R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

News:


Recent Posts

Pages: 1 2 [3] 4 5 ... 10
21
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
22
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.
23
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.
24
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.
25
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.
26
Outside of the subjective evaluations, are there any differences in objective measurements like THD+noise, phase response, noise floor or bandwidth?
27
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.
28
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).
29
Klaus Heyne's Mic Lab / Re: U67 Voltages And Other Questions
« Last post by uwe ret on August 22, 2018, 10:57:08 pm »
Before changing either of the two series resistors (6.8k∩ and 8.2k∩) please check the value of the shunt resistor, as together with the tube's condition is most responsible for the accurate high voltage value. According to my calculations I predict it is close to 205k∩ instead of the desired 68k∩. I further predict that you must have the early version of NU67, because with the later version the high voltage would not be able to rise to 239V, even with the shunt resistor open. When replacing R4 (the shunt resistor, KH) make sure it has a powerr dissipation rating of 1W or 2W!
BTW all calculations use simple Ohm's and Kirchhoff's laws.
30
Klaus Heyne's Mic Lab / Re: N52a question
« Last post by klaus on August 22, 2018, 10:26:42 pm »
Itís located exactly midpoint between the two big silver transistors, sitting upright to be adjusted from the sides rather than head on.
Pages: 1 2 [3] 4 5 ... 10