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

Neumann U67 Reissue: Complete Tear Down and Analysis

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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. 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.

The human side of the problem is that circuit designers test their work under certain practical conditions, and can't always anticipate the conditions that might occur when the technology changes in the future. 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; in the 1960s, consoles from the 1950s were still in widespread use, with input circuits based on the types of microphones prevalent in U.S. studios even earlier.

It's fundamentally problematic to evaluate the "sound" of a component that can't be isolated from the influences of other components. 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 know little or nothing about that type of microphone, but OK, you plug it in. Say that it sounds harsh, or muffled, whatever. Normally you would think that you'd just heard "the microphone" and that you now know "what it sounds like." You would then have an opinion based solidly on first-hand listening experience--exactly what you feel that you can count on. You might never suspect that your impression resulted from 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 "biased"--by which I don't mean "based on favoritism". People can be 100% fair and open-minded, but when uncontrolled factors skew the outcome, listeners are misled by the very thing we want to trust the most, our own direct experience. Our confidence in our own opinion will then be diametrically opposite to the actual validity of that opinion--but we won't be aware of it.

That's what I think may be going on when people say that the 200 Ohm setting of a microphone sounds different to them (qualitatively) from the 50- or 150-Ohm setting of the same microphone. Basically, if any such difference in sound quality is perceived, rather than drawing conclusions about the "sound" of one impedance setting or another, alarm bells should go off in people's minds; such conclusions aren't likely to 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

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?

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.

uwe ret:
A few good measurements will always trump multiple opinions...

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!!!!!

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