noise vs gain -- DAV or Presonus Firepod

[pianoman]

Hello, all --

I've been trying to optimize the amount of gain from my DAV preamps vs. the trim setting on the Presonus Firepod. The conclusion (you all probably know this qualititatively already) is to use the DAV as much as possible and then dial down the Presonus even to the point where the trim indicates negative gain. I finally bit the bullet and measured this stuff in my recording environment -- shades of my geeky engineering past (oops). I thought I'd document this here for the benefit of the group.

The procedure was to not use microphones and measure the electonic noise alone in recording to channels in Logic Pro as a function of different preamp settings. The noise was stochastic, but measurable to within 10% for higher noise and 25% for the lowest noise. When noise went off my Logic scale, I scaled the gain +12 ,24 or 36 dB using Logic's gain function and measured again, taking an average of the ratios as a scaling factor. Not perfect, but I no longer have access to the lab with all the cool HP equipment...

Shown in the pdf are two curves. The different lines are different preamp channels. The differences are due to high pass filter settings. Note that the measurement error is greatest for the lowest noise.

It is clear that for constant Presonus gain (in this case +6 dB), the DAV curve is quite smooth over the full range of gain I could measure (28 dB). It takes more than 20dB of DAV gain for the noise to increase by a decade. By contrast, the Presonus curve for constant DAV gain (+66 dB) is not as smooth and it takes only 8dB or so (if one believes the gain scale on the Presonus) for a decade rise once the minimum gain levels are passed.

The conclusion is to use the DAV gain as much as possible and reduce the Presonus gain until a safe recording level is achieved, despite the appearance of "throwing away signal" if one believes the Presonus Firepod charts.

If this belongs in the Computers and Music section instead, could someone move the post appropriately?

Chain: DAV --> Presonus Firepod --> Logic Pro 7, Mac G4 733 MHz

[MichaelPatrick]

I get the idea in a relative sense, but it would help me if the noise axis was also in dB because I don't understand "decades" very well.

An unterminated input makes some preamps quite unstable. Do you think that affected your tests? Wouldn't it have been better to short the preamp input so that the results would measure the DAV noise under conditions that are closer to normal? Dynamic mic impedance is typically low. I don't know about condensers, but I imagine they are too.

[pianoman]

Originally Posted by MichaelPatrick

I get the idea in a relative sense, but it would help me if the noise axis was also in dB because I don't understand "decades" very well.

An unterminated input makes some preamps quite unstable. Do you think that affected your tests? Wouldn't it have been better to short the preamp input so that the results would measure the DAV noise under conditions that are closer to normal? Dynamic mic impedance is typically low. I don't know about condensers, but I imagine they are too.

Good point. Since the result came so close to what I qualitatively see to be the case, I just went with it once I started quantitatively measuring. I hadn't really set out to do this full experiment; it just happened.

The point is not that DAV is noisy (I really have to crank the DAV up to see the noise). The point is that for a given gain increase the Presonus noise increases much faster (and less smoothly/predictably) than the DAV noise, so for a given recording level you want to rely as heavily on the DAV as it will let you, even to the point of apparently needing to attenuate the signal on the Presonus (and attenuation is supposed to be bad; you're losing signal).

Here's the same data with noise (arbitrary units) on a dB scale.

[MichaelPatrick]

You've clarified everything I was wondering about, and your conclusion makes sense.

It'll be hard to overload the Presonus when it's really trimmed down like you suggest, but I wonder if such a deep trim affects sonics in any negative way? What does it sound like down there with the DAV feeding more?

I don't have any DAVs but I just bought a Firepod to install at a place where I record frequently. It'll be for weekly run-of-the-mill --not high value-- recordings.

This info will help me use it wisely. Much appreciated.

[AdamZuf]

Last friday I did an acoustic recording in a cave. While I'm ok with the Firepod's not so quiet preamps in most usages, when high gain needs to be picked up from a distance, it is really noticable.
I was thinking of going to another interface, but honestly, I like the Firepod, it does what I need, and to get 16 channels with the FireStudio (or any of th Motu, RME etc) is much more expensive and the FireStudio doesn't work well with my laptop anyway.

Are there any cheap, ultra quiet preamps available? Maybe DIY something?
Thanks
Adam

PS: I found Rod Elliot's P66:
Low Noise Balanced Microphone Preamp

Will it be sufficient, when the Firepod is fed with line level signal, or is the Firepod too noisy to start with quiet, quality recordings?