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633 Pre-Amp Noise


echo

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I have a recently purchased 633 and on set today I noticed some noise. I immediately thought the issue was with the mic but it seems to be the 633. I unplugged the xlr input on channel two, and sure enough the noise was still there. It was a very intimate and whisper like scene so preamps were boosted quite a bit, roughly 3 o'clock on pre fade and just past 12 on the post fade. All three Mic inputs have this noise. Phantom is off. Noise boosts with either fader. This may be a stupid question but anyone know why this is and if this is normal?

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You may also be hearing noise from the headphone system..

 

It's also a signal to noise ratio thing in regards to listening through a mic.

 

I know you disconnected the mic, but...

 

Things heard through a mic... Low signal (or material you are trying to record)... High noise floor heard...  You gain it way up...

 

High signal...low noise floor heard.  Gain lower.. less noise...Just saying...

 

With nothing plugged in, just the pre amps.. you will always hear "noise"...

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I have a recently purchased 633 and on set today I noticed some noise. I immediately thought the issue was with the mic but it seems to be the 633. I unplugged the xlr input on channel two, and sure enough the noise was still there. It was a very intimate and whisper like scene so preamps were boosted quite a bit, roughly 3 o'clock on pre fade and just past 12 on the post fade. All three Mic inputs have this noise. Phantom is off. Noise boosts with either fader. This may be a stupid question but anyone know why this is and if this is normal?

Hi Woodley,

There is a minimum noise floor for any input that is caused by the thermal motion of electrons in the input components. Over the audio bandwidth, this energy is about -132 dBm and is an absolute lower limit. The only practical way of getting around this thermal noise is to cool the input with something like liquid nitrogen. The other methods are even less practical.

 

So, if you have applied 70 dB of gain to the input, then the noise level -132dBm noise floor is now boosted to -62 dBm. Interestingly, this is about the noise level of high quality 1/4" tape decks from the golden years. My guess is that this is about the level of noise you were hearing. A level at -62 dBm is very easy to hear on headphones. These numbers are for a perfect pre-amp and following headphone amplifier. Your equipment may only approach perfection.

 

Quick answer: The hiss is normal and impossible to eliminate.

Best Regards,

Larry Fisher

Lectrosonics

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Thanks for pointing out the "elephant in the room" problem with our often misplaced love affair of analog tape recorders, Larry!

 

Normally, much less than 70dB of gain would be need to be added, but even if was needed in Echo's case, I doubt it would seem problematic while normal level dialog was going on when using the mic at typical distances. I assume the noise Echo heard was much more obvious than that.

 

I see three possibilities:

 

1) Another unterminated input was open (such as input 4, 5, and/or 6).

 

or

 

2) Echo said that phantom power was off, which means that it was a dynamic mic, which would account for approximately 15dB more noise compare to a typical condenser mic. That shouldn't have been a problem if the mic was used close to the mouth like most dynamic mics in this business are intended. But if using a dynamic mic at the same distance that we typically use condenser mics, or if just comparing equal ambience levels between a dynamic and condenser mic, the noise would be easily apparent.

 

or

 

3) Maybe it's broke. 

 

GT

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matt: " Does the noise floor change whether you click the HPF in/out on the open channel? "

well, maybe, it depends...

but it may sound like it changes when the setting controlling freq response are changed...

I had a very strange issue with a new 664 whereby the noise floor would raise significantly when the HPF circuit was switched in. I could also hear (in low frequency "bumps") data being written to the cards, again only when the HPF was enabled. I thankfully had the unit replaced as it was pretty much brand new but wondered if it was a similar problem that the OP was having.

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