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Gear / Technical Help => Battery Boxes, Preamps, Mixers, ADCs, and Processors => Topic started by: BusDriver on November 27, 2010, 12:15:43 AM
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I'm a new user of a used mixpre. I've read endless comments about how quiet, and low noise this unit is. I have not had chance, yet, for field recording. I've been busy listening to ambient house noise - frying onions, refrigerator hum, guitar, etc..... When nothing is plugged in (no mics), and I listen only to headphones , and increase the gain past 7, 8 o'clock, what is the noise / hiss I am listening to? Will this all be masked by signal when one is introduced? It seems quite strong, likely to color quiet passages of recording. Is this a service issue? an undisclosed artifact at time of sale? or just what it is?
Sorry if this has been addressed someplace else. I could not find a thread.
Thanks in advance .....
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Someone correct me if I am wrong but it probably has something to do with the input transformers. Once connected to a set of mics the transformers balance out because of the signal. When nothing is plugged in the transformers aren't balanced..... I lack the technical terms at the moment to explain it any better..... :P
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Two thoughts:
- If you listen to quiet noises around the house you may hear the hiss, but when you are recording loud music that will completely mask the self noise of your preamp.
- There are some devices in this world which have great mic preamps feeding the recorder, but noisy headphone amps. It's easy to forgive the headphone noise if the signal going to the recorder is pristine.
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Busdriver > you're listening to the inherent thermal noise (or Johnson-Nyquist noise) coming from the resistive and capacitive elements of the mixpre, being amplified by the absence of a low source impedance on the input. If you "short" the pin 2 and 3 of your inputs with a low noise (metal film for instance) 50 ohms resistor, the noise level will drop dramatically. The same will happen to the noise level when you'll connect a microphone (and power it if it needs power, or else the source impedance won't be correct).
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Also, the headphone amp introduces noise that would not be present on a recording from the line outputs, which you need to take into account.
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^My guess is the headphone amp..... if so, it's nothing to worry about as it won't affect your signal. I ran a MixPre for a while, and I noticed the same........ but it is a great headphone amp........ much much gain. And it provids enough volume to actually be useful in a loud music atmosphere.
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Again, thanks for the input .... thanks to the pointer, and google, I read my fill on Johnson-Nyquist. Even though my brain no longer works on that level, it was quite understandable, none-the-less.
I finally was out with the unit and recorded a relatively quiet folk duo with multiple stringed instruments - guitar, violin, banjo .... mics 2- 3 feet from source, and results were fine. the noise associated with 'nothing' plugged in was absent.
Thanks for 'today's lesson' and prompt responses. This place is dangerous. Going to try my favorite 'absolutely no taping' club tomorrow. Used to be wide open 20 -25 years ago when I was last at this game. Pile enough stuff on the table ... nobody knows nothin' ... It helps when my partner comes along, as she always knits so it gets hard to tell wires from yarn.
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Testing a preamp with no microphones connected can't tell you whether the preamp will be noisy or quiet in actual use. The input circuits of most preamps are designed to be shunted with a low impedance, and if they're left "open" (= very high impedance) they will be noisy. That doesn't indicate anything bad at all about the preamp.
duch, I just noticed that you made the same point earlier--thank you! The idea of testing a preamp or recorder for noise with nothing connected to its inputs deserves to die, die, die. If you want to estimate or measure the noise of a preamp, its gain has to be to something like what you would actually use for recording, and something needs to be connected to the input that has an impedance like that of the microphone that you expect to use.
Otherwise the test can only be misleading. Even if preamp X is distinctly noisier than preamp Y when nothing is connected to its inputs, preamp X could still be distinctly quieter than preamp Y when an actual microphone is connected to its inputs.
--best regards