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Author Topic: U87ai transformer strapping  (Read 459 times)

Donn

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U87ai transformer strapping
« on: April 26, 2019, 03:35:44 am »

I have heard that some U87ai owners occasionally request that their U87ai have its transformer strapped to 50 ohms instead of 200. It seems it was more common to do it the other way around on classic U87's that came strapped to 50 ohms, but apparently some do it "backwards" with Ai's to reduce output if they are using it through a sensitive mic pre. Besides possible S/N ratio changes, is there any tonal downsides, or shifts in tonality whatsoever from this modification? Are there other reasons besides reducing output that one might strap to 50 instead of 200? Does it affect high or low frequency response in any way? Thanks for any insight you might be able to offer.

Donn
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klaus

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Re: U87ai transformer strapping
« Reply #1 on: April 26, 2019, 01:38:53 pm »

I am sure David Satz's ears are already perking up as he will shortly compose and post another detailed explanation about impedance matching of mics and mic pres.

I will address the 'tonal" issue, based of MY personal experience.
Consider how Neumann and others achieve the output impedance settings: two secondary coils of the output transformer are either connected in series (200Ω) or parallel (50Ω).

All else being equal (and it never is!) I prefer the timbre of the two coils connected in series. I will get an audible robustness in the mids with 200Ω, versus a glassy, a bit anemic overall timbre with 50Ω. 

Try both and report back!
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Klaus Heyne
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David Satz

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Re: U87ai transformer strapping
« Reply #2 on: April 26, 2019, 05:04:03 pm »

> I am sure David Satz's ears are already perking up ...

No, actually your post was on line for well over an hour before my radar detected it.

Look, I can't (and don't want to) contradict anyone's listening experience. All I want to do is point out two things that are fundamental, well known, and (I hope) non-controversial.

[1] Impedance interactions between microphones and preamps can be complex, especially where transformers are present on either or both ends. The impedances of audio equipment with input or output transformers are specified as if they were purely resistive, but they're really not. So if you change the output impedance of a microphone, the audible result that you get with one preamp may not carry over (or not 100%) to other preamps, because different preamps have such different input circuits.

This is especially true if the input impedance of any cable+preamp combination that you use is less than about 10 times the output impedance of your microphone at any frequencies that occur in your testing. That is a recipe for "the sound of the microphone" to depend more on the details of that preamp's input impedance. If you have a preamp with adjustable input impedance, and you set it to its lowest-impedance setting (say, 600 or even 300 Ohms), you'll increase the likelihood of hearing some result of an impedance interaction. But would you really characterize a microphone on the basis of an "edge case" where you aren't operating it the way it was designed and specified to operate? That wouldn't make sense to me.

[Side note 1a] If a microphone's output impedance is high enough and the preamp's input impedance is low enough and/or the cable's capacitance is high enough, the output stages of some microphones that have output transformers can misbehave due to slew rate limiting. This can be audible at high levels and high frequencies, and the risk is greater when the output transformer is set to higher-impedance settings.

[Side note 1b] There are preamps--especially some that use input transformers with a large voltage step-up--that have audibly different high-frequency response depending on the impedance that's driving them. When that occurs, their impulse response is likewise affected.

[2] Whenever you strap the output stage of a microphone to a different impedance setting, you change its sensitivity (i.e. its output voltage as a function of the SPL). To compare one strapping versus another requires a preamp gain adjustment that is equal and opposite to the change in the microphone's sensitivity. Otherwise any listening comparison will be thrown off. Even experts may perceive small differences in level (that are too small to register consciously as such) as differences in sound quality.

In summary (ladies and gentlemen of the jury), the idea that you can restrap a microphone, plug it into a preamp, listen to the combination and say, "This microphone now sounds more ___ (or less ___) than it did before" just isn't realistic.

Again, I'm not disputing what anyone hears when I say that. But why characterize the microphone as sounding a certain way, if that perception depends on the relationship between its impedance AND the preamp AND the cable AND possibly the preamp's impedance setting? I mean, use any combination of anything that you like--but don't attribute what you hear to the wrong thing.

--best regards
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Donn

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Re: U87ai transformer strapping
« Reply #3 on: April 26, 2019, 11:04:04 pm »

Thanks for the replies. From this information, I see no need to change the current impedance. None of the preamps I'm using are so sensitive that a simple gain adjustment won't work.  Besides, I'm in the process of having the daughterboard removed and the mic modded to a single FET, and I want to hear the difference one modification at a time before doing anything else.

Donn
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klaus

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Re: U87ai transformer strapping
« Reply #4 on: April 27, 2019, 12:09:40 am »

Quote
...why characterize the microphone as sounding a certain way, if that perception depends on the relationship between its impedance AND the preamp AND the cable AND possibly the preamp's impedance setting? I mean, use any combination of anything that you like--but don't attribute what you hear to the wrong thing.

I feel comfortably confident to characterize the sound of an output transformer's secondary in series or in parallel as previously stated: Regardless of choice of mics (tube or FET), pre's, cables and other variables you mention, the specific timbres of these two settings appear to remain consistent.
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Klaus Heyne
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