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Old January 10th, 2009, 01:33 AM   #1
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Struggling with a Beachtek DXA-FX

When I bought my Sony FX7 used a while ago, it came a Beachtek DXA-FX XLR adapter/pre-amp (dual XLRs, adjustable -10dB to +15dB).

I finally got around to trying out the DXA-FX over the last few days, with the aim of pairing a Senn G3 wireless lav and receiver kit with either my AT822 mic on-camera (for ambience) or else my AKG CK98 on a boom (for interviews).

I listened to Guy C.'s wireless tutorial on Vimeo, hooked everything up, turned on the Beachtek ... and was kinda appalled at the amount of hiss! Figured I had something set wrong somewhere, so have been fiddling ever since.

To get any reasonable volume out of the AT822 and CK98, I need to have their Beachtek channel on at least +10dB and the camera audio level at about "5" (not actually numbered on the FX7). At those settings the Beacktek is producing about a -40dB noise floor to my recordings. Yuk!

If I crank up the wireless AF level to +6 or +12dB and turn the Beachtek gain down to 0dB and the camera level down to "3", as Gary recommends...I can get reasonable recordings from the lav, with the noise down at around -52dB ..... but the AT822 and CK98 are barely audible.

In contrast, if I plug either the AT822 or CK98 directly to the FX7's miniplug, bypassing the Beachtek, I can achieve good volume (with the camera at about "5.5") and lower noise than either of the above two scenarios.

So .....I'm trying to decide if my expectations were too high, or whether the Beachtek might be faulty, and/or whether it's me that's faulty (likely! ha!). And whether a Juicedlink adapter would be worth investing in as an alternative to the Beacktek???

Any advice would be appreciated.
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Old January 10th, 2009, 09:36 AM   #2
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Hello,
At first I thought it was a phantom power issue since this particular unit does not seem to provide phantom. Then I read that the AT mic runs on a battery and that phantom applied to the mic could damage it. The Akg does need phantom so that could be the issue on that mic. The Beachtek requires a nine volt battery so make sure that isn't run down. Not a big fan of the Beachtek devices, no real experience with them but they seem over priced for what you could get in the Juicelink. No experience with those either but seems like a better product. And yes, the Beachtek could be defective. I've heard of these types of problems before.
Cheers,
Bernie
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Old January 10th, 2009, 09:46 AM   #3
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Thanks for the reply. The AT mic is running on its internal battery. The CK98 is plugged into a portable phantom power pack, with an XLR from that to the camera. All the batteries are fresh...

What kind of noise level would be expected with a unit like this?
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Old January 10th, 2009, 10:08 AM   #4
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How are you connecting the AT822 to the Beachtek? If you're using a conventional XLR cable, XLR-F on one end and XLR-M on the other, that's going to be part of your problem. The AT822 is a stereo mic. Although it has an XLR output connector, it is NOT wired the same as a conventional balanced XLR would be. As a stereo mic it puts the left channel on one pin of the XLR (probably pin 2), the right channel on another (pin 3), and ground on pin1 so the XLR is sending two unbalanced audio channels, not one balanced mono signal such as a conventional XLR such as the one on your CK98 would be. The Beachtek XLR inputs, on the other hand, expect a balanced mono input with the signal hot on pin 2, signal cold on pin 3, and ground on pin1. If you try to send the AT straight in, the Beach is going to flip the phase of one channel and try to combine it with the other, thus leading to partial cancellation and all sorts of other weirdnesses. You cannot use a standard XLR cable to connect it to the Beach.

Did you buy the AT new? If so, it should have come with 2 cables, one providing 2 1/8 TS mono plugs and the other TRS stereo plug.

If you connect your CK98 into the Beachtek all by itself, setting the Beach output switch (labeled M/S) to mono and leaving the wireless disconnected for now, how does it sound? You said you had connected both mics directly into the camera and they sounded fine - did you use the XLR to TRS cable that came with the AT mic for the direct connection trial with the CK98? If so, be aware there's going to be a problem with that as well if you try to use it for actual production work. The CK98 is sending a balanced mono signal as described above. The cable you got with the AT connects pin 2 to the mini tip, pin 3 to the mini ring, and pin 1 to the mini sleeve. In the camera tip goes to the left channel, ring goes to the right, and sleeve to ground. This is exactly what should happen with the stereo AT822. But for the mono CK98 it means that the single mono signal from the mic is going to both the left and right channels on the tape BUT with the right channel inverted in phase with respect to the left. If you ever do a mono downmix, the entire signal will vanish like magic.

I have a Juicedlink. I like it.
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Old January 10th, 2009, 05:29 PM   #5
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Steve I think you've put your finger on the problem - if I remove the AT822 from the configuration then I can get the CK98 (on its own balanced cable) sounding good either alone or in combination with the Senn lav. (I actually throught I had tried that combination early on, but I guess not.)

So - thanks! Great forum, guys.

Are there any commercial cables available that could "properly" combine the two AT822 channels to a mono XLR output, so that I could use that mic as planned on-camera alongaside the wireless?
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Old January 11th, 2009, 05:29 AM   #6
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Quote:
Originally Posted by Graham Hickling View Post
Steve I think you've put your finger on the problem - if I remove the AT822 from the configuration then I can get the CK98 (on its own balanced cable) sounding good either alone or in combination with the Senn lav. (I actually throught I had tried that combination early on, but I guess not.)

So - thanks! Great forum, guys.

Are there any commercial cables available that could "properly" combine the two AT822 channels to a mono XLR output, so that I could use that mic as planned on-camera alongaside the wireless?
Unfortunately a mere cable won't do it. You'll need some more circuitry to combine the two signals into one.

The cheapest is a device called a "microphone combiner" but for your application it's going to be a PITA - a quick Google turns up a bunch but they all are for use with XLR mics - that means you'll have to lead a cable away from the camera-mounted mic using the AT supplied XLR to dual mono plugs cable, through a pair of mini to XLR adapter plugs into the combiner, then cable from it to one channel of the DXA where it joins the wireless receiver on the other, then back to the camera mic input. Not very tidy and a lot of places where things could go wrong from cables getting snagged, connections getting dirty, etc.

A cleaner solution be to replace the Beach with a 4-channel Juicedlink or other true mixer with three or more inputs. Unlike the Beach, the Juicedlink is a true mixer as well as XLR/mini converter and each of its input channels can be panned left, right, or centred on the output. You'd still use the XLR-to-dual-mini cable supplied by AT plugged into a mono mini-to-XLR plug like the VXLR adapter from Rode - RØDE Microphones - That would plug into two channel inputs that would be both panned to the same output channel to combine them. Meanwhile the wireless would go to a third input channel on the mixer where it would be panned to the opposite output channel from that of the AT mic or Boom mic. (You never, ever, want to mix the wireless with an on-camera or a boom mic due to phase and reverb artifacts caused by the arrival time differences produced by the differing distances the mics are from the sound source. They each need to be recorded on their own separate isolated tracks.) Personally this is the route I'd go but I'd take it a step farther. The AT supplied cable is pretty long, 10 feet or so I think, and that's a lot of wire to coil up when you have it on the camera. I'd either make myself or have made-up for me at pro audio shop a short "Y" cable that would have a XLR-F on one end to plug into the mic, splitting to two XLR-M to plug into the mixer. It would be wired with pin 2 on the XLR-F to pin 2 of the left XLR-M, pin 3 on the XLR-F to pin 2 of the right XLR-M, pin 1 of the XLR-F to pin 1 of both left and right XLR-Ms, and pins 3 of each XLR-M jumpered to their pin 1. You might be able to find such a cable available commercially but I doubt it. That way you can have a more convenient cable length and avoid cable adapters, always a good idea. So the bottom line would be a 3 or 4 channel mixer with the AT left to one input channel panned left, the AT right to a second input channel also panned left, and the wireless to a third input channel panned right.

All that being said, to be honest I'm not sure I would bother with all that and I'd just lose the AT altogether when using the wireless. Mixing the left and right channels of the AT effectively means you're using it as an ordinary mono mic and you lose any stereo effect anyway. Other than music, very little of what you see is recorded in stereo anyway. With only two channels available for recording and the overriding need to use one of them exclusively dedicated to the wireless, due to the aformentioned phase and timing issues that arise when you try to mix them with a mic that's farther away from the subject than the lav, a mono mic on-camera would be just as effective, weigh a lot less, and be a lot less awkward to handle. Use your CK98 on the camera when you're not booming with it or take its amplifer module and combine it with the CK93 hypercardioid or CK91 cardioid capsule and use that for your on-camera ambience mic. If you really want stereo ambience, use the AT with a stand-alone recorder of some sort.
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Last edited by Steve House; January 11th, 2009 at 07:09 AM.
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Old January 11th, 2009, 10:38 AM   #7
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What is a Sennheiser G3? Never heard of that model, although I have heard rumors of that model.

Dan
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Old January 11th, 2009, 07:01 PM   #8
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> I'd just lose the AT altogether

Steve, I appreciate the time you put into your post - lots of good points to keep in mind in future! And I agree with your summary that in the final analysis it's not worth the bother. There was a new CK93 on Ebay today that I picked up for $87, so I'll try with that.

Dan - sorry that was a typo - it's a G2 Sennheiser wireless.
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