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Author Topic: AKG C451EB 48V Mod?  (Read 630 times)

Spindrift

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AKG C451EB 48V Mod?
« on: March 30, 2019, 03:47:42 pm »

This post is related to the recent thread regarding snipping the jumper on the C414EB to lock it to 48V and ease the load on phantom supplies: 

https://repforums.prosoundweb.com/index.php/topic,37331.msg538131.html#msg538131

Does the AKG C451EB have a similar modification which can be performed which limits it to a 48V supply?  48V is the standard in phantom supplies in this day and age and I'm looking to lessen the load on my phantom supplies when employing these mics. I understand that the C452 was the strict 48V model but I'd like to not have to go hunting down more mic bodies.

Thanks,
Keith Banning
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David Satz

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Re: AKG C451EB 48V Mod?
« Reply #1 on: April 20, 2019, 04:10:43 pm »

I've been reluctant to reply to this posting because I don't have the specific information at hand. The C 451 was basically a 12-Volt microphone with protection to let it work safely with higher supply voltages as well. But when fed at 48 Volts it drew about 5.5 mA, back when the DIN standard prescribed only 2 mA per microphone.

The page shown below is from AKG's 1979 catalog. A few years after the C 451 was introduced, after problems and complaints arose, AKG introduced a variant model C 452 exclusively for 48-Volt phantom powering. It drew 3 mA--still greater than the value set by the DIN standard--but it brought fewer power supplies to their knees. I remember looking at its circuit and noting that it differed from the C 451 in the value of just one resistor.

Unfortunately I can't find the schematics right now, so I can't tell you which resistor to change to what value. But someone else here probably will know that--and if so, I hope that they'll post here in reply. The changeover can be made quite easily.

--best regards
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klaus

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Re: AKG C451EB 48V Mod?
« Reply #2 on: April 20, 2019, 10:52:08 pm »

In the 414EB, the conversion from 9-12VDC to 48VDC phantom powering is achieved by cutting a wire shorting bridge right at the line from the transformer center tap. This activates an 8.2kΩ resistor in series with the 330Ω resistor already present.

The 451also has the 330Ω resistor in the same position, but does not have the 8.2kΩ resistor option for phantom.

I am missing the parts supply sheet for the 452, but its schematic-identical with the 451- also shows one resistor in the same position as the 451's 330Ω resistor. I assume that resistor is an 8.2kΩ value, or thereabouts.

Conclusion: replace the 330Ω resistor in your 451 with an 8.2kΩ - 1/4 watt resistor, and your mic should be well suited for phantom supplies.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Kai

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Re: AKG C451EB 48V Mod?
« Reply #3 on: April 25, 2019, 01:50:06 pm »

I suggest to measure the voltage on the upper part of the 330 Ohms resistor in the circuit, before and after the mod, with high level audio signal present.
If this voltage does not change with a higher value resistor everything is fine.
You can even try a higher value (use a trimmer), until the ( B+ ) voltage starts to drop, and then dial back 30%.

The only question that arises: why bother, I don't know any device that cannot supply sufficient phantom power.


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David Satz

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Re: AKG C451EB 48V Mod?
« Reply #4 on: April 26, 2019, 05:24:55 pm »

Kai, I'm a little surprised; there are plenty of preamps, mixers and recorders (especially older and/or portable ones, but not exclusively) that can't supply 5.5 mA per microphone. One example is up on eBay right now: the Neumann N 80--a lightweight, low-cost two-channel AC supply with a limit of 2 mA for both channels combined. Neumann's BS 945 battery power supply had a 1 mA limit!

The Nagra IV-S recorder could only supply about 2 mA per microphone when set for 48-Volt phantom powering, unless specially modified; for years I used 12-Volt phantom microphones with it for that reason.

--I have some phantom power testers (Schoeps PHS 48) each of which has a green LED that lights up when the tester is plugged into a microphone input that can power a 48-Volt Schoeps "Colette"-series microphone (4 to 4.5 mA). I've sometimes brought four of them with me to AES conventions, and plugged them in (with permission, of course) to the XLR mike sockets of mixers, preamps or recorders that I saw there. Many times they didn't light up. Or the first one would light up when I plugged it in by itself, but then when the second one was plugged in, the first one would go out and the second one wouldn't light up at all.

Just a few months ago I looked at a small portable recorder that had 48-Volt phantom powering but didn't specify the available current. So I brought a tester to the dealer's, explained the situation, and asked their permission to try it. The tester didn't light up, and I bought another recorder instead.

--best regards
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Kai

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Re: AKG C451EB 48V Mod?
« Reply #5 on: April 26, 2019, 05:45:42 pm »

...there are plenty of preamps, mixers and recorders (especially older and/or portable ones, but not exclusively) that can't supply 5.5 mA per microphone. ...
The Nagra IV-S recorder could only supply about 2 mA per microphone when set for 48-Volt phantom powering, unless specially modified; for years I used 12-Volt phantom microphones with it for that reason.
 ...
Unmodified the 451 can work with almost every voltage available.
If a P48 PSU cannot supply 5.5mA @ 48 V it often can practically supply this current at a lower voltage, working perfectly fine with the C451.
Or, like the ZOOM stuff, can be switched to 24V phantom power.

Anyway the C451 series aren't serious field recording mics IMO.
And modifying them to come down to 3.3mA from 5.5mA doesn't help much.
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