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Author Topic: Noisy Pre-amps & ADCs  (Read 976 times)

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Offline gormenghast

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Noisy Pre-amps & ADCs
« on: August 04, 2016, 12:12:12 PM »
Something I never really considered was how noisy a pre-amp or ADC can be.  I've read about it but didn't think much about it until I heard something in a recording of mine.  Slight faint hiss.  Got me to thinking where does it come from.  The PA?  Mics?  Pre or the ADC?  Or a combo of some or all.  If it's the PA I guess there isn't much I can do about that, but if it's the equipment I could fix that.   I'm curious what is the all in one recorder available that has the lowest noise pre-amps?  ADC?  I use a M10 exclusively, but I'm considering another deck for P48.

 
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Offline MakersMarc

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Re: Noisy Pre-amps & ADCs
« Reply #1 on: August 04, 2016, 09:51:36 PM »
I tend to think that you're not hearing much self noise, but rather PA noise. It's amazing what you can hear that you didn't catch at the performance. Most pre and ADC noise, I think, is way swamped by the music.
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Offline DSatz

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Re: Noisy Pre-amps & ADCs
« Reply #2 on: August 04, 2016, 11:20:52 PM »
I think that if you start digging into the topic of noise in recordings, you will soon realize that it's quite complex. For example there's really no such thing as "the quietest preamp" as such, since noise levels depend so much on details such as the microphone's output impedance, the gain settings on the preamp, how you "weight" the noise signal as to its frequency spectrum, and whether or not you smooth out the short-term impulse noise the way most manufacturers like to do. Preamp "A" could measure quieter than preamp "B" under a given set of conditions--but then under another, equally plausible set of conditions, the reverse could well be true.

I have seen this myself. At 60 dB gain, one of my preamps is several dB quieter than another one--but at 35 dB gain (far more typical of what I actually use), the second preamp is quieter than the first.

Noise specifications for preamps are nearly always measured with the gain turned all the way up. That usually gives the best numerical result, but doesn't tell you ANYTHING about how the preamp will behave at other gain settings. There is truly no way to estimate, from the noise figure at 60 dB gain, what the noise figure at 30 or 35 dB gain might be. You have to actually measure at the gain levels that you're going to use.

More fundamentally, the noise level of a preamp is essentially irrelevant when other noise sources in the recording "chain" predominate. Many people don't seem to realize that the self-noise of a microphone--even a first-rate, professional-quality studio condenser microphone--will often exceed the self-noise of the preamp channel that it is connected to. The net effect depends on a number of factors, though. For example, the noise from condenser microphones that you can measure at low frequencies is considerable, but is usually completely inaudible unless the signals are amplified greatly, because our ears and brains just don't register low-level, low-frequency sound with anything near the same sensitivity as we register midrange sound.

So these are just some thoughts for starters.

--best regards
« Last Edit: August 27, 2016, 11:08:12 PM by DSatz »
music > microphones > a recorder of some sort

Offline bombdiggity

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Re: Noisy Pre-amps & ADCs
« Reply #3 on: August 05, 2016, 01:43:47 PM »
As always so much relevant food for thought (thanks!). 

My observations on a piece or two:

Noise specifications for preamps are nearly always measured with the gain turned all the way up. That usually gives the best numerical result, but doesn't tell you ANYTHING about how the preamp will behave at other gain settings. There is truly no way to estimate, from the noise figure at 60 dB gain, what the noise figure at 30 or 35 dB gain might be. You have to actually measure at the gain levels that you're going to use.

The most important aspect relative to our use...  I had not realized the specs are based on all the way up. 

The noise level of a preamp can also be essentially irrelevant because other noise sources in the recording "chain" predominate. What surprises a lot of people is that the self-noise of a microphone--even an first-rate, professional-quality studio condenser microphone--can very well exceed the self-noise of a preamp channel that it is connected to. However, again, the net effect depends on a number of factors--for example, the noise from condenser microphones that you can measure at low frequencies is considerable, but is usually completely inaudible unless the signals are amplified greatly, because our ears and brains just don't register low-level, low-frequency sound with anything near the same sensitivity as we register midrange sound.

My observation is that the noisiest extraneous elements tend to be:

1) any number of things in the stage/PA amplification chains at amplified shows (music or crowd will cover anything in my signal path at anything of even low to moderate volume).

2) the mics at unamplified shows (there are certain shows I have to use my SP's for and when I bring the levels up in post there's certainly hiss there, though I do like the mics).  No comparison to the Schoeps in terms of self noise, though that is to be expected.  Using the Schoeps at very quiet shows it's usually some buzz in a stage amp that is the offender. 

I don't think the Tinybox or the (Oade modded) R-44 add any noise I can hear, though I'm sure there is some residual noise added by everything in the chain. 
Gear:
Audio:
Schoeps MK4V
Nak CM-100/CM-300 w/ CP-1's or CP-4's
SP-CMC-25
>
Oade C mod R-44  OR
Tinybox > Sony PCM-M10 (formerly Roland R-05) 
Video: Varied, with various outboard mics depending on the situation

 

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