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Author Topic: Understanding Microphone Preamplifier Noise (link)  (Read 341 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; the hugeness of this effect probably has to be experienced in order to be properly appreciated).

What the article doesn't mention, though, is that equivalent input noise also varies considerably at different gain settings. There isn't a fixed amount of input noise that's simply "amplified more" when you increase the gain; on the contrary, EIN is generally highest at low gain settings. That's why most manufacturers specify it at the maximum gain that their preamps offer. Unfortunately, when that's the only information you have, you can't infer anything about noise performance at any other gain settings; two preamps with identical EIN at 60 dB gain might have 10 dB difference in EIN at 30 dB gain, which is a much more typical setting for live recording with condenser microphones.

But most actual microphones are significantly noisier (in at least some part of the frequency range) than either a 150-Ohm resistor or the input of a good preamp. (Complication 1: Noise levels can vary considerably at different frequencies, as does the ear's sensitivity--see https://en.wikipedia.org/wiki/Equal-loudness_contour. This is why "weighting curves" are generally applied in noise measurements.) (Complication 2: Noise that's smooth and steady can be tolerated at higher levels than noise that has distracting, moment-to-moment "features". Some types of noise measurement take that into account a lot more than others. Look for "quasi-peak" or "qpk" rather than "RMS" values if you want better realism in noise specifications.)

Then finally, consider live recording environments. Again, all the different variables come into play--frequency and time distribution of the noise--but environmental noise (especially in live, public situations) nearly always "swamps" microphone noise.

So the environmental noise is generally greater than microphone noise, which in turn is generally greater than preamp noise (though there can be exceptions). 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: February 07, 2018, 10:11:22 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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