Gain structure: dBu, VU, dbFS

Tom Morrow · May 22nd, 2011, 01:08 PM

After having used the mixpre a little bit I'm seeing its limitations and wondering if a 302 or mixpre-d might work better for me. But that inspired me to research a bit more and find out there's no single ideal solution.

With a multimeter I measured the output with my custom-adjusted cable on the mixpre at a level of -32dbu (when the mixer outputs a 1khz tone nomincally at 0dbu). That is the level I had determined is best to align my 60d camera's 0dbFS point with the mixer 20dbu output level, so that the mixer limiting happens right before clipping, with the 60d camera preamps all the way down.

But it looks like using any other attenuating scheme, other than my custom cable for the mixpre, would be nonideal in that I would have to turn up the camera preamps more because the 302 or mixpre-d can't natively output a -32dbu signal. The DSLR level output on the mixpre-d is fixed at -36dbu, and the 302 has a dead zone where it can't be adjusted for output between -16dbu and -40dbu. So I'd have to turn the noisy camera preamp up by 4db with the mixpre-d, or 8db with the 302. Probably not a dealbreaker, but I'm concerned about the difficulty of remembering and verifying that my camera preamp level is set correctly every time I shoot, given that the scale is unmarked and I would have to remember something like "adjust all the way left then ten clicks to the right".

The SEScom cable incidentally produces an output of -42dbu from the mixpre, so it would require 10db increase on the camera.

I am curious to hear how others handle this issue, especially how people got the levels matched while using a 302 with a DSLR, or how they are planning to match levels with the Mixpre-d and a DSLR.

Tom Morrow · May 22nd, 2011, 01:34 PM

As a reminder to myself: It looks like feeding the sescom cable from the XLR outputs would increase the output by 10db making it just perfect in level for me, and this could be a good option for someone who just wants to buy something off the shelf and not hassle with adjustments. But finding and living with the adapters/cables to convert the 1/8" to XLR might be annoying.

Rick Reineke · May 24th, 2011, 03:18 AM

The 302's XLR outputs are variable all the way down to -50dB (or -60) via the software menu.

Tom Morrow · May 27th, 2011, 01:29 AM

Yes IIRC the 302 can gain down all the way from -40 to -56dbu but unfortunately can't gain down from -16 to -40dbu.

The more I think about this maybe it's not worth worrying if I have to boost the camera's gain by 8db. I could always do a test recording and see how that changes the noise floor as opposed to my cable.

May 22nd, 2011, 01:08 PM	#16
Tom Morrow Major Player Join Date: Feb 2011 Location: San Francisco, CA Posts: 416	Re: Gain structure: dBu, VU, dbFS After having used the mixpre a little bit I'm seeing its limitations and wondering if a 302 or mixpre-d might work better for me. But that inspired me to research a bit more and find out there's no single ideal solution. With a multimeter I measured the output with my custom-adjusted cable on the mixpre at a level of -32dbu (when the mixer outputs a 1khz tone nomincally at 0dbu). That is the level I had determined is best to align my 60d camera's 0dbFS point with the mixer 20dbu output level, so that the mixer limiting happens right before clipping, with the 60d camera preamps all the way down. But it looks like using any other attenuating scheme, other than my custom cable for the mixpre, would be nonideal in that I would have to turn up the camera preamps more because the 302 or mixpre-d can't natively output a -32dbu signal. The DSLR level output on the mixpre-d is fixed at -36dbu, and the 302 has a dead zone where it can't be adjusted for output between -16dbu and -40dbu. So I'd have to turn the noisy camera preamp up by 4db with the mixpre-d, or 8db with the 302. Probably not a dealbreaker, but I'm concerned about the difficulty of remembering and verifying that my camera preamp level is set correctly every time I shoot, given that the scale is unmarked and I would have to remember something like "adjust all the way left then ten clicks to the right". The SEScom cable incidentally produces an output of -42dbu from the mixpre, so it would require 10db increase on the camera. I am curious to hear how others handle this issue, especially how people got the levels matched while using a 302 with a DSLR, or how they are planning to match levels with the Mixpre-d and a DSLR.

May 22nd, 2011, 01:34 PM	#17
Tom Morrow Major Player Join Date: Feb 2011 Location: San Francisco, CA Posts: 416	Re: Gain structure: dBu, VU, dbFS As a reminder to myself: It looks like feeding the sescom cable from the XLR outputs would increase the output by 10db making it just perfect in level for me, and this could be a good option for someone who just wants to buy something off the shelf and not hassle with adjustments. But finding and living with the adapters/cables to convert the 1/8" to XLR might be annoying.

May 24th, 2011, 03:18 AM	#18
Rick Reineke Inner Circle Join Date: Aug 2008 Location: New York Posts: 2,039	Re: Gain structure: dBu, VU, dbFS The 302's XLR outputs are variable all the way down to -50dB (or -60) via the software menu.

May 27th, 2011, 01:29 AM	#19
Tom Morrow Major Player Join Date: Feb 2011 Location: San Francisco, CA Posts: 416	Re: Gain structure: dBu, VU, dbFS Yes IIRC the 302 can gain down all the way from -40 to -56dbu but unfortunately can't gain down from -16 to -40dbu. The more I think about this maybe it's not worth worrying if I have to boost the camera's gain by 8db. I could always do a test recording and see how that changes the noise floor as opposed to my cable.