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Author Topic: EF14 Power Supply for VF14 Mic  (Read 656 times)

industrialarts

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EF14 Power Supply for VF14 Mic
« on: February 08, 2021, 06:49:03 PM »

A friend would like to mod an aftermarket supply he has that was for an EF14 U47 (which he no longer owns) to be used with a U47 with a VF14.  Would it be sufficient to just remove the EF14 heater supply voltage or would it be necessary to connect the microphone pin 4 to ground (this supply is using pin 4 for the heater voltage).  Since it's tied internally in the mic is the additional grounded required?  This power supply is not wired like the Oliver Archut alternative, it has a 5 pin XLR

Thanks
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klaus

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Re: EF14 Power Supply for VF14 Mic
« Reply #1 on: February 08, 2021, 10:57:44 PM »

Changing the wiring scheme is going to be the least of your problems when adapting a power supply with separate heater and B+ (high voltage) outputs to generate a single voltage that can power a VF14- equipped U47.

Typically, the B+ section of these supplies cannot produce 40mA+ @105VDC required by the U47 with VF14. You would have to investigate the B+ side of the existing supply's input transformer, output, to confirm it has enough current capability. Then you have to probably beef up filtering, then...
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Klaus Heyne
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industrialarts

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Re: EF14 Power Supply for VF14 Mic
« Reply #2 on: February 09, 2021, 07:45:09 PM »

Yes, I thought about the current draw.  I found the specs on the B+ transformer so I would have to do some math  :P to check if it can supply enough current but my first thought is no since I believe the secondary is rated around 120vac@50ma. 

Are there other microphones that could use this supply?

Thanks for your help

Mark
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klaus

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Re: EF14 Power Supply for VF14 Mic
« Reply #3 on: February 09, 2021, 07:50:56 PM »

Not sure what you mean with "this supply".
Plenty of tube mic supplies use a variant of 6.3V heater and 120V B+, but I know of no other supply aside of Scott Hampton's that delivers 105VDC@40mA.
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Klaus Heyne
German Masterworks®
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Kai

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Re: EF14 Power Supply for VF14 Mic
« Reply #4 on: February 10, 2021, 12:44:10 PM »

I found the specs on the B+ transformer so I would have to do some math  :P to check if it can supply enough current but my first thought is no since I believe the secondary is rated around 120vac@50ma. 
If the rating is correct it could work.
But - the result wouldn’t exactly equal that of the original Neumann NG PSU.

With 125 V AC secondary (estimate, as you draw 40 mA instead of the rated 50 mA, and don’t load the 6.3 V windings) you end up at ca. 175 V DC after the rectifier.
This leaves 70 V for RC filtering.
70 V drop appears at 40 mA through a total of 1750 Ohms filter resistors.
You would need ca. 3 times higher uF filter caps, albeit with only 200 V max. voltage.

The exact value for the filter resistors need to be determined empirical, and the open circuit DC voltage needs to be checked to specify the filter caps.
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klaus

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Re: EF14 Power Supply for VF14 Mic
« Reply #5 on: February 10, 2021, 02:39:21 PM »

Just get the Hamptone and you are done. Nothing finer this side of an original NG.
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Klaus Heyne
German Masterworks®
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industrialarts

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Re: EF14 Power Supply for VF14 Mic
« Reply #6 on: February 10, 2021, 08:15:28 PM »

Quote
The exact value for the filter resistors need to be determined empirical, and the open circuit DC voltage needs to be checked to specify the filter caps.

Actually, it's a very nicely built, regulated supply with separate transformers for each supply, so I think I'll leave it alone.  Theoretically, I guess it could work but I'll tell the owner to leave it as is.

Quote
Not sure what you mean with "this supply".
Plenty of tube mic supplies use a variant of 6.3V heater and 120V B+

I meant the U47,EF14 supply I have here on my bench. The schematic for the U47 shows a B+ of 105v while a mic like the KM56 or M49 have B+ of 120v.  I can't think of any other mics with B+ 105 but then I am not familiar with the working voltages of all that many mics, just the ones that have crossed my bench.  Also, if a mic were to use the separate 6.3 v heater supply then it wouldn't require 40ma, am I correct?

It might be nice to have this as a backup supply for other mics instead of selling it

Thanks for your replies and advice
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klaus

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Re: EF14 Power Supply for VF14 Mic
« Reply #7 on: February 10, 2021, 09:33:07 PM »

I can only quote myself from your quotes of myself:
Quote
Plenty of tube mic supplies use a variant of 6.3V heater and 120V B+
Quote
I know of no other supply aside of Scott Hampton's that delivers 105VDC@40mA.

It follows that it would be prudent to keep the EF14 supply around, if it was well built, well stabilized and well filtered, as many dual-voltage tube mics will work with that voltage configuration.



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Klaus Heyne
German Masterworks®
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