U67 de-emphasis network

[GeorgeToledo]

I know this would be a form of distortion of course, but I mention it becuase of reading various comments / threads / info discussing the nasty hard clip various mics have internally, like the 87AI... and this got me thinking about THIS as a way around that.

Much of the objectionable sound from U87AI doesn’t come from clipping the FET, it comes from clipping the output transformer. It is worth engaging the pad whenever there is even a chance of it being needed.

 

[kingkorg]

King Korg said
"It is exactly the same as using eq in post"
(sorry the qoute feature was messing up)

Or is it?
Consider that the negative feedback signal is a selected range of the audio signal that's captured by the capsule, phase reversed and being fed directly into the very same transducer that captured it. The most obvious effect was stated in the U67 marketing. Stating that the there was no need for a pop filter as pops were intercepted before the amplifier.
In other words, the low frequencies that caused the pop are eliminated by the reversed signal pushing the diaphragm in the opposite direction to avoid the diaphragm collapsing. Imagine what it's doing at high frequencies...
This is not just eq its also affecting the behaviour of the capsule.

You are suggesting some kind of dynamic action. Or maybe i got it wrong? It doesn't work that way, IMHO. Both low and high end curve are static and not dependent on the signal input. Easy to test by feeding the signal into the circuit. Also vintage k67 are easy to collapse anyways which they eventually fixed by increasing the diaphragm tension. However i see the point, and it is absolutely plausible the fed back signal does help counter the mechanical action. I'm not sure to what extent tho, and what the path is in u67.

@soliloqueen you are in the middle of testing this "issue" with vintage capsules. Maybe see if you get different result if the capsule is in actual 67 circuit?

 

[kingkorg]

@OPR one more question. What cap is responsible for low end feedback to the capsule? Is there even path for low end attenuation? @RuudNL maybe you could chime in. I'm not really familiar with u67 circuit. I just don't see it.

 

[Khron]

What cap is responsible for low end feedback to the capsule? Is there even path for low end attenuation?

I believe it's described here..?

I don’t know it is of interest but here is the patent paper explaining the low frequency feedback arrangement.
https://patentimages.storage.googleapis.com/63/fe/ce/b47b9b1fca6f3d/US3207848.pdf

 

[kingkorg]

I believe it's described here..?

So through S2 switch. Obviously i was focusing on the signal coming back from the transformer. Thanks!

 

[OPR]

You are suggesting some kind of dynamic action.

Yes

It doesn't work that way, IMHO. Both low and high end curve are static and not dependent on the signal input.

How so? If an AC voltage(Audio signal in this case) is fed in to a transducer would it not cause it to react in some way?

IMHO. Both low and high end curve are static and not dependent on the signal input.

Again, the negative feedback is an audio signal so is dynamic in nature.

Also the same kind of thing is happening with the U87 circuit. Yes you can get a similar effect with Eq in post but not identical.

Interesting circuits...

 

[OPR]

I believe it's described here..?

https://patentimages.storage.googleapis.com/63/fe/ce/b47b9b1fca6f3d/US3207848.pdf
Thanks for sharing Khron!
I hadn't come across this. It kinda blows a hole in the whole mic modelling concept as we currently know it, as it's mostly reliant on post processing, eq curves and pattern control. and not by directly influencing the transient behavior of the capsule it's self, which is where the true character of a mic resides.

 

[OPR]

Much of the objectionable sound from U87AI doesn’t come from clipping the FET, it comes from clipping the output transformer. It is worth engaging the pad whenever there is even a chance of it being needed.

Also the capsule is tuned differently in later k67 as well a 48v polarization in the old 87 vs 60v in the Ai. Polarization voltages exert electrostatic forces on the diaphragm drawing it towards the back plate also damping the diaphragm limiting its compliance, resulting in a harder sound relatively speaking.
This doesn't mean higher polarization voltage = bad sound.... Higher voltages can be used but it's best if the capsules is designed to accommodate the extra electrostatic forces in a complimentary way.

 

[GeorgeToledo]

“Higher voltages can be used but it's best if the capsules is designed to accommodate the extra electrostatic forces in a complimentary way.”

The basic capsule design was already specced for the higher polarization voltage, look to the U67 for an example of that. The original U87 had lower polarization voltage to the capsule as a limitation of not yet having figured out how to get 60v via dc converter.

Point taken about the sonic difference in general though. My grammar was a little imprecise, I didn’t mean to suggest that hitting the transformer harder and the resulting distortion from somewhat under-specced output transformer was the *only* difference between U87 and U87Ai.

 

[OPR]

The basic capsule design was already specced for the higher polarization voltage, look to the U67 for an example of that. The original U87 had lower polarization voltage to the capsule as a limitation of not yet having figured out how to get 60v via dc converter.

Yes that's correct the k67 was specked for 60v and compliments the 67 with it's tube circuit and negative feedback tuning. The fet circuit has less headroom and a more strident sound, relatively speaking, so the lower 48v polarization voltage added some extra headroom for the fet as well as a more laid back sound from the capsule, when compared to the later AI. Regarding capsules and polarization voltages, working back from the maximum voltage generally allows the diaphragm to be more compliant which sounds ,subjectively,more pleasing but at the expense of output. No such thing as a free lunch...

 

[ZHOUJUN]

I used several old K67s from different periods and two old K87s to test on the same old U67, and found that their sounds were somewhat different. I tried changing different polarization voltages and biases, and something interesting happened. The difference in sonic color between them became very small and very close. Now that I have not changed the de-emphasis network, next I plan to try changing these and test their frequency response.

 

[Khron]

Thanks for sharing Khron!

Well, it was just on the previous page of this thread

directly influencing the transient behavior of the capsule it's self

Is it, though, physically / electromechanically affecting the diaphragm, or "just" the signal going "through" it ("through" as in through any other capacitor)... As in, is it really preventing the diaphragm itself from physically bottoming out, or just preventing the (electric signal of the) LF of the plosive itself reaching all the way to the output?

 

[soliloqueen]

You are suggesting some kind of dynamic action. Or maybe i got it wrong? It doesn't work that way, IMHO. Both low and high end curve are static and not dependent on the signal input. Easy to test by feeding the signal into the circuit. Also vintage k67 are easy to collapse anyways which they eventually fixed by increasing the diaphragm tension. However i see the point, and it is absolutely plausible the fed back signal does help counter the mechanical action. I'm not sure to what extent tho, and what the path is in u67.

@soliloqueen you are in the middle of testing this "issue" with vintage capsules. Maybe see if you get different result if the capsule is in actual 67 circuit?

the capsule is, purely electrically speaking, a really, really, really really bad capacitor with a lot of mechanical energy leakage. this has no choice but to go in both directions. in fact, there's no way this couldn't happen. if you feed a signal into a charged capsule, it's going to move. this is true of every physical part in reality. i was working on a preamp recently injecting test tones and if you got the level high enough across the transformer, you could physically hear the test tone coming out of it. and this was in a "well-behaving" part. in a capacitor as "leaky" as a capsule, of course it would act this way. is it meaningfully different than a low and high pass in terms of sound quality? maybe not so much, depending on how fast the diaphragm is, but it might save the diaphragm from slamming into the backplate like trying to slam the door to a room with no windows. i'd need to test this in the u67 directly, and i might later when i have time, but if the signal really is going back into the capsule i have no reason to doubt the behavior in theory. you shouldn't need to test this in a u67. you could just throw another feedback circuit together and run it into the capsule and see what it does. maybe you could use a laser or something.

not saying you don't get the same curve with a regular capacitor in its place, just that, being that the capsule is a leaky capacitor, it can (probably?) "leak" that curve to the outside just like it leaks anything. that's just kind of what it does. i don't know for sure, but i wouldn't be surprised.

 

[Paul W]

How efficiently does the capsule act as a little electrostatic speaker at low frequencies?

How efficient a transducer is it in the normal direction, for that matter?

My understanding (correct me if I'm wrong) is that a 180-degree delayed version of the LF of the output is fed back into the capsule connections, which act as an electrical summing bus, and that's sufficient to accomplish the high-pass filter thing electrically.

How much of an additional effect you get from driving the capsule mechanically with that signal (in opposition to whatever air pressure changes are making it do) should depend on how efficient a speaker it is. (Which shouldn't be too hard to measure.)

 

[kingkorg]

How efficiently does the capsule act as a little electrostatic speaker at low frequencies?

How efficient a transducer is it in the normal direction, for that matter?

My understanding (correct me if I'm wrong) is that a 180-degree delayed version of the LF of the output is fed back into the capsule connections, which act as an electrical summing bus, and that's sufficient to accomplish the high-pass filter thing electrically.

How much of an additional effect you get from driving the capsule mechanically with that signal (in opposition to whatever air pressure changes are making it do) should depend on how efficient a speaker it is. (Which shouldn't be too hard to measure.)

The fed back signal is in the mV range and has to be proportional to the "input" signal capsule provides. Otherwise it would become dominant and the whole feedback thing would fall appart.

This is the reason i don't agree with OPR, because that level of feedback is miniscule in order to combat the attraction force of polarization voltage.

I don't think there's anything magical going on there and besides some microscopic differences in THD, a simple EQ in post would do the same job. The trick is to have exact curve manufacturer had in mind. And then you can experiment with linear phase stuff, etc...

This is all very easy to test, i'm just too lazy and busy to do it right now.

My measurements of k67 show it can put out substantial voltage at plosives, like above 10Vpp. At that point the tube craps out, and can't supply sufficient amount of NFB, and you get distorted waveform back to the capsule. This won't help prevent collapsing.

 

[kingkorg]

I asked chatGPT to summarize my thoughts about this as concise as possible:

Charge from Polarization Voltage: The polarization voltage is applied to the backplate to establish the electrostatic field required for the microphone to function. The feedback signal is typically a small AC signal superimposed on this DC polarization voltage. This AC signal modulates the potential on the backplate, slightly affecting the electrostatic attraction between the diaphragm and backplate.

However, the magnitude of the feedback signal is relatively small compared to the polarization voltage. Its primary role is to fine-tune the microphone's frequency response and reduce distortion, not to significantly affect the diaphragm's motion or charge distribution.

Summary:

The low-end feedback signal indirectly influences the diaphragm's movement by modulating the backplate voltage, but it does not directly affect the physical spacing or the steady-state charge set by the polarization voltage. Instead, this feedback is used to optimize the microphone's acoustic and electronic performance, particularly in controlling low-frequency response and distortion.

 

[Khron]

How efficiently does the capsule act as a little electrostatic speaker at low frequencies?

That was the exact thing i was wondering (or at least strongly hinting at) with

As in, is it really preventing the diaphragm itself from physically bottoming out, or just preventing the (electric signal of the) LF of the plosive itself reaching all the way to the output?

 

[Murdock]

The fed back signal is in the mV range and has to be proportional to the "input" signal capsule provides. Otherwise it would become dominant and the whole feedback thing would fall appart.

This is the reason i don't agree with OPR, because that level of feedback is miniscule in order to combat the attraction force of polarization voltage.

I don't think there's anything magical going on there and besides some microscopic differences in THD, a simple EQ in post would do the same job. The trick is to have exact curve manufacturer had in mind. And then you can experiment with linear phase stuff, etc...

This is all very easy to test, i'm just too lazy and busy to do it right now.

My measurements of k67 show it can put out substantial voltage at plosives, like above 10Vpp. At that point the tube craps out, and can't supply sufficient amount of NFB, and you get distorted waveform back to the capsule. This won't help prevent collapsing.

If I understood the patent paper correctly this way of high pass Filter reduces the chances of the tube “closing” under high pressure level. You can’t get that with an eq afterwards.

 

[kingkorg]

If I understood the patent paper correctly this way of high pass Filter reduces the chances of the tube “closing” under high pressure level. You can’t get that with an eq afterwards.

Under extreme conditions yes, not under normal operation. Feedback definitely DOES reduce THD at those frequencies, but we are arguing about if feedback level is enough to physically push the diaphragm away from the backplate, and i say absolutely not.

We are talking about miniscule differences that happen below 1% THD under normal operation, and TBH i don't care about those. Hence my opinion that using an EQ in post is giving the same results. But you have a good point that if we approach ''breakup'' region feedback will have larger impact. But how many people are using this type of mic in that manner? IDK...

Also philosophical approach... The paper came out in the period when least possible THD was desired. Today we use THD as a tool, for nostalgia... I don't think anyone today uses tube mics because they want less THD. IDK, it's complicated.

Soliloqueen brought up an interesting thing up in a private conversation we had. What happens when clipped LF signal at the plate gets fed back to the capsule when in-phase signal coming from the capsule isn't clipped? I have no idea.

 

[Paul W]

If I understood the patent paper correctly this way of high pass Filter reduces the chances of the tube “closing” under high pressure level. You can’t get that with an eq afterwards.

I don't know what you mean when you talk about the tube "closing." I'm guessing you're talking about the tube amp saturating/clipping.

I would think this circuit would be limited in its ability to keep e.g. a plosive from saturating the circuit, because the LF has to get through the circuit forwards before it can be fed back phase-rotated to cancel itself out. Wouldn't the feedback be too late to keep the rising edge from clipping?

Edit: I posted this before seeing kingkorg's post above it. I don't know if we're talking about the same thing.