R/E/P > Klaus Heyne's Mic Lab

Neumann U67 Reissue: Complete Tear Down and Analysis

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afterlifestudios:

--- Quote from: 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.

--- End quote ---

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.

klaus:
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.

klaus:

--- Quote from: Jim Williams on August 23, 2018, 12:06:24 PM ---Outside of the subjective evaluations, are there any differences in objective measurements like THD+noise, phase response, noise floor or bandwidth?

--- End quote ---

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.

David Satz:
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

soapfoot:

--- Quote from: 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

--- End quote ---

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?)

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