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Author Topic: Understanding Microphone Preamplifier Noise (link)  (Read 775 times)

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Offline huskerdu

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Understanding Microphone Preamplifier Noise (link)
« on: January 03, 2018, 07:22:43 PM »
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Offline voltronic

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Re: Understanding Microphone Preamplifier Noise (link)
« Reply #1 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.
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Offline DSatz

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Re: Understanding Microphone Preamplifier Noise (link)
« Reply #2 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 (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. And any comparisons made with unmatched levels carry a high risk of being deceptive (i.e. the results may seem utterly convincing in the moment, but once you match levels, the outcome could be completely different. This is broadly true in audio comparisons generally, not just about preamp noise).

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. And 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. (Complication 1: Microphone noise varies considerably at different frequencies, and so does the ear's sensitivity to that noise. Frequency weighting curves are generally applied in microphone noise measurements, but any such weighting is only appropriate for one particular SPL range. When the noise is at a markedly different level (in good modern microphones, the noise is typically 25+ dB lower than the "A" curve was designed for), the weighting curve can skew things enough to produce misleading results. And complication 2: Smooth, steady noise of a given average level is far more tolerable than noise with the same average level but with prominent, momentary "spikes." Some noise measurements (e.g. "quasi-peak") respect that difference far more than others ("RMS")--guess which figure most manufacturers publish?

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
« Last Edit: May 16, 2018, 09:59:49 AM by DSatz »
music > microphones > a recorder of some sort

Offline aaronji

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Re: Understanding Microphone Preamplifier Noise (link)
« Reply #3 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...

Offline voltronic

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Re: Understanding Microphone Preamplifier Noise (link)
« Reply #4 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.
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I am hitting my head against the walls, but the walls are giving way.    ///    If a composer could say what he had to say in words he would not bother trying to say it in music.
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