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Author Topic: "Stephen Paul Audio" U47-To-FET Conversion?  (Read 6293 times)

Murray Trider

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"Stephen Paul Audio" U47-To-FET Conversion?
« on: April 30, 2016, 02:32:36 PM »

Hi Klaus,

Last night I purchased this Neumann U47.  It had a FET conversion done by Stephen Paul Audio in 1989.  The capsule has the SPA 3 micron modification.  It sounds fantastic, even without the VF14 M telefunken tube. 

Ideally, I would like to fully restore this mic to it's original "tube" state but retain the SPA 3 micron capsule. 

Have you seen this FET conversion before?  How much of this mic has been modified? How difficult would it be to make this a tube U47 again?

Thanks so much!
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soapfoot

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Re: SPA U47 FET Conversion?
« Reply #1 on: April 30, 2016, 02:37:24 PM »

Out of curiosity--

Why is it that you seem sure you'd like to retain the 3 micron capsule? Is it merely a matter of cost savings?

Excellent K47 capsules are still available new from Neumann, so for a modest (but substantial) investment you could easily get a proper K47 in there-- the one that was designed to work with the BV8 transformer and the VF14 tube.

It's worth pointing out that nobody ever thought anything was wrong with the U47 in its original capsule configuration!
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Jim Williams

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Re: SPA U47 FET Conversion?
« Reply #2 on: May 01, 2016, 12:56:15 PM »

Tube to fet conversions were popular back in the 1980's. Some could not find replacement tubes, some wanted more reliability, some wanted something different. Fet 47's should have answered that need.

The thin diaphram mods were also popular in LA circles back then. Unlike today, folks back then were searching for more transparency and less colors. Back then it was all about the artist, now it's all about the gear. It was also popular back then to remove audio transformers from the signal path
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klaus

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Re: SPA U47 FET Conversion?
« Reply #3 on: May 01, 2016, 02:18:21 PM »

...folks back then were searching for more transparency and less colors...

The question, back then as now, is: do thinner diaphragms achieve that goal? Wouldn't "more transparency" indicate higher (musical) fidelity? I never found that to be the case with thin-film LD diaphragms. They seemed to add, rather than remove, an undesirable component from the audio: glassiness and a lack of musical texture.
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Klaus Heyne
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David Satz

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Re: "Stephen Paul Audio" U47-To-FET Conversion?
« Reply #4 on: May 01, 2016, 04:13:45 PM »

There's always a problem when an experiment has multiple variables: You can never trace observed effects to any particular cause.

I had several conversations with the late Stephen Paul about his microphone modifications. He emphasized that it wasn't simply a matter of exchanging heavier diaphragms for lighter ones, but that he also reworked the capsule backplates. And while I certainly don't know everything that he did, I've never heard of a case in which he modified the capsule of a microphone without also altering its audio circuitry. His U 87 modifications, for example, undid some of the high-frequency filtering of the original U 87 circuit.

As a result I don't think that his customers are in any position to judge how much sonic effect, for better or worse, is due to his capsule alterations, and how much comes from circuit modifications. Even if they switch capsule heads for stock ones, they couldn't know how much difference is due to lighter diaphragm material and how much is due to remachining the backplate and other possible adjustments to the very finely-balanced dimensions of a capsule design.

Apparently many people consider it "intuitively obvious" that reduced diaphragm mass will directly yield better high frequency response and "quicker" transient response. And this "intuitively obvious" understanding is no doubt good for sales. But it is based on vague or mistaken notions of how the capsules in condenser microphones work--even setting aside the whole fundamentally mistaken concept of "quicker" vs. "slower" transient response (as if impulse response could ever be separated from bandwidth and frequency response).

The motion of the diaphragm in a condenser microphone is constrained by its stiffness (not its mass within a reasonable range), by the damping effect of the air in the chambers on both sides of the backplate, and by the friction which the air behind the diaphragm undergoes as it passes in a back-and-forth manner through the holes and other "aerodynamic" features of the backplate (e.g. the grooves of the "linear admittance" K 64 capsule). And as far as diaphragm mass is concerned, unless you operate a microphone in a vacuum or underwater, its diaphragm always moves in tandem with layers of air both in front and in back of it--the mass of which is significantly greater than that of the diaphragm.

As a result, once a microphone diaphragm has been placed under the tension needed to create the design-appropriate resonant frequency, its mass (within a reasonable range) doesn't play a primary role in the capsule's behavior--just as its material composition, strictly speaking, doesn't. These things count only as second- or third-order details.

We might also consider the fact that Neumann has had 3-micron Mylar available to them for use as a diaphragm material for over fifty years now, and they've actually used it in their small-diaphragm capsules since 1964. If it gave audibly superior performance in large-diaphragm microphone capsules, they could easily have chosen it when they designed (for example) the K 89 capsule--but they didn't. Meanwhile, at least one other manufacturer uses 6-micron Mylar for their small-diaphragm capsules, and they get ruler-flat response up to 20 kHz with very clean impulse response.

So this is not as simple and "intuitively obvious" a scenario as the sales talk might lead people to imagine. Physics that requires some study seems to be involved ...

--best regards
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klaus

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Re: "Stephen Paul Audio" U47-To-FET Conversion?
« Reply #5 on: May 01, 2016, 09:26:53 PM »

... And while I certainly don't know everything that he did, I've never heard of a case in which he modified the capsule of a microphone without also altering its audio circuitry.

Aside of a few U87 where the high frequency feedback was altered, every SPA mic I have seen here, and there have been many including lots of U87, had original circuitry and original values of components, but many of these mics featuring different materials for passive components like coupling capacitors and high impedance resistors.

I agree with David that SP's thin-film capsules are a complex issue not easily reduced to just the 3µ or thinner diaphragms. But the sound I associate with these capsules and described earlier, regardless of how much thickness he also shaved off the backplates, is somehow absent when thin diaphragms are replaced with standard 6µ original AKG diaphragms (I did that a few times with SP CK12 capsules, where the diaphragms had collapsed): here, the prominence of the presence range was noticeable, rather than the texture or high frequency or "speed".
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Klaus Heyne
German Masterworks®
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Murray Trider

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Re: "Stephen Paul Audio" U47-To-FET Conversion?
« Reply #6 on: May 03, 2016, 12:20:31 PM »

Thanks to all for this wealth of information!  I'm really enjoying the tone and character of this mic even with these modifications.  As much as I'd love to fully restore it, I feel like sourcing a VF14m tube at this point may be a futile pursuit.
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hasbeen

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Re: SPA U47 FET Conversion?
« Reply #7 on: May 10, 2016, 10:39:00 AM »



Excellent K47 capsules are still available new from Neumann, so for a modest (but substantial) investment you could easily get a proper K47 in there-- the one that was designed to work with the BV8 transformer and the VF14 tube.

Is this true? Can it be purchased with a U47 head basket as a plug and play? And if so, is this direct from Neumann USA? Thanks
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klaus

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Re: "Stephen Paul Audio" U47-To-FET Conversion?
« Reply #8 on: May 10, 2016, 12:15:07 PM »

No plug & play. You buy the capsule separately and gather the other parts necessary to assemble the head, either as old stock or replica: capsule mount, switch plate assembly and basket.
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Klaus Heyne
German Masterworks®
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Recording Engineer

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Re: "Stephen Paul Audio" U47-To-FET Conversion?
« Reply #9 on: August 31, 2016, 02:14:25 AM »

I too had several full-day conversations with the late Stephen Paul a couple years prior to his passing and one of them included his approach to his U67 mods and he really seemed to emphasized how thin-diaphragms, backplate modifications, as well as circuit modifications (including resistors manufactured specific for SPA) was really all a total system. It reminds me of quite a few other great microphone designers talking about the sound of vintage microphones, such as the U47, is a result of a total system. Make sense to me!
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