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 on: September 05, 2018, 11:22:43 am 
Started by klaus - Last post by Jim Williams
That will affect the response. I noticed in similar mics with a domed mount that the shape does create reflections, resonances and some nulls too.

Mounting the capsule a few mm higher can overcome some of those reflections. A complete frequency response plot with the differences would also be informative but that isn't likely to happen here.

 on: September 04, 2018, 01:30:30 pm 
Started by klaus - Last post by RuudNL
I was a bit surprised that I didn't read a word about the change in shape of the base below the microphone capsule.
In the 'old' U67 there was a dome shaped piece just below the capsule itself.
In the 'new' U67 the capsule sits higher in the headbasket, but the base where the capsule is mounted on, is flat.
IMHO this could influence the sound. Any opinions?

 on: September 03, 2018, 06:19:15 pm 
Started by Eomsy - Last post by opacheco
Any comments?


 on: September 03, 2018, 03:41:00 pm 
Started by klaus - Last post by brucekaphan
Klaus, THANK YOU for doing such a thorough job of analyzing and interpreting the U67 reissue—I have to say I would have expected no less, but that doesn't change my appreciation for the obvious amount of time, energy, and thought you put into your analysis. And I am a big fan of using all of one's senses, not just one's intellect, when it comes to judging anything having to do with music/recording. Not being a spring chicken myself, I know how hard fought the battle is to learn how to deeply hear, and even more so to learn to deeply hear and be able to make useful judgements based on hearing deeply. I was one of those foolish folks who bought into the M149 release, before it had been tested and (dis-) proven both by experts such as yourself, and by the marketplace. I wasn't going to make that mistake again! There's a lot of food for thought that you've served up here—so very helpful! Thank you!!!!!

 on: September 02, 2018, 01:09:11 pm 
Started by klaus - Last post by uwe ret
A few good measurements will always trump multiple opinions...

 on: September 02, 2018, 12:46:24 pm 
Started by klaus - Last post by Jim Williams
An objective analysis will tell "the rest of the story". Load effects on frequency response, THD effects from various loads, peaking of the response and other effects can then be easily documented for those without the time nor test gear to do their own research. This, like many other audio subjects is crying out for answers that can easily be obtained with sufficient effort.

 on: September 02, 2018, 09:02:09 am 
Started by klaus - Last post by soapfoot
Thanks, David!

And at risk of continuing the topic drift just a bit too long--what, in your mind, are some practical means of addressing (or circumnavigating) this issue in a methodical, but not-overly-technical way?

Let's say there are several recordists at a studio who have great ears and make great recordings, but whose background is more musical than technical (i.e. they have music degrees, and not EE degrees). How might they be made aware of this problem in a way they might grasp intuitively, and how might they test or control for it if they sense that it's becoming a factor with a given signal chain?

 on: September 01, 2018, 01:56:14 pm 
Started by klaus - Last post by David Satz
soapfoot, this board has always seemed to tolerate small amounts of "topic drift" as long as it doesn't get out of hand. Ultimately it's up to Klaus, of course. So I'll just post this one, tiny message (like all my postings), and then we can all return to the topic, OK?

The easy part first: Impedance matching (source impedance = transmission line impedance = load impedance) simply isn't relevant here. Leave that for RF circuitry, where it maximizes power transfer. As you said, we want to maximize voltage transfer, so a bridging approach is called for (with the load impedance at least an order of magnitude greater than the source impedance).

The technical problem is that real-world microphones and real-world preamps--especially where transformers are used on one or both sides--don't have purely resistive impedances. Thus their interactions can lead to audible frequency response variations and other problems. It seems misleading to specify impedance as a single number in such cases; a curve, or at least a numeric range, would be far more appropriate IMO. (Note that there are also microphones where the problem goes the opposite way--e.g. some ribbons have impedance curves that exceed 1 kOhm around their primary resonance. Those microphones are as "preamp sensitive" as microphones with very low impedance.)

The human side of the problem is that circuit designers test their work under certain practical conditions, and can't know whether their designs remain valid under other conditions or not. Langevin, for example, may have tested the preamp shown in Temmer's paper only with dynamic microphones, which were far more prevalent in American broadcasting at the time. American studios back then tried to keep equipment running for as long as they could--so in the 1960s, consoles from the 1950s were still in widespread use, with circuit designs based on microphones prevalent in U.S. studios even earlier.

There's a fundamental problem in evaluating anything that interacts with other things. Say you're a studio engineer, and you've always liked the sound of a certain preamp that you have. A client brings his or her own favorite microphone to a session and you plug it in, never having used that particular type of microphone before. Say it sounds harsh or muffled, or whatever--but not as good as people say that it's supposed to sound. You would normally think that you had just heard "what the microphone sounds like." You might never suspect that your impression was based on an impedance interaction like the ones we're talking about--essentially a malfunction in your trusty preamp. If you're an audiophile, and particularly if the preamp is very expensive, you might even think, "My preamp is so great, it lets me 'resolve' differences between microphones that other people don't hear with their inferior, 'lower-resolution' preamps." Both reactions are unconsciously biased but completely honest, and the experience will probably be very convincing for the person who has it.

This is what I think may be going on when people say that the 200 Ohm setting of a microphone sounds different (qualitatively) from the 50- or 150-Ohm setting. Basically, if any such qualitative difference is perceived, alarm bells should go off in people's minds, rather than stopping there and drawing conclusions about the "sound" of one impedance setting or another; such conclusions may not be generally valid.

--Back to practical solutions: If your preamp's frequency response depends on the microphone's output impedance as Temmer's paper shows, you might try to find out or figure out what kind of load the secondary winding of the input transformer is "working into" (driving). Those frequency response variations can sometimes be tamed by placing a shunt resistance, or possibly a parallel RC network, across the secondary. Or if you can't (or don't want to) get into the preamp circuitry, you can do what Hardy does when you're going to use a very-low-impedance microphone. That "pads down" the signals coming from the microphone, but not too severely, and it reduces the microphone's self-noise as well as any noise due to interference in the cable, exactly as much as it reduces the wanted signal. So unless your preamp is rather noisy, it won't harm the signal-to-noise ratio of the recording, and it may also help avoid preamp overload.

--best regards

 on: September 01, 2018, 08:30:54 am 
Started by klaus - Last post by soapfoot
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?)

 on: August 27, 2018, 04:01:56 pm 
Started by Mickeyrouse - Last post by eheiserman
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.

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