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Buy a cheap mic booster; actually reduce the noise floor?

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Buy a cheap mic booster; actually reduce the noise floor?

Postby dfira » Tue Mar 02, 2021 4:24 pm

So there's a really cheap mic booster going around called the Klark Teknik CT1.

People on youtube love it, but honestly, I suspect a great deal of them don't have much of an idea what they're doing.

Ideally I'd want some specs, and to know what I'm looking at, but does anyone know/think if a mic booster will actually reduce the noise floor? if my pre-amp is not terrible (but not that clean either: Scarlett Solo 3rd gen), I wonder if a Behringer style booster while allowing me to increase the amount of gain, will actually give me the intended effect of a lower noise floor?

This is the typical Behringer experience. You kinda get what's advertised, but always in some way also kinda don't... Then you always end up thinking why Behringer, why did I let you do this to me again!?
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Re: By a cheap mic booster, actually reduce the noise floor?

Postby resistorman » Tue Mar 02, 2021 4:43 pm

I have a Crimson Audio booster I bought for my passive Royer SF12 ribbon mic so I can use it with standard mic preamps. Since it uses transformers and FET transistors, it gives a nice sound to dynamics and they can be used for more sensitive situations. But I don’t think the inherent noise level is improved.
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby Hugh Robjohns » Tue Mar 02, 2021 5:02 pm

dfira wrote:...does anyone know/think if a mic booster will actually reduce the noise floor?

If it did, tell Scotty because it would be 'agin the laws of physics, cap'n! :lol:

I wonder if a Behringer style booster while allowing me to increase the amount of gain, will actually give me the intended effect of a lower noise floor?

No. Not possible. Any amplifier raises the wanted signal level lots, and the residual noise floor very little, you hope -- thus improving the signal-noise ratio. But it can't actually reduce the noise floor.

However, if you are struggling to achieve a good signal level, a device like this can be helpful by proving the interface preamp with a signal that's typically 20dB larger than it would otherwise have been, thus reducing the workload of the interface preamp and putting a large signal through the converter.
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby DC-Choppah » Tue Mar 02, 2021 6:14 pm

If you have to crank up the preamp gain for your (low output) mic, then you are closer to clipping. That means you have less head room. Head room is the room to absorb big spikes.

So by adding 25 dB of gain right after the mic, you can set the preamp lower and the mixer lower, and absorb more big input spikes without clipping. More headroom.

The preamp adds a bit to your noise floor actually. But the point is to get more headroom when recording so you don't clip when using mics that need lots of gain.

I use an extra preamp like this when I use an SM57 to record something that is 3-4' away - cause I like how it sounds.
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby Sam Spoons » Tue Mar 02, 2021 6:54 pm

Surely clipping is dependent on the signal level reaching the preamp so if you are, say, 20dB below clipping then adding 20dB of gain from the preamp's gain knob will cause clipping, as will leaving the gain knob alone and inserting a 20dB booster into the mic cable?

WRT noise floor, doesn't it depend on where the noise is coming from? If it is inherent in the first stage of mic preamp then increasing the gain will increase the noise floor, adding a booster will allow you to reduce the preamp gain by the same amount so reducing the noise floor. But, if the noise comes from the mic or the room then adding a booster will make no difference to it.

The point of a booster is that it is very low noise so allows you to use a quiet source without having to push the gain in the preamp beyond the point where the preamps noise becomes audible.
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby shufflebeat » Tue Mar 02, 2021 8:03 pm

Sam Spoons wrote:Surely clipping is dependent on the signal level reaching the preamp so if you are, say, 20dB below clipping then adding 20dB of gain from the preamp's gain knob will cause clipping, as will leaving the gain knob alone and inserting a 20dB booster into the mic cable?

I've heard something similar discussed elsewhere, particularly with regard to feedback in live environments.

I'm with Spoons but people seem to have strong opinions (it was a while back in "another place") so would like to hear if there is anything in it.
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby blinddrew » Tue Mar 02, 2021 10:00 pm

Sam Spoons wrote:WRT noise floor, doesn't it depend on where the noise is coming from? If it is inherent in the first stage of mic preamp then increasing the gain will increase the noise floor, adding a booster will allow you to reduce the preamp gain by the same amount so reducing the noise floor.
[pedantry] Nope. It will reduce the noise as a proportion of the signal, but it won't actually reduce the noise. The noise floor doesn't change. That being those pesky laws of physics.[/pedantry]
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby Sam Spoons » Tue Mar 02, 2021 10:08 pm

:thumbup:
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby blinddrew » Tue Mar 02, 2021 10:17 pm

Sorry, winky face missing off the end of my post there. But you got the spirit of it. :)
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby Sam Spoons » Tue Mar 02, 2021 10:24 pm

blinddrew wrote:Sorry, winky face missing off the end of my post there. But you got the spirit of it. :)

If it had been on 'the other place your post or mine would probably have started a flame war by now :D
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby Hugh Robjohns » Tue Mar 02, 2021 10:48 pm

DC-Choppah wrote:But the point is to get more headroom when recording so you don't clip when using mics that need lots of gain.

I don't think that's the point back here on planet Earth! ;) Your's seems a rather odd and confused idea , to me.

Clipping is simply related to the signal level passing through a gain stage, and it's possible to clip either the input stage, or the output stage, or both!

Input clipping is obviously related to the input signal level versus the capability of the first active element of the preamp -- some preamps are equipped with switchable input pads to prevent input clipping/overload in the relevant situations (loud source, high-output mic).

Output clipping is usually dependent on the applied gain which raises the output signal closer to the power rails and thus clipping. So the idea is to set the gain such that the signal is as big as necessary, while maintaining a sensible headroom margin at the preamp's output.

A cascade preamp stage, like a Fethead or Cloudlifter, etc, typically boosts the signal level by around a fixed 20dB, and it's only used when the normal preamp has insufficient gain for the required situation. So if you need 75dB of gain, but the preamp only offers 60dB, a cascade preamp will get you there, with some extra gain available in hand. (20+55=75dB)

But there would be zero benefit -- and actually both a noise penalty and risk of input overload -- if you only needed 30dB of gain but still used a cascade preamp. (20+10=30dB) So rather than improve the headroom along the signal path, in that situation you'd actually be reducing it by risking input overload!

In most practical situations the audible noise floor is set by the recording environment's acoustic noise, rather than the mic or preamp's electronic noise. However, every mic generates some electronic noise (Johnson noise), as does every active gain stage.

In radio frequency engineering the electronic noise of an amplifier is specified in terms of the number of dB of noise added to the signal -- so a 1dB noise figure would be impressive and a 6dB figure rather unimpressive. All very straight forward and simple... But oddly, we don't do that in pro audio. Instead we have a slightly more obscure method which relates to the theoretical Johnson Noise of a 150 Ohm source impedance, which we call the Equivalent Input Noise, or EIN figure.

So, a 150 Ohm resistor generates a Johnson Noise level of a gnat's wotsit under -131dBu at room temperature (measured between 20Hz and 20kHz). So if the preamp added 1dB of noise it would be specified with an EIN figure of -130dB. If it was adding 6dB of noise, its EIN would be -125dBu. Most decent pro mic preamps have EINs of -127dBu or better (ie, they add 3 or 4dB of noise) and anything worse than -124dBu will be obviously noisy!

Of course, the lower the source impedance, the less Johnson noise, and so some manufacturers specify the EIN with a shorted (0 Ohm) input impedance -- so beware unusually impressive EIN numbers and/or specs that don't state the source impedance!
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby ef37a » Tue Mar 02, 2021 10:57 pm

Well, which "noise floor" are you talking about? I will check the figures tomorrow but this is what I recall from tests on my KA6 some months ago.

Mic gain at minimum n=-100dB fs. Mic gain at max n=-85dB fs. It follows therefore that if an external booster allows me to keep gain near minimum AND the booster adds less noise than the internal pre amp, I will end up with a lower noise floor.

I have just ordered one of those cheap boosters and I already have a Fethead, got it a couple of months ago but for several reasons have done nothing much with it. Give me another couple of weeks.

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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby dfira » Tue Mar 02, 2021 11:11 pm

Sorry, I meant increase the signal to noise ratio, and as a result, be able to lower the gain on my preamp.(which means a lowered noise floor?) (Is that technically correct?) I hope that was clear.

But I am counting on:

1) my preamp being the cause of much of the noise, - i suspect it is because with no mic plugged in, but the gain at where i would have it anyway, the noise is floating around -66dBU* when i plug the mic in it increases only very little, and that includes ambience.

2) the focusrite pres to be noisier than the cheap mic booster. If the booster is even equally as noisy as my preamp, then i get the benefit of being able to record quieter sources, but not a lowering of the actual noise, which is what i would like to achieve here.

* Don't catch me our on incorrect decibel annotation... I'm writing it from memory! :)
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby wireman » Tue Mar 02, 2021 11:26 pm

Sam Spoons wrote:WRT noise floor, doesn't it depend on where the noise is coming from? If it is inherent in the first stage of mic preamp then increasing the gain will increase the noise floor, adding a booster will allow you to reduce the preamp gain by the same amount so reducing the noise floor. But, if the noise comes from the mic or the room then adding a booster will make no difference to it.

The point of a booster is that it is very low noise so allows you to use a quiet source without having to push the gain in the preamp beyond the point where the preamps noise becomes audible.

From a perspective of noise introduced by the amplifier(s) and signal to noise ratio you would want most of the gain in the first stage (otherwise you are amplifying the first stage noise in the second stage. So the problem with this proposal is the "cheap" part [Edit.. as you want a low-noise stage first]. Secondly if the existing preamp has nonlinear noise so that it is much more noisy at high gain them multi-stage would make sense, I have a feeling that was an issue with my AW4416 preamps.
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Re: Buy a cheap mic booster; actually reduce the noise floor?

Postby James Perrett » Tue Mar 02, 2021 11:40 pm

dfira wrote:1) my preamp being the cause of much of the noise, - i suspect it is because with no mic plugged in, but the gain at where i would have it anyway, the noise is floating around -66dBU* when i plug the mic in it increases only very little, and that includes ambience.

With nothing plugged in the electronic noise will be higher than with the mic plugged in. How much noisier depends on the design of the mic preamp.
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