U67 de-emphasis network

[dmp]

Question can anyone confirm or enlighten ,my tertary winding measures 12 ohms on my multimeter does tha sound right ?

Well if it measures 12 ohms its not open - so that's good. I found both of my BV12 had at an open winding. The windings that were not open did have the right ratio (Actually ratio should be P/S 6.92:1, P/T 21.6:1.) Both bobbins can be measured separately.

Best way to test your bv12 is to connect a test tone at the primary, measure the voltage at the secondary & tertiary and calculate the ratios.
My opinion is there is definitely something off about the Bv12 but I am going to try to get it working, as it fits the PCB footprint. Even with correct coils that are not open (probably from poor winding), they have a lower thickness of laminations (less than 7.1mm) If I were starting a new build, I would go with Moby's for sure.

 

[dmp]

To start with, I never measured a microphone capsule that had a high frequency boost of 9 dB...

How do you measure microphone capsules? What high frequency boost have you measured in k67 capsules?

A measurement on a U67 microphone through the 'messeingang' shows -5 dB @ 15 KHz.

The messeingang is the calibration input, described in the '63 manual. Why do you think the measurement (always) shows -5dB @ 15 kHz?

 

[RuudNL]

How do you measure microphone capsules? What high frequency boost have you measured in k67 capsules?

The messeingang is the calibration input, described in the '63 manual. Why do you think the measurement (always) shows -5dB @ 15 kHz?

I have measured many K67/K87 capsules in a 'flat' circuit. Usually the HF boost was in the region of 5 dB.
The 'messeingang' measures the complete (de-emphasis) circuit, with the microphone capsule in place.

 

[Delta Sigma]

Reviving this old thread to get my head around the de-emphasis circuit. The low freq feedback makes sense to me. So does the the C17.

It's the main tertiary feedback that I don't quite get. Is the frequency response of the brown/white winding what provides the bulk of the de-emphasis?

In other words, is that winding's frequency response ~5dB higher at 15kHz when terminated by 470R?

 

[Murdock]

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

 

[Delta Sigma]

I think it's a pretty slick low f feedback arrangement. Wish I saw this before I did the ol' analyze in my head thing.

 

[jBam]

Thanks guys. Very interesting approach. I learned something new thanks a ton!

Me too!... I know this is an old thread, but... is that the standard approach in mics? To filter (well... modulate I guess) the backplate voltage to reduce the sensitivity (or whatever is going on) as opposed to filtering the signal like an eq!?

It's intriguing... not what I expected. I assume there's a legit benefit to this approach... perhaps transient response/ phase coherency??

 

[tomas.borgstrom]

Me too!... I know this is an old thread, but... is that the standard approach in mics? To filter (well... modulate I guess) the backplate voltage to reduce the sensitivity (or whatever is going on) as opposed to filtering the signal like an eq!?

It's intriguing... not what I expected. I assume there's a legit benefit to this approach... perhaps transient response/ phase coherency??

As far as I know U67 / m269 are the only microphones with this sophisticated feedback network to reduce high frequencies. I'm convinced that this is the reason for the unique smooth top end of these microphones.

It's very common though to just put a capacitor between plate and ground to achieve a LPF.

 

[Khron]

It's very common though to just put a capacitor between plate and ground to achieve a LPF.

... Which is a pretty "barbaric" way to achieve that (imho), as opposed to a capacitor in parallel with the plate resistor, for example.

 

[tomas.borgstrom]

... Which is a pretty "barbaric" way to achieve that (imho), as opposed to a capacitor in parallel with the plate resistor, for example.

Interesting, I have never seen it in a schematic. How does the curve look?

 

[Khron]

Interesting, I have never seen it in a schematic. How does the curve look?

Perhaps / likely the same? With less chance of distortion, and probably(?) less dependence on the output impedance of the tube.

Instead of "bleeding" the HF to ground (while still having the tube amplify those too), it reduces the amplification of those high frequencies "at the root" (so-to-speak), in the first place.

 

[tomas.borgstrom]

Perhaps / likely the same? With less chance of distortion, and probably(?) less dependence on the output impedance of the tube.

Instead of "bleeding" the HF to ground (while still having the tube amplify those too), it reduces the amplification of those high frequencies "at the root" (so-to-speak), in the first place.

What are the typical values of this capacitor?

 

[Khron]

What are the typical values of this capacitor?

I could be wrong, but I'm reasonably sure you can figure that out from the f = 1 / (2piRC) formula; the frequency response would end up being a high-frequency shelf attenuation, from whatever the nominal gain is, down to unity at HF.

Or there's LTspice

 

[MagnetoSound]

A capacitor in parallel with the plate resistor does exactly the same thing.

It is still a shunt to ground, by virtue of the filter cap behind the resistor.

 

[Khron]

It is still a shunt to ground, by virtue of the filter cap behind the resistor.

Hah, never thought of it that way, but it kinda makes sense, indeed...

 

[jBam]

Interesting... very.

So filtering aside, could feeding back part of the actual signal to the backplate be used, for example, to minimise hard clipping internally within the mic?? i.e. at vey high SPL, introducing a soft clip type circuit (tuned to activate only at higher V limits) to reduce sensitivity of the actual capsule at high SPL / signal levels essentially minimising or avoiding a true internal hard clip scenario...? Or would that somehow encourage the diaphram to be more likely to actually collapse or hit the backplate or something nasty??

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.

I like the idea of a mic that can sound sweet at low level and cop some serious SPL without square clipping internally. Even though the above would "distort" the waveform, I never found decent soft clipping to sound obnoxious. A downside would be not knowing when it's engaging... I'm picturing a switch and an led on the mic that illuminates when soft clipping is active lol... already sounds complicated.

 

[jBam]

So this concept of applying filter to the backplate still has my head buzzing.

Might be hard to answer definitively, but:

Is there a sonic benefit to this approach... as opposed to e.g. and eq approach after the capsule.

Specifically I wonder if this approach has an identical phase coherence to e.g. an eq after recording (or in the signal post capsule), or if it actually enables a better phase response (i.e. more accurate recording) within the mic with the benefits of de-emphasis...?

Does that make sense?

Paraphrased: if you put a k67/87 in a flat circuit, and eq matched the filter response after recording... would that be roughly the same result in phase response, or does this little trick of applying the internal filter to backplate allow a different more accurate phase response by reducing sensitivity in the filtered band range whilst still receiving the signal "in phase"...

I keep flipping between the two options in my head.

 

[kingkorg]

So this concept of applying filter to the backplate still has my head buzzing.

Might be hard to answer definitively, but:

Is there a sonic benefit to this approach... as opposed to e.g. and eq approach after the capsule.

Specifically I wonder if this approach has an identical phase coherence to e.g. an eq after recording (or in the signal post capsule), or if it actually enables a better phase response (i.e. more accurate recording) within the mic with the benefits of de-emphasis...?

Does that make sense?

Paraphrased: if you put a k67/87 in a flat circuit, and eq matched the filter response after recording... would that be roughly the same result in phase response, or does this little trick of applying the internal filter to backplate allow a different more accurate phase response by reducing sensitivity in the filtered band range whilst still receiving the signal "in phase"...

I keep flipping between the two options in my head.

It is exactly the same as using eq in post. But in this case would it be practical for a mic company to advertise a mic noting that you have to put specific eq curve for it to sound as intended? It is emphasis/de-emphasis "trick" commonly used at that time. It is basically the same thing as Dolby NR. It's found in tons of gear, in some form. Turntables as well.

If they used a simple filter such as cap to ground to get the same effect this would slightly increase the distortion at filtered frequencies. When you do the same thing using feedback, you reduce the distortion at affected frequencies.

 

[jBam]

Ah ha!... makes sense to me. Thanks @kingkorg ! It's a practical trick, with a benefit in the way it's applied... but effectively nothing particularly incredible is going on. Now I don't have to spend another day thinking about it lol

 

[OPR]

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.