Measuring a preamp's self noise

wheelema

wheelema

Boner-obo
There are other threads on this topic but I wanted to see if we could get some feedback from other members. I'm not pretending that this is going to be accurate or scientific but hopefully it will be interesting.

With no microphone attached, your recording app running, and the gain on your pre set to the minimum value tell us what the meter reads, then ramp your pre up to maximum gain and tell us what the meter tops out at.

Don't forget to mention what pre you're using.

Sound Devices USBPre 2: -87db to -45db
 
You must have the preamp input terminated in some way. Something like a 150 to 200 ohm resistor across pin 2 and 3 of an XLR or tip and ring of a TRS plug.

Paul
 
You need to know two other facts: the minimum and maximum gain of the preamp, and the 0dBFS to dBV calibration of your converters. Also, you should measure the self-noise of the converter (terminated, of course) because at minimum preamp gain the preamp output noise might be lower than the converter input noise.

Allow me to demonstrate:

Using a converter calibrated to 0dBFS = +5dBV, a gain range of +1dB to +47dB, and converter noise floor of -112dBFS, I get a noise range of -99dBFS to -84dBFS (so I can ignore converter noise for my example). Thus, at maximum gain (really mostly what we care about), equivalent input noise is -84 - 47 + 5 = -126dBV. That's unweighted; A-weighted would be 3 or 4dB lower. Thermal noise of the 150 ohm termination is -133dBV, which is far enough below -126dBV that we can safely ignore its noise contribution. Any lower though and you'd want to subtract the thermal noise contribution.
 
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So for the majority of home users there is no simple or relatively accessible way to determine if their pre is contributing an unacceptable degree of noise to the signal chain other then to swap it out and compare different results?

Ouch.
 
wheelema said:
So for the majority of home users there is no simple or relatively accessible way to determine if their pre is contributing an unacceptable degree of noise to the signal chain other then to swap it out and compare different results?

Ouch.
Click to expand...

By what standard are you classifying unacceptable? What is an acceptable number?
you could simply look at the specs which should give you the noise floor of the gear (assuming it is working properly) and you ears are probably the best way to tell if the noise is unacceptable.
Even then unless there is something seriously wrong with the design, as soon as you plug in a mic and turn up the gain to get the level you want into the system, the ambient noise floor of a fairly typical home recording room is going to be significantly above the self noise of the preamp and mic and any other gear in the chain combined.

And having said all of that, some amazing records, many of which still get regular play on mainstream radio, tv, webshows, movies etc were recorded on equipment far, far noisier than even a cheap entry level preamp available for a few bucks today but it really doesn't seem to be a problem

If you were to plugin the gear that Eddie Kramer used to record/mix the Stones or Hendrix, or the gear that Alan Parsons used on Dark Side of the Moon or that Geoff Emerick used with the Beatles and so on and so on and so on, your DAW meters would be sitting significantly higher than if you were to plug in a fast track USB.
They were knowledgeable enough with gain staging to not have it be a problem though
 
wheelema said:
So for the majority of home users there is no simple or relatively accessible way to determine if their pre is contributing an unacceptable degree of noise to the signal chain other then to swap it out and compare different results?

Ouch.
Click to expand...

*If* you trust the company's specification, then all you need to do is read their spec. If it is not stated as "equivalent input noise" or "EIN", then it's not a useful spec. Some companies will spec dynamic range instead, which is nice but not as useful (more important for converters). You need to know EIN.

SD is a very reputable company; they say -127dBu (which is -129dBV), they imply it's unweighted but don't specifically state that--they say "22Hz to 22kHz bandwidth", which should mean unweighted, so there you go. Unweighted or not, that is a plenty low noise spec, so you should be fine.
 
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