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Author Topic: U67 Filter Caps: Helpful? Harmful? Audible?  (Read 3122 times)

Je55

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U67 Filter Caps: Helpful? Harmful? Audible?
« on: May 23, 2014, 06:58:26 pm »

Hi Klaus.. & co

Ive built a u67 from scratch all point to point inside a u87ai.

I have one question about the 2000pf Filter caps across every pin to grid. In the original these are obviously feedthrough capacitors on the lower board and i have some wima film types which i could easily install but space is limited and really how relevant is using a capacitor here? I notice that Oliver doesn't utilize capacitors in this position on his u67 recreations.

Cheers,

Jesse
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klaus

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Re: u67 Filter Caps
« Reply #1 on: May 23, 2014, 09:12:06 pm »

These caps were put in so the mics could comply with German public broadcast specs. Neumann complied with all requirements, but the broadcasters still rejected the model, because it used the "wrong" tube.

I normally remove the two caps associated with the balance audio outs and leave the others in. They don't harm anything.
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Klaus Heyne
German Masterworks®
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Je55

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Re: u67 Filter Caps
« Reply #2 on: May 23, 2014, 09:38:54 pm »

Cheers Klaus,

I'm going to wire it without any of the caps for now and maybe add them later across the voltage lines.

Appreciate the reasoning behind the addition to these, makes perfect sense :)

Side thought:
Has anyone ever considered implementing a couple of toroid filters in parallel with capacitor inside the mic across B+ and heater?

as such: https://www.youtube.com/watch?v=xDUoxsMpfYQ

Cheers,

Jesse
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Kai

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Re: u67 Filter Caps
« Reply #3 on: May 26, 2014, 03:54:51 am »

Has anyone ever considered implementing a couple of toroid filters in parallel with capacitor inside the mic across B+ and heater?
The inductors would be installed in series with the B+ and heater supply lines, not in parallel to the caps.
If you need such a filter at this point something is wrong inside the PSU.
If you have a clean supply you don't need to filter.
If you have hum the filters wouldn't help, as the inductance of those DIY coils is too small.
And- a coil can even pick up hum from stay magnetig fields!
It could help if you have interference from cellphones, but usually the U67 isn't prone to this problem, as it's made for broadcasters.

Regards
Kai
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Uwe

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Re: u67 Filter Caps
« Reply #4 on: May 26, 2014, 11:17:09 am »

The ceramic feedthrough capacitors C12 through C16 are there to shunt potentially interfering RF signals to ground, which may be picked up by the microphone cable unintentionally function as a long-wire antenna.

Assuming that both capacitors in the audio path appear in parallel to the 200 Ohm source impedance, the corner frequency of the resulting low pass filter will be nearly 630 kHz, well above the perception of even most gilded 'Golden Ears'. Aside from keeping RF-interference at bay none of these filters have any adverse effect on the audio signal. Even the minuscule phase shift at the highest audio frequencies is only equivalent to less than 1/8" or 3 mm movement between the source and microphone.
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klaus

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Re: u67 Filter Caps
« Reply #5 on: May 26, 2014, 04:43:37 pm »

(...)
... well above the perception of even most gilded 'Golden Ears'. (...) none of these filters have any adverse effect on the audio signal.

Subjective observation is an established component of the scientific principle. A whole field of scientific advancement-medicine- is to a large extent based on subjective feedback from the patient. Clinical Phase One and Phase Two trials are routinely abandoned the moment patients report adverse effects, regardless of whether these effects can be "objectively proven" or not.

I am pleading, once again, for more tolerance  towards empirical, subjective, data reported from listeners. I am not advocating to embrace anti-science, but for open and respectful acceptance of a vital component of audio: subjective listening experience. There are plenty of fine points of human hearing that fall through the cracks of our (so far) relatively primitive state of measuring parameters that purportedly tell us all about what we hear and how we ought to perceive it.

Case in point: I remove the capacitors in the balance audio path of U67 outputs (I leave the other 2000pf caps in), because I perceived them to be detrimental to good sound, even if only in a minor way. But when desired, capacitors in that circuit position can do some good for the overall audio of a mic, too: In the U87, I lower the 330pf value of the two ceramic disc capacitors to under 100pf., using polystyrene caps, with pleasant results.

All of this as a result of what I hear, rather than how logic or illogic is seems on paper.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Jim Williams

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Re: u67 Filter Caps
« Reply #6 on: May 27, 2014, 11:02:27 am »

Have any owners used a fully modern, regulated power supply and have they detected sonic differences from that change?

In my experience, dynamics do tighten up with a low impedance, regulated power supply on the B+ lines. Noise and ripple are less too. The fuel supply effects the performance as it does with engines.

The results and differences may also be subjective, or, what ever floats your boat.
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klaus

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Re: u67 Filter Caps
« Reply #7 on: May 27, 2014, 12:55:53 pm »

"Dynamics tighten up" Can you explain? I define "dynamics" in mics, or in audio in general, as the volume difference between loudest and softest (highest and lowest output of the mic). Is that what you mean? and how are dynamics affected, in your opinion?
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Klaus Heyne
German Masterworks®
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Kai

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Re: u67 Filter Caps
« Reply #8 on: May 30, 2014, 10:47:07 am »

I define "dynamics"... as the volume difference between loudest and softest...
If you take it scientificly.

For me there is even a more subjective "dynamic" which consists of a mix of several sound parameters:
- Tight and deep bass.
- Clean and open attacks and transient response.
- Transparent midrange, even at low and high levels.
- Overall more defined sound.
These are the ones that I found linked to PSU differences in general.
I cannot link those to technical parameters like more or less distortion or such.
It's purely subjecive and often not possible to do A/B comparison, as it's not a 1 second job to reverse the changes.

E.g. I had a big AMEK mixer that's sound drastically changed after replacing the provisional power line cabling to proper dimensions.
This does not mean it worked with undervoltage before!
OK, this thing draws almost 2KW from the power line, a bit more then a tube mic.
I found similar sound changes when comparing power amps PSU's, e.g. a BGW where I replaced the M-core transformer with a toroid.
There are simple technical explainations for this - more stability in the PSU lead to more stable signal processing in the audio chain.
I did not yet find the same thing with mic's.
I can't even "see" the audio on the supply line of a mic using a scope, but who knows.

I was astonished lately what a (my) human ear can differentiate.
I had to check two "in the box"- mixes of a song for equality.
The null-test (difference mix1 - mix2) showed a difference of -80dB below peak level.
The residiual signal sounded like noise with no musical content.
I auditioned both mixes from my DAW, switching blind between two tracks.
Each time I opened my eyes I had choosen the same mix for better sound, no exception!
I did that 10 times to be sure, because I couldn't belive it.

It was he same mix, the only difference might have been either the software version it was bounced from or the dithering used to go to 24(!) bit.

Regards
Kai
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Jim Williams

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Re: u67 Filter Caps
« Reply #9 on: May 30, 2014, 12:08:29 pm »

"Dynamics tighten up" Can you explain? I define "dynamics" in mics, or in audio in general, as the volume difference between loudest and softest (highest and lowest output of the mic). Is that what you mean? and how are dynamics affected, in your opinion?

A good analogy is a tube guitar amp. Those older designs with tube rectifiers are known for their soft transients. Change the tube rectifier out with 1N4007 diodes and larger filter caps those player dynamics 'tighten up'.

The same happens with tube mics or tube mic preamps with low impedance regulated power supplies. You get sharper transients, better bass control without smear or sloppiness and better low level high frequency details.

It's something one need to compare in real time to fully understand those changes. Fuel does affect sonics.
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