must move.

And in order to create the required cancellation, around 5 6 db in the LF it would need around half of the input voltage to bring the low end back to flat.
try it in a DAW canceling audio against it's self and it might make more sense to you can roughly get an idea of the ratio blend required for it to function

 

[kingkorg]

In order for the capsule to pass the NFB audio in the form of AC voltages the diaphragm must move.

Maybe this is the part where we diverge? The diaphragm doesn't have to move. If this were true the NFB wouldn't work if the capsule was replaced with a plain capacitor.

 

[OPR]

Maybe this is the part where we diverge? The diaphragm doesn't have to move. If this were true the NFB wouldn't work if the capsule was replaced with a plain capacitor.

And neither would the mic...

Well then ask your self the question... is a microphone capsule a plain capacitor?

 

[Paul W]

And neither would the mic...

Well then ask your self the question... is a microphone capsule a plain capacitor?

Mostly. It is a capacitor and an inefficient transducer. The feedback voltage will serve to oppose the LF motion of the diaphragm, but only a little. Most of the diaphragm motion will still happen, and the signal the capsule puts out will be further high-pass filtered electrically.

Most of the high-pass filtering effect will happen whether or not the capsule acts as an inverse transducer and resists the LF motion, because you're feeding an out-of-phase LF signal from the amplifier output back to the amplifier input.

 

[OPR]

Mostly. It is a capacitor and an inefficient transducer. The feedback voltage will serve to oppose the LF motion of the diaphragm, but only a little. Most of the diaphragm motion will still happen, and the signal the capsule puts out will be further high-pass filtered electrically.

Most of the high-pass filtering effect will happen whether or not the capsule acts as an inverse transducer and resists the LF motion, because you're feeding an out-of-phase LF signal from the amplifier output back to the amplifier input.

Well it sounds like you mostly agree with me.
At risk of repeating my self...
Take a look at the curves. If the NFB was to defy the laws of physics and have no impact on diaphragm motion. then why would the low frequency
NFB effect the frequency response all the way up to 10K ,and possibly beyond, as this is where the HF NBF kicks in, which is still intact in this measurement. Could it be the result of extra diaphragm damping due to the LF NFB?... possibly.

 

[Paul W]

Well it sounds like you mostly agree with me.
At risk of repeating my self...
Take a look at the curves. If the NFB was to defy the laws of physics and have no impact on diaphragm motion. then why would the low frequency
NFB effect the frequency response all the way up to 10K ? could it be the result of extra diaphragm damping due to the LF NFB? possibly.

I don't see the connection. As I understand things, there are two separate questions.

There's circuitry that low-passes and 180 degree phase shifts the output of the amplifier, and feeds that out-of-phase LF back to the input of the amplifier, but also to the capsule which is connected there as well.

One question is how much of that high-pass filtering is accomplished by the feedback voltage applied to the capsule opposing LF diaphragm motion, and how much is done by simple electrical cancellation at the input of the amplifier.

It seems like a whole different question why either effect extends way up the spectrum. I would guess it's because the low-pass filter at the output of the amplifier doesn't have a steep slope across the middle frequencies, so the feedback (to both the capsule and the amplifier input) has middle frequencies in it, as well as LF.

If that weren't true, I don't know why either high pass filtering effect (by capsule backpressure OR by electrical cancellation) would extend up that high in the spectrum.

And I'm curious why the slope is so gradual across the middle frequencies. Is it designed that way as a gradual "tilt" EQ, rather than a normal high-pass filter?

 

[jBam]

Some very interesting discussion and reading in this thread since I last checked in!... It's an interesting concept / question: is it different to modulate the capsule vs. the signal-at-the-amp (or anywhere after).

The way I see it simplified is without thinking about a filter at all... just a raw feedback of the signal... e.g.

If you had a raw 1kHz test tone being recieved by a mic capsule (as in, out of a speaker, into the mic capsule), and you bypass the filter and:

a) feedback a small portion of the recieved signal to the amp input's signal (but inverse phase) then you're effectively reducing the level of the signal.
b) feedback a small portion of the recieved signal to the capsule polarisation to modulate it against the incoming signal, then, for me, that's the question: "is it the same outcome as a) above"?

a) is solid.

b)... well it's hard to grapple (to me, with limited knowledge but keen interest). My mind starts to play tricks on me, as I wonder how this feedback would work... this system would feedback the recieved voltage, which instantly lowers the signal level recieved at the diaphram by reducing the polarisation voltage, but that lowered level is the levels that's then actually recieved... which is fed back to lower the sensitivity which lowers the signal, and on and on.... I don't understand how this would be stable. But I don't understand a lot of things lol... and it has to be stable, because even with the ACTUAL filter used, this is what's going on (right?)... I imagine feedback to the capsule has a sort of logarhythmic reaction, where the effect of feedback lowering the level of the signal diminishes over and over until it's not "really" diminishing anymore and is effectively "stable", which is all instantaneous and what we end up hearing (as opposed to the signal instantly turning itself off due to constantly reducing it's own signal in e.g. a linear fashion).

So - my understanding above is possibly roughly correct, or just flat out wrong ... Eitherway, the question for me remains: "Is the result of this feedback in option b) the same outcome as option a) above"?

For feedback to the capsule to be identical to feedback at any point after that, it would entirely depend on the capsule having a linear reponse to voltage changes in the same way that it does with option a) above... a consistent sensitivity change for varying polarisation voltages (let's say "in normal operating conditions"). That's either how it is, or it's not... as a result, I see two options for what's actually happening:

1. having the polarisation voltage modulated by the recieved signal slightly pushes and pulls the diaphram based on +/- incoming signals in a manner that restricts it's movement resulting in a change in signal that is the same amount as e.g. lowering the signal at any point after the capsule... i.e. a linear response.
2. OR it is not a linear response, and the signal becomes distorted to some degree... i.e. it is NOT identical to doing this after the capsule due to non-linear ways polarisation voltages effect the recieved signal.

Man... but it kind of has to be Option 1... How else could a microphone with feedback networks recieve any filtered signal accurately with low distortion if it wasn't...

If it's Option 1, then the key benefits I see would be in diaphram protection / longievity... it physically restricts signals at the diaphram... sure not a huge amount physically, but certainly an audible / electronically signficant amount... and if those signals are areas that might cause damage like big plosives and such, then that has a benifit right?... HF filtering...? well less critical from that perspective... but if you're already filtering to the polarisation voltage, why not keep at it!?

 

[OPR]

I don't see the connection. As I understand things, there are two separate questions.

There's circuitry that low-passes and 180 degree phase shifts the output of the amplifier, and feeds that out-of-phase LF back to the input of the amplifier, but also to the capsule which is connected there as well.

The the LF NFB connects to the capsule back plate and not directly to the amp so the capsule is part of the signal path.

One question is how much of that high-pass filtering is accomplished by the feedback voltage applied to the capsule opposing LF diaphragm motion, and how much is done by simple electrical cancellation at the input of the amplifier.

Again The the LF NFB connects to the capsule back plate and not directly to the amp so the capsule is part of the signal path. there is no direct path between the LF NFB and the amp.

It seems like a whole different question why either effect extends way up the spectrum. I would guess it's because the low-pass filter at the output of the amplifier doesn't have a steep slope across the middle frequencies, so the feedback (to both the capsule and the amplifier input) has middle frequencies in it, as well as LF.

Here are the details for the 67 amp FR. The1500 should be 15k for the the HF roll off the lows are at 40HZ so dont extend that far up.