Using the BeachTek DXA-8 with the GL2 at DVinfo.net

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Old February 23rd, 2005, 12:24 AM   #1
Fred Retread
 
Join Date: Jul 2004
Location: Hartford, CT
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To Know It Is to Love It: Testing the BeachTek DXA-8 with the GL2

While I was set up to make the measurements for my "Secrets of the GL2 Mic Jack" post, I decided to do some measurements with the DXA-8 in the loop.

Here is what I learned:

1. My DXA-8 had a measured gain range of -12 dBV to + 14.3 dB, close to BeachTek's specifications of -10 to +15 dB. The error differences are probably mine.

2. Conveniently, the mid range position of the Beach's gain control knobs is the zero gain position. I didn't know that. Therefore, if you want to use the GL2's level meters to "measure" the input voltage with the table in my "Secrets of the GL2 Mic Jack" post, it can be done with the DXA-8 installed and running. Just set the DXA-8 for zero gain.

3. This delighted me: Contrary to what I had assumed, the DXA-8's LEDs do not indicate limiting activity, which struck me as vague, confusing, and therefore not useful information. They indicate the Beach's output voltage. The LED is dimly lit at an output of about 4 mV and becomes its brightest at an output of about 9 mV. Thiis is GREAT NEWS! A very useful indication. With MIC ATT OFF, I know that the GL2 input loves the range 4-9 mV, and now with the DXA-8 LED's flashing I have a clear, direct indication that the input is getting what it wants.

4. So the limiting takes place in the background (i.e., without overt indication). I found that the limiting was capable of reducing the gain by -21 dBV with only about 0.5% THD (Total Harmonic Distortion), -24 dBV with 4% THD and as much as -34 dB with high levels of THD.

5. With its gain control set at mid range, the DXA-8's limiters clamped its output voltage (which is the input to the GL2) at 9-10 volts, even as the input to the DXA-8 increased to as high as 160 mV. Above 160 mV the output voltage started to climb. At an input of 230 mV the Beach's output was 14 mV, the maximum the GL2 can tolerate with MIC ATT OFF.

6. The BeachTek doesn't know or care whether the GL2 is in MIC ATT OFF or ON mode (I detected no loading effects), it's output depends only on it's input. I was surprised to learn that the DXA-8 can be used with the GL2 in MIC ATT OFF mode with my (and most) mics. With hot mics in loud environments it might be advisable to use MIC ATT ON mode. For instance, the AT4073 can put out 700 mV at 110 dB SPL.

7. As its specs indicate, the DXA-8 CAN clip the signal when the input to it gets too high, and the higher the gain is set, the lower the input voltage at which it will clip. However, this is balanced by the fact that the hotter the mic the lower you will have to set the gain. The margins designed into the DXA-8's performance mean that clipping by the Beach will be a rare concern. And clipping at the GL2 input will never be a concern with a properly set up DXA-8--that's the whole point of it's limiters.

8. Setting up the DXA-8 with the GL2
   A. Set the Beach's gain adjusment knob to mid range
   B. With representative sound happening, attenuate the incoming signal and/or adjust the DXA-8's gain up or down as necessary to obtain frequent LED flashes (but not constant bright illumination). Try to achieve this state not too far above or below the zero gain position.
   C. Adjust the GL2's audio gain knobs to suite your tastes. I like -6 to -3 dB
   D. Note: Tweaking the Beach's gain control knob up or down from the position set above may not swing the indicated levels much. This used to bother me--gave me the sense that I must be operating something out of range. Not anymore.

I love my DXA-8

[Edited 2/23]
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Old February 23rd, 2005, 05:43 PM   #2
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Join Date: Nov 2004
Location: New Zealand
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Hey Fred - nice work.

Would you consider doing the same test on a sound devices mix pre?
That would be most helpful for us keen on buying one to compare.

Thanx heaps :)
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Old February 23rd, 2005, 06:24 PM   #3
Fred Retread
 
Join Date: Jul 2004
Location: Hartford, CT
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I'd love to get my hands on a MixPre. I'm sure I'd find that it lives up to its specs like the BeachTek, but I like to discover the little things like those in my post above that enhance usability by helping the whole picture to make sense.

I don't think the two can be compared because the higher priced MixPre is more feature rich to meet the demands of the pros. But maybe I could gain some insight as to whether the difference is warranted for a "committed amateur" like myself.
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Old February 25th, 2005, 02:04 PM   #4
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Just an addition (and repeat). I have found that A-T 8202 Attenuators work well with the DXA-8 where required. They pass phantom voltage and are -10, -20, -30 dBa selectable. The price from B&H is good too.

They are a little long (5 inches?), but on a VX2100, they do not interfer with the normal lens operation if the box is mounted in the threaded moun on the bottom.
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