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Old July 13th, 2010, 07:11 PM   #1
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Blocking Capacitor what value? dynamic stick mic to tx beltpack.

Hi
so there's been talk before about this but what value cap' shall I get?

It is to connect a Beyer M58 to an old Micron tx502 belt pack.

It is working fine with the xlr wired to 8 pin lemo but do not want it to break down.
I thought I might pop down Maplins tomorrow and get one. Just need to know what type and value.

cheers and hope John will chime in on this.
Jim
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Old July 13th, 2010, 10:35 PM   #2
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10uF tantalum, at least 10V rating. put in series to block the DC, + lead toward the transmitter
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Old July 13th, 2010, 10:44 PM   #3
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What value should I use for the capacitor(s)?

Short answer:
something between 1 to 5 microfarads @ 5v.

Long answer:
The capacitor value depends on:
1) The input impedance of the camcorder
2) The low-frequency rolloff (3dB down frequency)

I use this web page...
Opamp Labs Inc <> RFC Calculator <> www.opamplabs.com
to calculate the capacitor value.

For example, if your camcorder input impedance is 2500 ohms and you want frequency response down to 20 Hz, then you would use a 3.18 microfarad capacitor.

You can type in any two numbers and it will solve for the third. For example, if I still assume 2500 ohms input impedance, and plug in 1 microfarad, then I get a 63.6 Hz 3dB down point.

Note that most camcorders don't have published specifications for mic input impedance.

Note also that you may not want a lot of low frequency response unless you are recording flamenco dancers on a hardwood floor or a steam engine, or a pipe organ. Most low frequency sound out there is undesirable noise (at least for the purposes of video!)

Any capacitor which has the required capacitance and voltage values can be used. If you want to put the cap inside the XLR connector shell, you will need a pretty small one.

Surface-mount capacitors of appropriate capacitance and voltage values are now readily available, but their tiny size means that you must be careful handling and soldering to them.

From my web page on the subject...
Camcorder Mic Adapters
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Old July 14th, 2010, 06:24 AM   #4
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Quote:
Originally Posted by Greg Bellotte View Post
10uF tantalum, at least 10V rating. put in series to block the DC, + lead toward the transmitter
Thank you Greg.
I've just done this and al is good. I'll have a look to see if it has acted as a hi pass filter a little later. The set up is currently on a soak test to check it lasts. I have an important job I'm using this for so I'm very gratefull.

It did work without the cap' but I had not had it powered up for more than 5 minutes and I would rather avoid stray current on the mic and lead. This is for a job outside Buckingham Palace and around London vox popping the public.

cheers
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Old July 14th, 2010, 12:01 PM   #5
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I've not measured the high pass of 10uF, but it's the value I found in a Sennheiser CM-1 cable. Glad it worked for you.
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Old July 14th, 2010, 02:56 PM   #6
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Out of interest what would happen if the capacitor was wired in series with the opposite polarity?
ie + facing the mic?
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