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11
Klaus Heyne's Mic Lab / Re: Neumann U67 Reissue: Complete Tear Down and Analysis
« Last post by soapfoot on September 01, 2018, 08:30:54 am »
There is always a preamp involved. If you try different microphone impedance settings with a variety of different preamps, I think you are likely to get a variety of different impressions.

For example, I imagine that many readers of this forum use preamps that have input transformers. Particularly if those transformers have a high turns ratio (producing a significant voltage step-up within the transformer), the high-frequency response can vary quite audibly as a function of the microphone's impedance. In effect such preamps work properly for only a specific range of driving impedances (generally ~150 Ohms), and anything outside that range produces either a rising or a falling response, with corresponding phase distortion.

Attached is the first part of a 1960s Gotham Audio bulletin showing some aspects of this situation in particular cases (bulletin 10a).

This problem is why, for example, John Hardy's "M-2" and "Twin Servo 990" preamps have switches for use with very low-impedance microphones (e.g. Schoeps and some other transformerless microphones, which can be in the 25 Ohm range). And many other preamps have input transformers with higher turns ratios than the Jensen JT-16-B that Hardy uses, so their potential for having this problem is correspondingly greater.

If your preamp doesn't have this problem (or not enough to worry about), then the choice of 50 vs. 200 Ohms becomes more a matter of practical engineering, e.g. the lower impedance setting helps to isolate the microphone's output circuit from the effects of cable capacitance (high frequency losses, reduced high-frequency headroom); it also decreases the likelihood of overloading the input of the preamp; and if your preamp has a low input impedance, the lower output impedance setting in the microphone will reduce losses due to improper loading, which may well be frequency-selective (as in the Gotham bulletin).

But if you use the microphone with a variety of preamps, the sonic consequences of the different possible settings will be more or less a toss-up, I think. If/when such effects occur, it would be a mistake to attribute them to the microphone alone, out of context.

--best regards

This is great, David.

So, a question-- using a mic preamp with a transformer-coupled input, is it better to aim for matched impedance with the microphone, rather than bridged impedance?

I was always operating under the (misconception?) that, as long as the preamp had an input impedance roughly 10x the microphone's source impedance, I'd maximize the microphone's performance  (excepting some special cases). I guess my assumption was based on the idea that we'd be most interested in voltage transfer, and less interested in power transfer in this application.

I'm sure this is incomplete, but if it's actually incorrect I'd appreciate the clarification.

(Mr. Heyne, if this is too far off topic, perhaps a thread split--duplicating Mr. Satz's post--is warranted?)
12
Klaus Heyne's Mic Lab / Re: Heisermann Capsules
« Last post by eheiserman on August 27, 2018, 04:01:56 pm »
Hey all,
I thought I would chime in here.. I will try and stay technical as I can. I don't want to make this an add. :)
So my k47 ish capsule was really just a fun experiment at first that actually sounded pretty good. So a little history.
All of my machining is done in the states by a local guy here in NC, I have him disk off the brass and add the more complicated small threading that I can do on my machine so I essentially get blanks with no holes drilled for 87/67 style capsules. Then I drill the holes on the face. Anyway long story short is I wanted a capsule and only had the 67 style blanks, thought why not give a 47 hole pattern a try. As I'm sure you all know one half of a 67 is only around 4mm thick and a true k47 is 6mm or so.. I had to play with hole size depth etc to get it to sound good but so far I am happy. It's not a true k47 by any stretch. It's a bit more modern sounding, more 10k and above really, But the over all sound is pretty close. Great in a fet47.  No clone parts here.. 100% usa.
13
Klaus Heyne's Mic Lab / Re: Neumann U67 Reissue: Complete Tear Down and Analysis
« Last post by David Satz on August 24, 2018, 11:38:53 am »
There is always a preamp involved. If you try different microphone impedance settings with a variety of different preamps, I think you are likely to get a variety of different impressions.

For example, I imagine that many readers of this forum use preamps that have input transformers. Particularly if those transformers have a high turns ratio (producing a significant voltage step-up within the transformer), the high-frequency response can vary quite audibly as a function of the microphone's impedance. In effect such preamps work properly for only a specific range of driving impedances (generally ~150 Ohms), and anything outside that range produces either a rising or a falling response, with corresponding phase distortion.

Attached is the first part of a 1960s Gotham Audio bulletin showing some aspects of this situation in particular cases (bulletin 10a).

This problem is why, for example, John Hardy's "M-2" and "Twin Servo 990" preamps have switches for use with very low-impedance microphones (e.g. Schoeps and some other transformerless microphones, which can be in the 25 Ohm range). And many other preamps have input transformers with higher turns ratios than the Jensen JT-16-B that Hardy uses, so their potential for having this problem is correspondingly greater.

If your preamp doesn't have this problem (or not enough to worry about), then the choice of 50 vs. 200 Ohms becomes more a matter of practical engineering, e.g. the lower impedance setting helps to isolate the microphone's output circuit from the effects of cable capacitance (high frequency losses, reduced high-frequency headroom); it also decreases the likelihood of overloading the input of the preamp; and if your preamp has a low input impedance, the lower output impedance setting in the microphone will reduce losses due to improper loading, which may well be frequency-selective (as in the Gotham bulletin).

But if you use the microphone with a variety of preamps, the sonic consequences of the different possible settings will be more or less a toss-up, I think. If/when such effects occur, it would be a mistake to attribute them to the microphone alone, out of context.

--best regards
14
Klaus Heyne's Mic Lab / Re: Neumann U67 Reissue: Complete Tear Down and Analysis
« Last post by klaus on August 24, 2018, 04:10:10 am »
Outside of the subjective evaluations, are there any differences in objective measurements like THD+noise, phase response, noise floor or bandwidth?

Aside of the (measurable) output difference between the two secondary strappings, (4-6db), you may be the better source to measure finer points of tranformer strapping choices with your beloved precision analyzers.

I would be curious to know whether different strappings for secondaries would show up on a sophisticated display.
15
Klaus Heyne's Mic Lab / Re: Neumann U67 Reissue: Complete Tear Down and Analysis
« Last post by klaus on August 24, 2018, 04:02:57 am »
You are correct. I took this photos before restrapping for 200Ω.

This vintage U67 which I used for comparison tests was originally strapped for 50Ω (two wires in parallel, as shown on your photo). 50Ω was the low-output strapping for all Gotham-imported U67. Together with an audio pad network installed in NU67 of the same period, the output was down 12dB, compared to U67 systems sold for European delivery.
16
Klaus Heyne's Mic Lab / Re: U67 Voltages And Other Questions
« Last post by afterlifestudios on August 23, 2018, 07:13:48 pm »
Thanks Klaus and Uwe.  R4 shunt resistor was measuring closer to 100k.  So I replaced it with a 68k as per the schematic and now I've got a stable 209vdc. 

I must take this moment to express my gratitude for your expertise and generousity with your help and information.
17
It is strapped for 200Ω (the usual studio standard for mics) and so was the Reissue, which came that way from the factory.

Thank you, Klaus.  I've been diving into my own u67 in attempts to better understand it.  I believe(d) mine was strapped for 200ohm until I saw the photos you posted in this review of your selected vintage u67 which you say is strapped for 200ohm as well. 

Here is a close up of your vintage u67 from your tear down article above.  Is that how it looks when set for 200ohm?  (Mine does not have the two horizontal jumpers, rather one vertical jumper on the right side of the board where it says 200ohm.) 

I've done quite a bit of internet searching on how to configure the strapping for u67, but can't find anything definitive.  (I found a very detailed description from you about the 87 strapping, but that's a different animal...)

Thanks, and please feel free to move this somewhere else if you don't want it cluttering up this tear down thread.
18
Outside of the subjective evaluations, are there any differences in objective measurements like THD+noise, phase response, noise floor or bandwidth?
19
Nice! I didn't know this. Thanks John!
About one hour of data exporting, importing, manipulation and some graphic artwork can be spared :)

Best regards

Dear boggy,

I read all your advices for measuring each added port to the measure in turn but I have designed a HR and I am not getting a very clear curve of the resonator working in the area that I have designed until:

1- Did you use a graph smoothing(i.e. 1/1, 1/2, 1/3...) firts to do the A-B graph??

2- Where do you place the source(loudspeaker, sub or the noise source) relative to the HR(in front of??, directed to the HR??, Behind of HR??, to one, two, three meter of??.. )??,

3- Do you have another advises or notes in order to get the best capture working curve of a resonator??

I will appreciate any comment.

Thanks anticipated,
opacheco.
20
Klaus Heyne's Mic Lab / Re: Neumann U67 Reissue: Complete Tear Down and Analysis
« Last post by klaus on August 23, 2018, 09:38:15 am »
It is strapped for 200Ω (the usual studio standard for mics) and so was the Reissue, which came that way from the factory.

I do not like the 50Ω strapping, because it puts the two secondaries in parallel, with a slightly glassier sound and slightly less midrange texture than when the secondaries are connected in series (200Ω).
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