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Misunderstandings about preamp noise
Old 28th March 2021
  #91
Gear Maniac
 
Stewix's Avatar
 
To make it even clearer, while also underlining the relation between this thread and my question...

In this thread is "stated" that, very broadly speaking:

raising the gain of a preamp -> less headroom (aka increased risk of unwanted distortion), but better S/N ratio...

lowering the gain of a preamp -> more headroom, but a bit worse S/N ratio...

Now, letting aside all the possible corrections, clarifications, and whatnot, what I'm asking is:

is there any other possible "downside", apart from a bit worse S/N ratio, when lowering (a lot) the gain of a (budget) preamp?...

Simple as that...
Old 28th March 2021 | Show parent
  #92
Lives for gear
 
Ty Ford's Avatar
 
🎧 10 years
Quote:
Originally Posted by Stewix ➡️
My question is in the following form:

Should someone be careful ONLY when doing X, or ALSO when doing Y?...

Maybe it was not clear...



Ok, but this is not what I was asking for...
I thought you were asking about how to get less noise. Gain staging is always important. Every circuit has a curve along which are the optimal positions for reduced distortion. Unless you know what the designer(s) had in mind, it's difficult to tell. Most people have found that using anymore than 80% of a gain knob twist boosts gain but also increases distortion. Use your ears.
Old 28th March 2021 | Show parent
  #93
Gear Head
 
Quote:
Originally Posted by Stewix ➡️
To make it even clearer, while also underlining the relation between this thread and my question...

In this thread is "stated" that, very broadly speaking:

raising the gain of a preamp -> less headroom (aka increased risk of unwanted distortion), but better S/N ratio...

lowering the gain of a preamp -> more headroom, but a bit worse S/N ratio...

Now, letting aside all the possible corrections, clarifications, and whatnot, what I'm asking is:

is there any other possible "downside", apart from a bit worse S/N ratio, when lowering (a lot) the gain of a (budget) preamp?...

Simple as that...
Depends how the preamp is optimised. A lot of the chip preamps get best performance at minimum gain but there are plenty of preamp designs that get better thd wise as the gain drops but then rise again at the lowest settings. Usually not nearly as bad as the high gain distortion though.
Old 28th March 2021
  #94
Lives for gear
 
Generally the less gain you use the less flavour of the amp you get. If you like what the pre-amp does, you'll want to optimise towards using as much gain as you can get away with - if not - do the opposite and add digital gain after the event.
Old 29th March 2021 | Show parent
  #95
Gear Maniac
 
Stewix's Avatar
 
Thank you all

Quote:
Originally Posted by machdisk ➡️
Depends how the preamp is optimised. A lot of the chip preamps get best performance at minimum gain but there are plenty of preamp designs that get better thd wise as the gain drops but then rise again at the lowest settings. Usually not nearly as bad as the high gain distortion though.
Thanks, this was the kind of "information" I was searching for...
Old 29th March 2021 | Show parent
  #96
Lives for gear
 
nosebleedaudio's Avatar
 
🎧 15 years
Quote:
Originally Posted by Stewix ➡️
To make it even clearer, while also underlining the relation between this thread and my question...

In this thread is "stated" that, very broadly speaking:

raising the gain of a preamp -> less headroom (aka increased risk of unwanted distortion), but better S/N ratio...

lowering the gain of a preamp -> more headroom, but a bit worse S/N ratio...

Now, letting aside all the possible corrections, clarifications, and whatnot, what I'm asking is:

is there any other possible "downside", apart from a bit worse S/N ratio, when lowering (a lot) the gain of a (budget) preamp?...

Simple as that...
Headroom is more about the Level After the gain, a high input on the pre needs Less gain but could still have Less headroom..
The higher the max output of a pre the higher the headroom compared to a pre with lower max output.'
A pre that runs on +-24v will have more headroom than one that runs off +-18V..
Any amp stage has more distortion at high gain..Thats why some pres have two amp sections each with lower gain to lower distortion..instead of say 50dB from one amp block, each one has 25dB each...Another reason some pres have a input transformer, they can have any where from 6dB to 20dB of voltage gain, requiring less gain from the amp section...

I think its just better to have at least a good level (+4dB to +10dB) for other reasons, some stages/sections in a mixer/console may have a unity gain that needs a good level to drive headphones aux sends ect....

Last edited by nosebleedaudio; 29th March 2021 at 02:54 AM..
Old 29th March 2021 | Show parent
  #97
Gear Maniac
 
Stewix's Avatar
 
Quote:
Originally Posted by nosebleedaudio ➡️
Headroom is more about the Level After the gain, a high input on the pre needs Less gain but could still have Less headroom..
The higher the max output of a pre the higher the headroom compared to a pre with lower max output.
Ok...

Quote:
Originally Posted by nosebleedaudio ➡️
Any amp stage has more distortion at high gain..Thats why some pres have two amp sections each with lower gain to lower distortion..instead of say 50dB from one amp block, each one has 25dB each...
Sometimes it seems to me that, when gain staging my pre, at some point(s) I experience a bit of "non-linearity"...

What I mean is that there are a couple of "spots" where, when raising the gain by a very little amount, this causes a small drop in the level, instead of an increase; but after that, raising the gain knob a bit more, produces the "normal" behaviour of an increased level again...

Does this has to do with the fact that maybe, in those "spots", one is "crossing the border" of different amp blocks?...

And, in this case, that little raise of gain could be a setting that has a better headroom, being the "min" of the following block (instead of the "max" of the previous one)?...

Or I'm getting this wrong, considering that, for example, the previous amp block is fully used anyway?...

Quote:
Originally Posted by nosebleedaudio ➡️
I think its just better to have at least a good level (+4dB to +10dB) for other reasons, some stages/sections in a mixer/console may have a unity gain that needs a good level to drive headphones aux sends ect....
I'm not sure I understood you correctly: do you mean that one should be "less concerned" about lowering the gain "too much" when tracking, and "more concerned" for things like headphones aux sends and other similar things?...

Anyway, sorry for all these questions...

I believe that, while "using the ears" is fundamental, to have a better understanding of gear is also very important (especially when the ears are "suggesting" different and conflictual "stories", which need the correct interpretation of what's going on)...

Thanks!...
Old 29th March 2021 | Show parent
  #98
Lives for gear
 
nosebleedaudio's Avatar
 
🎧 15 years
As for the pot issue, could be a faulty/borderline pot, a cheap pot..
Its not uncommon for a good pot to be a little jumpy esp at the higher gain setting.

As for levels. I can't think of any good reason to have a lower than normal signal..Its just good common practice to get a good level, esp on a console, some amp sections after the mic pre are either unity gain or have a trim gain (10 to 15dB) that helps for too low or too high levels..
Old 29th March 2021 | Show parent
  #99
Gear Maniac
 
Stewix's Avatar
 
Quote:
Originally Posted by nosebleedaudio ➡️
As for the pot issue, could be a faulty/borderline pot, a cheap pot..
Its not uncommon for a good pot to be a little jumpy esp at the higher gain setting.

As for levels. I can't think of any good reason to have a lower than normal signal..Its just good common practice to get a good level, esp on a console, some amp sections after the mic pre are either unity gain or have a trim gain (10 to 15dB) that helps for too low or too high levels..
Thanks again...
Old 4 weeks ago | Show parent
  #100
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ulysses's Avatar
 
🎧 15 years
Quote:
Originally Posted by esldude ➡️
Look at the attached graph. It is the EIN curve vs gain for three different preamp circuits.

With the simple one if you were sitting at 60 db and decided to drop it to 42 db plus a FEThead at +18db you'll be worse off because the FEThead will boost a now even noisier signal than when you had only the pre setup for 60 db gain.

Let us suppose you had the 1570 preamp circuit. You lower gain from 60 db to 42 db and make it up with 18 db from the FEThead. There will be almost no difference at all. Actually a fraction of a db noisier.

I don't see any scenario where adding a FEThead lowers noise. In the right condition it will almost not change it. It can supply gain without additional noise penalties, but it can't lower noise. No matter how many times people think it can.
It's been a few months, but you're misunderstanding something important about what EIN means and I'd like to clear it up for the benefit of other readers. Let's look at the graph again, and as an example we'll first focus on just the lowest-noise preamp at 30 and 60dB of gain. The EIN appears to be about -125dBu at the 30dB gain setting, and -128dBu at the 60dB setting. YEs, the EIN number is lower at 60dB. But what does this mean?

EIN, Equivalent Input Noise, is the noise level that would need to be injected into the input in order to produce that amount of noise at the preamp's output. This is the preamp's self-noise and is independent of the signal and noise in the real input signal.

At 60dB gain, the preamp produces self-noise equivalent to injecting an input noise signal at -128dB. That noise is then amplified by the 60dB gain of the preamp, which means the self-noise at the output is now at -68dBu.

At 30dB gain, the preamp's self noise is equivalent to a -125dBu signal, but it's only being amplified 30dB, so that self-noise appears at the output at -95dBu.
There's less preamp noise at the output (where it matters) with the 30dB setting than with the 60dB setting.

You can look at the graph and see that for each of the preamp types shown, and for all gain settings, the difference in EIN from one gain setting to another is always smaller than the actual gain change. For all three preamp types shown, the preamp's self-noise performance is better at lower gain.

Remember, you set the gain to get the output signal up to whatever nominal level you've decided on. The noise of the preamp is only relevant in its relation to that output signal level. If you can somehow increase the input signal, you turn down the gain control to compensate, and you're turning down the preamp's own noise along with it. If the thing you used to increase the input signal (cloudlifter or fathead) makes its own noise, that'll be amplified along with the microphone signal. But the preamp's contribution to the noise floor will be reduced when you turn down its own gain control.
Old 4 weeks ago | Show parent
  #101
Gear Nut
 
So a fethead may help if it’ ein is lower than preamp’s...
Is there a math formula to calculate mic’s noise contribution to the above scenario?
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