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Old October 22nd, 2004, 10:59 PM   #1
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Impedance and level considerations

I am about to buy my first mic, so all I know is what I have read. Based on the recommendations in this forum, I have decided to buy an AT4073 for my Panasonic PV-DV953. Now this camcorder only has a passive minijack input, and the microphone requires 11-52V phantom power, which I am planning to draw from a BeachTek DXA-6.
  1. Would a cheaper preamp defeat the purpose of having a mic of this quality?
  2. Is more phantom power better (with regard to the preamp) and why?
  3. Are these devices impedance matched/compatible? How do you tell?
  4. Are these devices voltage level matched (i.e., will I get clipping)? The preamp appears to have no indicators!
  5. Is the mic jack on the camcorder really that fragile?
  6. Should I always have some form of wind protection when outdoors, like I would put a UV filter over my lens?
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Old October 23rd, 2004, 12:56 AM   #2
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Re: Impedance and level considerations

<<<-- Originally posted by Emre Safak : I am about to buy my first mic, so all I know is what I have read. Based on the recommendations in this forum, I have decided to buy an AT4073 for my Panasonic PV-DV953. Now this camcorder only has a passive minijack input, and the microphone requires 11-52V phantom power, which I am planning to draw from a BeachTek DXA-6.
  1. Would a cheaper preamp defeat the purpose of having a mic of this quality?

    Possibly. More probably, it would significantly reduce your flexibility by only allowing you to use a single input, whereas a BeachTek allows you to use the left and right channel independantly. You can use your AT shotgun on one channel, and a wireless lav or a handheld vocal mic on the other, for example.
  2. Is more phantom power better (with regard to the preamp) and why?

    No. There was a time when Phantom Power voltage wasn't standardized, and there is still plenty of older but high-quality equipment in regular use with that supplies or uses Phantom Power at something other than the now-standard 48v. As long as both pieces of equipment are compatible, you'll have no problem. All modern equipment, including the BeachTek, uses the standard 48v, which is within the usable range of the mic.
  3. Are these devices impedance matched/compatible? How do you tell?

    Yes. The BeachTek is designed for a low impedence mic input, and virtually all professional mics are balanced, low impedence mics, whether they're dynamic or condenser mics. Usually only cheap hobby/entry-level mics are high-impedence, unbalanced designs.
  4. Are these devices voltage level matched (i.e., will I get clipping)? The preamp appears to have no indicators!

    The process of getting the levels set properly across all components of your equipment is called "setting the gain structure". I wrote a bit about how to do that with a similar setup here:

    http://www.dvinfo.net/conf/showthrea...threadid=33332
  5. Is the mic jack on the camcorder really that fragile?

    Yes, it is. Treat it very carefully, and always use right-angle 1/8" connectors (these have the least leverage on the jack if they're pulled) and if you're using a mic or some other input that's directly connected to the jack, make sure you secure the cord to the carry handle of the camera with some slack in it, so that a tug on the cable won't go directly to the jack. With the BeachTek mounted to the bottom of the camera, this danger is greatly reduced.

  6. Should I always have some form of wind protection when outdoors, like I would put a UV filter over my lens?
-->>>

Outdoors, probably yes. As a rule, you always want the least amount of wind protection that is acceptable, but better to have it and have a small amount of "coloring" on the sound than to have wind whistling in your recordings. You need more protection as wind speed increases, of course.

-Troy
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Old October 25th, 2004, 12:55 PM   #3
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I did some more reading, and I would like to know if I correctly understood the following:

The Beachteks have maximum mic input voltage of 880mV. The AT4073a has an open circuit sensitivity of 70.8mV, and a maximum input sound level of 126dB SPL. Since Audio Technica uses a base pressure level of 1 Pa=94dB, does this mean that the peak voltage is 70.8 x 10^((126-94)/20)=2.82V?

If this is so, then the AT4073a is capable of overloading the Beachtek, and an XLR inline attenuator may be required. However this is rarely likely to occur in practice. According to my calculations, the attenuation factor has to be 10dB at maximum SPL.
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Old October 25th, 2004, 02:36 PM   #4
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The DXA-6 is a passive device, it supplies phantom and attenuates only, it provides no gain. I wouldn't worry about overloading it.
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Old October 25th, 2004, 08:09 PM   #5
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Emre,
I agree with your calculations except for one adjustment. When connected to the BeachTek the mic is no longer looking at an open circuit. The open circuit voltage will divide in proportion the the impedances. Since the mic has 100 ohms and the BeachTek about 600, the Beach would get most of it. I also agree with your assumption that you rarely have to worry about encountering 126 dB.

I didn't realize that there was a maximum input level spec on the DXA-6. My only guess is that above 880 mV the transformer core may start to saturate, causing the response to become nonlinear.
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