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High End Pre Noise Question
Old 10th July 2014
  #1
Gear Nut
 

High End Pre Noise Question

Hi folks, quick question for you.

With high end pres, is it normal for the noise floors to be uneven? i've never used an outboard stereo pre until now, all of the outboard stuff I've used aside from converters has been mono UA gear.

When I hook this pre up to the converters with the settings identical in each channel, the noise coming in is showing up as 10db difference in the DAW meters.

I'm just curious is this is normal.

Thanks!
Old 10th July 2014
  #2
Lives for gear
 

Hi
I would expect around 1dB 'difference' in HISS between preamps that ARE set to the same gain. Simply having pots set 'the same' does not necessarily mean the gains ARE the same. You must test this with tone and make sure you know what the gain really is.
You also need to really define WHAT the noise is. Hiss, hum, interference pickup, ????
Matt S
Old 10th July 2014
  #3
If it's a tube preamp then I would swap the tubes around and see if the noise moves to the other channel. Tubes sometimes vary greatly with the amount of noise (hiss) they generate.
Old 10th July 2014
  #4
Gear Head
 

If you have access to a spectrum analyzer, it can tell you the frequency range(s) of noise components....
Old 10th July 2014
  #5
Lives for gear
 
Geoff_T's Avatar
 

Hi

Any pre can be affected by external interference fields, depending on what is adjacent to the the pre. Some compressors, vintage or clone, can have EI core transformers that may not affect the compressor circuits but can wreak havoc with an adjacent pre at high gain. Similarly, powered speakers, power amplifiers in general, and many other products not using toroidal transformers can affect noise by inducing emf fields into the audio transformers.

You may find moving it around can mimimize the issue... I have seen scores of problems on gearslutz with folk with 500 series racks picking up interference from adjacent modules or power supplies.

It just needs a little detective work....

Old 10th July 2014
  #6
Moderator
 
TonyBelmont's Avatar
 

Quote:
Originally Posted by WarWeasel View Post
Hi folks, quick question for you.

With high end pres, is it normal for the noise floors to be uneven? i've never used an outboard stereo pre until now, all of the outboard stuff I've used aside from converters has been mono UA gear.

When I hook this pre up to the converters with the settings identical in each channel, the noise coming in is showing up as 10db difference in the DAW meters.

I'm just curious is this is normal.

Thanks!
Sure... Some odd and unpredictable results can be found at sub-audible levels when there is no input signal present/ input isn't terminated.

I would expect all of these odd sub-audible noise differences you are "seeing" to be negligible once a balanced input is presented to the unit.
Old 10th July 2014
  #7
Lives for gear
 

Hi
Mic inputs should be 'terminated' with a resistor, usually a 150 Ohm resistor is connected between pins 2 and 3 on an XLR for this. A simpler test but not quite as meaningful would be to 'short' pins 2 and 3 on the mic XLR.
The resistor or 'short' should be enclosed within the XLR plug, and use minimum cable length.
The EIN of a decent mic amp should be at least 127dB (20-20K RMS). A good mic amp can manage EIN of about 129.5dB IIRC. This is made up of the ACTUAL gain plus the measured noise.
Matt S
Old 11th July 2014
  #8
Gear Nut
 

Hmmm.

Interference fields aren't an issue. The unit is not near any other gear or speakers.

I've tried a few other things to see what I get...

With the in-gain at unity on both channels and output gain at maximum, I get a reading of -80 in the left channel and -78 in the right channel. This appears to be about what I would expect.

As I turn the output gain knobs down, the left channel noise descends to -96 in the DAW, the right channel actually goes up to -77. Left channel behaves as I would expect, this is odd of the right channel.

It's not a tube amp, so tubes aren't an issue.

Whether or not something like this will be audible in a mix, hmmm. Don't know. Some say absolute polarity is inaudible, some say it isn't. I can definitely feel the difference, even if my ears don't say "that's unnaturally flipped polarity." My concern pertains more toward the side of why this is happening, as it would seem that something is wired differently between the channels, and whether or not the difference in wiring may subtly alter the energetic transfer of audio. Numerous times we have encountered situations where slight compromises seemed like they wouldn't be a big deal and they turned out to be, especially in comparison with masters where all components performed flawlessly.

I expect multi-channel units to behave the same in both channels, especially in high end gear costing upwards of $2k.

I've done the tests swapping the exact same cables and using the same AD channel to make sure all other variables are the same.

I'll try with a spectral analyzer later this evening and see what happens.
Old 11th July 2014
  #9
Lives for gear
 

Hi
From the information given above it would seem that the amplifier stage AFTER the level pot has a bit of an issue. Knowing whether it is 'hiss' or possibly other noise would help diagnose it.
-80dB is reasonable for 'line level' unity gain, although not stellar it should not intrude on a final mix. Obviously -96 with the pot turned down is getting good and more like what would be expected (for what is probably a single line level stage with maybe 10dB gain available). If the unit is 'hand wired' it could be that the wiring is routed inappropriately, or there could be a failed transistor/IC in the output amplifier for the right channel.
Pictures and more information??
Matt S
Old 17th July 2014
  #10
Gear Maniac
 
nazaroo2's Avatar
Really sounds like a failed/failing cap, burned resistor, bad op amp, or drifted feedback loop or an inner-local powersupply problem with the right channel.

Another possibility is DC leaking into an output transformer, or an unusual amount or unbalanced amount of current somewhere.
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