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Author Topic: Nagra IV-S and Sennheiser MKH 110  (Read 10221 times)

Offline bibio

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Nagra IV-S and Sennheiser MKH 110
« on: November 02, 2008, 11:34:06 AM »
I recently bought the MKH 110 and have had to build a special battery box to power the mic as it runs on 8v. Part of the circuit has a 220N capacitor on the audio out to isolate the DC on the cable from the mic pre. However, this causes a low frequency roll off that starts too high making the mic bass-light, and in its prime, it is anything but with a frequency response going down to 1Hz. It also causes a pinch/boost at 250Hz.

If I take the 220N out of the circuit, it stops the bass roll off but now there is a voltage going to the amp. I haven't tested this with my nagra as it is too precious. I tested the circuit, sans 220N, with my behringer desk which seemed fine. I've been told that if the amp contains a DC blocking capacitor there will be no need for the 220N.

Does anybody know if the Nagra IV-S mic pre contains a DC blocking capacitor? Also, if it doesn't, will my circuit cause any harm if I don't isolate the DC voltage from the amp?

There is also a possibility that the mic's extended low end may cause saturation if there is no roll off at all as those low infra-sonic frequencies may cause problems, so perhaps a capacitor will be needed but does anyone know what capacitor will help with the roll off?

Please bear in mind that I know nothing about electronics, so be gentle with me!


Offline steveidosound

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Re: Nagra IV-S and Sennheiser MKH 110
« Reply #1 on: November 02, 2008, 04:37:08 PM »
Others please jump in here, but wouldn't you just need to increase the size of the series DC blocking capacitor to extend the low frequency response?
Any capacitor by definition will block the DC, but the larger it is the lower the roll off point will be. It represents a 6dB per octave filter with the termination impedance of the microphone input.
Steve Williams

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Offline ironbut

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Re: Nagra IV-S and Sennheiser MKH 110
« Reply #2 on: November 02, 2008, 10:14:21 PM »
Really interesting mic you have bibio. I googled it when I saw your post. That thing must really get down there! I have to wonder if the 110 can be powered with  8v or must be powered with 8v. I have some Sennheissers and Audio Technicas that can be powered with 6v but they can also be powered with 48v too. If this is the case with your MKH's you shouldn't have to block the power coming from your Nagra. Just a thought.
Another thing you might try is to run the battery box into the line in rather than the mic in. I do that all the time with the flash recorders I use.
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Offline ironbut

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Re: Nagra IV-S and Sennheiser MKH 110
« Reply #3 on: November 02, 2008, 10:46:28 PM »
Here's some battery box plans you might compare with yours.
http://taperssection.com/index.php/topic,72994.0.html
steve koto
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Offline bibio

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Re: Nagra IV-S and Sennheiser MKH 110
« Reply #4 on: November 03, 2008, 05:20:53 AM »
Others please jump in here, but wouldn't you just need to increase the size of the series DC blocking capacitor to extend the low frequency response?
Any capacitor by definition will block the DC, but the larger it is the lower the roll off point will be. It represents a 6dB per octave filter with the termination impedance of the microphone input.

I've been told the same thing, so yes, I should try that. Another issue that I've just realised is that I'm planning on using the dynamic mic settings on the nagra, one is 50 ohms and the other 200 ohms. The MKH 110 manual states that an amplifier can be used whose impedance is >2kohms. So I need to raise the impedance considerably, I have been told this will affect the frequency response of the mic.

Any comments on this folks?


Offline bibio

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Re: Nagra IV-S and Sennheiser MKH 110
« Reply #5 on: November 03, 2008, 05:24:05 AM »
Really interesting mic you have bibio. I googled it when I saw your post. That thing must really get down there! I have to wonder if the 110 can be powered with  8v or must be powered with 8v. I have some Sennheissers and Audio Technicas that can be powered with 6v but they can also be powered with 48v too. If this is the case with your MKH's you shouldn't have to block the power coming from your Nagra. Just a thought.
Another thing you might try is to run the battery box into the line in rather than the mic in. I do that all the time with the flash recorders I use.


I'm not really planning on using it as an infrasonic mic, but maybe one day... besides, I can't imagine the nagra I have will extend that low. Nagra made the IV-SJ for jobs like that, which was indeed the recorder used with the MKH 110 by recordists needing the infrasonic range.

The manual for the MKH 110 says it can run on 8v +/-1v, so I should stick to that.

If I run into the line, how will I get the gain? The box I have built is only a power supply.

Offline bibio

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Re: Nagra IV-S and Sennheiser MKH 110
« Reply #6 on: November 03, 2008, 05:24:48 AM »

Offline steveidosound

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Re: Nagra IV-S and Sennheiser MKH 110
« Reply #7 on: November 03, 2008, 01:33:03 PM »
http://www.sennheiser.com/sennheiser/old_manual.nsf/resources/MKH_110_oNr_oDatum_Sp3.pdf/$File/MKH_110_oNr_oDatum_Sp3.pdf

This shows a formula for determining  DC blocking capacitor size for a given mic input impedance and also some info on powering it off of higher voltage supplies such as your Nagra might already have.

The DC blocking capacitor effectively raises the native input impedance of your mic preamp going higher as the frequency goes lower.
But at mid frequencies the capacitor should have no effect - leaving the input impedance of the Nagra well below the 2K ohm min. specified. This means you are somewhat loading down the output of the mic too far and that could cause distortion issues at higher signal levels as the mic will not be able to properly drive the lower impedance load of the mic preamp in the recorder. The old way to deal with this was with an external transformer.
They were typically made to use an unbalanced high impedance mic with a balanced low impedance microphone preamp. Or the other way around.
But the high impedance unbalanced side of these were much higher than your mic requires - typically 50K ohms or even higher.
Your solution would need to be something that translates 2-3K ohms to match the Nagra mic preamp impedance settings  AND allows the phantom power of the Nagra to be changed to the 8V required by the mic. or blocks the external DC supply from the deck. A resistor could be used to "build out" the impedance to the proper load for the mic, but you will loose sensitivity (and or signal to noise) in the process.

It sounds as if you mic was designed to the sort of spec. of the modern mic input on camcorders and other small mini jack based devices which are typically stereo, medium impedance of a few K ohms and have a small DC power component  of a few volts riding on the mic connection. My Sony camcorder, old pro-walkman cassette, mini-disc and DAT recorders all have such stereo mini mic jacks with the trade name "plug-n-power" or some such thing for a series of little condenser mics made for use in those applications. 
Steve Williams

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Offline sound signal

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Re: Nagra IV-S and Sennheiser MKH 110
« Reply #8 on: November 03, 2008, 06:11:16 PM »
Hi Bibio,

The mike preamp in the Nagra IV-S has a five-position switch for each channel, to set the preamp for microphone types  "Dyn 50" (dynamic 50 Ohm), "Dyn 200" (dynamic 200 Ohm), "+12" (condenser +12V phantom power), "+48" (condenser +48V phantom power), "-12V" (condenser -12V Phantom power), and "T" (condenser 12V T-power).  Unfortunately you can't use any of these to power your MKH-110, it needs its own power supply which doesn't match any of these standards.  But you can send the signal into the mike pre.

It has an input transformer which is switched to set appropriate gain and input impedance for each of these microphone types.  "Dyn 50" and "Dyn 200" are, of course, 50 Ohm and 200 Ohm each.  Unfortunately the Nagra specs don't mention the input impedance for the condenser settings but an educated guess suggests that it should be fairly close to Sennheiser's specified minimum of 2k, so you should be OK.

It also has an input blocking capacitor that is switched into circuit for all the condenser types "+48", "+12", "-12" and "T", that is 22uF in value.

But you don't want the supply voltages from the Nagra going to your mike so you'll need an external  blocking capacitor in line anyway.  If you use another 22uF in line, you will have, in effect, 11uF which gives a roll-off below 3Hz.  If that sounds too low, don't worry, your Nagra introduces several other roll-offs before the signal ever hits tape.

So, this is my humble suggestion: Send the output from your mike (pin 1) via a 22uF cap to the Nagra mike pre input pin 3.  Send the 0V from your mike (pin 2) to the Nagra mike pre input pin 2.  Send the earth of your mike (connector shell) to the Nagra mike input pin 1.  Switch the mike pre to "T" and you should be well away.  If you find the mike is overloading, or is bass light, wire a 1k resistor in series with the Nagra mike input pin 2 or pin 3 and you should be OK.

Usual disclaimer applies. I am not responsible for damage to your Sennheiser, your Nagra, your physical or mental health, your pet or significant other incurred while attempting anything related to the above.  But I am confident you should be OK.

Do let us all know how you get on.

With best regards,
George Karaolides
Nicosia, Cyprus