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Gear / Technical Help => Battery Boxes, Preamps, Mixers, ADCs, and Processors => Topic started by: huskerdu on January 03, 2018, 07:22:43 PM

Title: Understanding Microphone Preamplifier Noise (link)
Post by: huskerdu on January 03, 2018, 07:22:43 PM
I found this pretty informative: http://www.sounddevices.com/tech-notes/microphone-preamp-noise (http://www.sounddevices.com/tech-notes/microphone-preamp-noise)
Title: Re: Understanding Microphone Preamplifier Noise (link)
Post by: voltronic on January 03, 2018, 07:43:29 PM
This is really great, thanks for posting.

The proper measurement technique has been explained here before by Jon and a few others, but it bears repeating.

This bit was news to me:
Quote
The very best EIN that can be achieved is -133 dBV, since this is noise purely from a 150 ohm resistor.

I also see that SD is now listing EIN for the MixPre series in both dBV and dBu.  Previously, they were only using the better-looking dBV figure.
Title: Re: Understanding Microphone Preamplifier Noise (link)
Post by: DSatz on January 04, 2018, 09:31:27 AM
The article recommends using a 150-Ohm resistor as a "surrogate microphone" when measuring preamp noise, and careful matching of gain levels (set with test tones and a meter, not just by ear) when making comparisons. Those are excellent suggestions.

It is indeed completely senseless to judge a preamp by turning its gain all the way up and listening with nothing connected to its input. You will hear differences among different preamps or recorders that way, but that tells you literally nothing about how the preamps will behave with a microphone attached, at gain settings that you would actually use for recording. Really, literally nothing. Don't do it.

What the article doesn't mention, though: Equivalent input noise varies considerably at different gain settings. Perhaps unexpectedly, in most preamps it is lowest at high gain settings--often by a lot. That's why most manufacturers specify it at the maximum gain their preamps offer--generally a far higher setting than one would ever use day-to-day. -- Unfortunately, when you only know the EIN of a preamp at one gain setting, you can't infer anything about its noise at other gain settings. The quieter of two preamps as measured at 60 dB gain (typical for a spec sheet) may not be the quieter of the two at 30 or 35 dB gain (typical for actual recording).

Also not mentioned in the article: Most microphones are significantly noisier than either a 150-Ohm resistor or the input of a good preamp.Then finally, in live recording environments the same variables occur--frequency and time distribution of the noise--but especially in live, public situations, environmental noise nearly always "swamps" microphone noise.

Thus the search for the quietest preamp can easily become an exercise in misplaced perfectionism (see https://en.wikipedia.org/wiki/Streetlight_effect).

--best regards
Title: Re: Understanding Microphone Preamplifier Noise (link)
Post by: aaronji on January 09, 2018, 09:21:15 AM
I also see that SD is now listing EIN for the MixPre series in both dBV and dBu.  Previously, they were only using the better-looking dBV figure.

As long as SD noted the proper units (which they did), I don't see them looking different at all.  Especially since it is such an easy conversion, even for the mathematically-impaired...
Title: Re: Understanding Microphone Preamplifier Noise (link)
Post by: voltronic on January 09, 2018, 07:29:55 PM
I also see that SD is now listing EIN for the MixPre series in both dBV and dBu.  Previously, they were only using the better-looking dBV figure.

As long as SD noted the proper units (which they did), I don't see them looking different at all.  Especially since it is such an easy conversion, even for the mathematically-impaired...

I agree completely, but there was a bit of a dustup on the GS Remote board accusing SD of engaging in marketing stat-fluffing.  Not worth linking to; it was ridiculous.