You are here

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

All about the tools and techniques involved in capturing sound, in the studio or on location.

Moderator: Moderators

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

Postby shufflebeat » Wed Mar 03, 2021 1:26 am

Hugh Robjohns wrote: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!

This relates to the discussion I referred to earlier. With a less forgiving preamp is there another price to be paid, not noise but character of sound, when hitting it towards the higher end of it's capacity with a 'lifter of some sort and would it not be better in that case to accept the noise which might be the lesser of two evils?
shufflebeat
Jedi Poster
Posts: 6139
Joined: Sun Dec 09, 2007 1:00 am
Location: Manchester, UK
"Dance, dance. wherever you may be, for I am the Lord of the damp settee..."

Do yourself a favour, wear earplugs at gigs.

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

Postby CS70 » Wed Mar 03, 2021 1:33 am

dfira wrote: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.

In general noise floor is due to thermal conditions and the current in the circuitry, and for a mic simply the fact that there's air molecules around the capsule hitting it at random.
So it stays where it is.

However a booster - basically using two clean gain stages instead of one - allow you to keep the signal in the best performing band of the final preamp (and nowadays, the A/D conversion analogue front end).

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.

Without anything plugged, you're just listening to the impedance noise, which means nothing: if the preamp isn't broken, when you plug in a microphone that noise will drop.

That's the reason why you mute any preamps you don't use.


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! :)

Preamplifiers are exactly designed to amplify very small voltages (mic level goes from about 0.001 to 0.01 volts) without adding much noise and distortion. By definition any good ones are _not_ noisy. That's one of the reason most stuff works at line level (higher voltage), because it's far easier (and cheaper) to make as it does not need be so good as a preamp.

A fixed-gain clean booster will not need the variable gain circuitry, so everything else being equal it will generally be a little less noisy than a regular preamp with an equivalent gain setting.. but just a little, down from an already very small value.

The benefit of using it is, as above, that by staging the gain that way you can keep the variable-gain preamp in the sweet spot where everything is running at best (and heat is kept down. A lot of electrical noise is due to heat in resistors - put some ice under the pre and it will be a trifle quieter).

It's very unlikely your Focusrite is particularly noisy, but if you have to crank the gain you will amplify ambient noise. Any noise you hear when you get the gain you need is ambience (or actual noise you're producing, clothes are a killer for that).

With microphones, the only way to reduce the signal-to-ambient noise ratio is to reduce the ambient noise, or work nearer to the mic.

And about the decibel notation - it is not something you can ignore. "decibel" is just a ratio, so if you don't say what you are taking a ratio of, the number gives no meaning. Being unsure of "what" decibel you're talking about is like going to a supermarket and say you want to buy "something".
User avatar
CS70
Jedi Poster
Posts: 7719
Joined: Mon Nov 26, 2012 1:00 am
Location: Oslo, Norway
Silver Spoon - Check out our latest video and the FB page

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

Postby ef37a » Wed Mar 03, 2021 9:18 am

Ey up chaps! Don't go putting ice (and therefore water) near your mic pres.

Cooling a 1k resistor, a value you might find in the feedback circuit, from 20C to 0C only drops the 20kHz bandwidth noise from -122.68dBu to -122.99dBu.

You have to get close to absolute zero to have a significant effect on noise and no electronics the likes of us can get our hands on will work down there!

Dave.
ef37a
Jedi Poster
Posts: 12836
Joined: Mon May 29, 2006 12:00 am
Location: northampton uk

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

Postby dfira » Wed Mar 03, 2021 9:27 am

Interesting stuff!!
dfira
Regular
Posts: 64
Joined: Tue Nov 03, 2020 3:56 pm

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

Postby CS70 » Wed Mar 03, 2021 9:35 am

ef37a wrote:Ey up chaps! Don't go putting ice (and therefore water) near your mic pres.

Cooling a 1k resistor, a value you might find in the feedback circuit, from 20C to 0C only drops the 20kHz bandwidth noise from -122.68dBu to -122.99dBu.

You have to get close to absolute zero to have a significant effect on noise and no electronics the likes of us can get our hands on will work down there!

Dave.

Ahah yeah it was a joke :D
User avatar
CS70
Jedi Poster
Posts: 7719
Joined: Mon Nov 26, 2012 1:00 am
Location: Oslo, Norway
Silver Spoon - Check out our latest video and the FB page

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

Postby Hugh Robjohns » Wed Mar 03, 2021 11:57 am

shufflebeat wrote:With a less forgiving preamp is there another price to be paid, not noise but character of sound, when hitting it towards the higher end of it's capacity with a 'lifter of some sort and would it not be better in that case to accept the noise which might be the lesser of two evils?

If you can engineer the gain structure appropriately neither overload distortion nor noise should bother you... if you can't you can make your choice. ;-)
User avatar
Hugh Robjohns
Moderator
Posts: 30631
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound
In my world, things get less strange when I read the manual... 

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

Postby James Perrett » Wed Mar 03, 2021 3:25 pm

I noticed that the Focusrite noise specs for the Scarlett Solo Gen 3 are a bit unusual. They say the figure is A weighted with no bandwidth mentioned. I would imagine that this would improve the number slightly. I seem to remember the last generation had a slightly disappointing noise performance (EIN of -120dBu) so I was wondering if this was a way of massaging the numbers.
User avatar
James Perrett
Moderator
Posts: 10538
Joined: Mon Sep 10, 2001 12:00 am
Location: The wilds of Hampshire
JRP Music - Audio Mastering and Restoration. JRP Music Facebook Page

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

Postby Hugh Robjohns » Wed Mar 03, 2021 4:23 pm

James Perrett wrote:They say the figure is A weighted with no bandwidth mentioned. I would imagine that this would improve the number slightly.

Typically by about 5dB in the case of an EIN measurement. The noise of a 150 Ohm source is at room temperate measured between 20Hz and 20kHz is -131dBu with a flat measurement, and -136dBu if A-weighted.

I was wondering if this was a way of massaging the numbers.

Yes, undoubtedly... although given that the noise floor is inherently low level, it could be argued that there is some pragmatic sense that A-weighting better indicates its subjective intrusion, I suppose...

The original Solo claimed an EIN of -125dB without any other details...
User avatar
Hugh Robjohns
Moderator
Posts: 30631
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound
In my world, things get less strange when I read the manual... 

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

Postby dfira » Wed Mar 03, 2021 5:16 pm

Having nothing to go buy except prior experience many moons ago with pro level gear, I have to say I have *felt* like these pres were on the noisy end of the spectrum. I know they can vary between device to device, so I thought perhaps I just got a particularly noisy one.

But then again, the only two mics I have used this interface with, also happen to be on the noisy end, so I'm not pointing fingers.
dfira
Regular
Posts: 64
Joined: Tue Nov 03, 2020 3:56 pm

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

Postby DC-Choppah » Thu Mar 04, 2021 6:37 am

My analog mixer has a PFL (Pre Fader Level) button. The procedure is that you hit the PFL button and set the mic preamp gain to get the level to the recommended spot - green going to yellow. After setting the mic preamp levels I set up the mix to get everything at musical levels so the monitor sounds musical, and then set the interface's levels so I am recording at approximately the balance I want for the song with the interface at green/yellow but not higher.

When I tried to use an SM57 on my piano (instead of my usual condensers), I could not get my PFL up to the recommended level for the piano. The piano was weak even with the mixer preamp set to max. Then I added more gain at the interface to be able to hear the piano in the monitor (headphone). But now when a bass drum hits from across the room, the interface clips! It feels like a lack of headroom to absorb the bass drum into the SM57 piano mic.

Adding the extra gain with an additional preamp to get the PFL to the right place solved the 'head room' problem. You guys helped me with that one a long time ago and I keep a tube preamp in my rack so I can try an SM57 on some things instead of a condenser.

I think the extra preamp just allows you to use a dynamic and condenser mic interchangeably without sacrificing dynamic range of your analog signal chain.
User avatar
DC-Choppah
Frequent Poster
Posts: 1744
Joined: Fri Jul 20, 2012 12:00 am
Location: MD, USA

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

Postby ef37a » Thu Mar 04, 2021 9:16 am

DC-Choppah, I would love to know the make and model of your mixer. interface and that valve pre amp?

Your, well reasoned explanation shows the basic fact that in any amplification chain, something will determine the clipping point. I am not surprised that it turns out to be the interface, for a couple of reasons.
1) Almost all AIs use a 'One Knob' gain control and we rarely get pads on them these days. With just one gain control the practical gain range is perhaps +10dB to +60dB and even with that range the setting of the control at the high gain end becomes difficult, i.e. the top end is very 'cramped'. Thus there has to be a trade off between maximum gain and headroom, most going for higher gain* Mixers on the other hand have effectively three levels of gain control. That around the mic pre amplifier, the channel fader which often gives another 10dB at max and another 10dB at the master fader. Thus, greater headroom can be 'dialed' in.

2) Interfaces often have lower voltage supply rails than mixers and external pre amps. Typically a modest mixer will run at +&- 15V maybe 17V. I have not measured inside any AIs but I doubt 'budget' devices run much above 2x 12 volts? "Bus" powered AIs could be even less.

Valves of course can have massively greater headroom than solid state equipment but I am rather surprised yours has such a good noise performance? Hence my curiosity!

*This, IMHO is as it should be because it allows the use of a dynamic microphone for vocal work although quiet V/O work might still be problematic especially with the likes of a 7b. Overload problems can be sorted quite cheaply with an external attenuator should the need arise.

Dave
ef37a
Jedi Poster
Posts: 12836
Joined: Mon May 29, 2006 12:00 am
Location: northampton uk

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

Postby Hugh Robjohns » Thu Mar 04, 2021 11:16 am

DC-Choppah wrote:When I tried to use an SM57 on my piano (instead of my usual condensers), I could not get my PFL up to the recommended level for the piano.

Yes. Insensitive mic, low level source, preamp with restricted gain range.

Then I added more gain at the interface to be able to hear the piano in the monitor (headphone). But now when a bass drum hits from across the room, the interface clips!

I'm not entirely clear what you're doing here -- is the bass drum miked and going through the desk (and with the channel gain aligned using the PFL method as you described), or is this ambient spill picked up by the SM57?

Either way, the problem is a lack of headroom in the interface because you've wound its input sensitivity up.

If it's acoustic spill, being picked up by the SM57, the most likely reason for the apparently excessive signal level is that the mic is in a resonant chamber formed by the piano and its lid that happens to over-emphasise the bass drum fundamental, and/or it's positioned in an anti-node of the room's standing waves that do the same thing.
User avatar
Hugh Robjohns
Moderator
Posts: 30631
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound
In my world, things get less strange when I read the manual... 

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

Postby Sam Spoons » Thu Mar 04, 2021 11:53 am

[pedantry] PFL = "Pre Fade Listen" aka "Solo" [/pedantry] :blush: used when live mixing to listen to an individual channel or bus without affecting the main output. On many mixers it also has the useful benefit of sending the signal to a meter (rather than just the 'signal present' channel LED) for more resolution when setting the gain structure.
User avatar
Sam Spoons
Jedi Poster
Posts: 15607
Joined: Thu Jan 23, 2003 1:00 am
Location: Manchester UK
Still taking this recording lark seriously (and trying to record my Gypsy Jazz CD)........

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

Postby Hugh Robjohns » Thu Mar 04, 2021 12:24 pm

Good point well made.

Adding further to the pedantry... although many budget mixers rename PFL as Solo, on high-end mixers they are actually quite different things.

A high-end, professional studio mixer normally has three channel monitoring modes: PFL, AFL and SOLO (or SIP -- solo in place!).

PFL monitors the signal from just before the fader (pre-fade listen), so it ideal for optimising the gain structure through the first part of the channel, and also for checking a source before fading it up. On a mono channel, PFL is also mono, of course, but may be mono or stereo on a stereo channel, depending on the architecture of the desk.

AFL (after fade listen) monitors the signal somewhere after the fader's buffer amp (to make up the signal level lost through the fader), so allows checking of the signal's contribution to the mix bus. Often AFL is actually derived after the pan-pot, so reflects any panning decisions too and in that case AFL would be in stereo of course, from both mono and stereo channels.

And then there's SOLO or SIP -- sometimes also rightly known as 'Destructive Solo'. On a broadcast desk this mode is automatically disabled when the studio's red TX light goes on.

Whereas PFL and AFL are monitored via separate and dedicated PFL/AFL busses, and can be used without affecting the main mix output in any way, the Solo function actually leaves the Solo'd channel alone, but mutes every other channel (and usually their aux sends, too) not selected to be either Solo'd or 'Solo Safe'. And instead of the monitoring switching to the PFL/AFL bus, it stays on the main mix output. So this is 'destructive' in the sense that the mix bus output is changed, with only the Solo'd channel(s) remaining audible.

Typically, things like FX return channels would be selected as 'Solo Safe' -- meaning that they won't be muted along with everything else -- so that a Solo'd channel can be auditioned along with any effects its contributing to. Different desks have different arrangements to control whether channels are Solo'd exclusively, or if multiple channels can be Solo'd and added or removed, and also in how the solo-safe system works.

Obviously, since the Solo system hijacks the main stereo mix outputs of the console, it is a stereo monitoring mode, and includes the effects of both fader levels and panning.

Some Solo systems get even more clever, and include a 'Solo-in-Front' mode where, rather than muting everything else, they just turn everything else down a bit... so this allows you to hear the solo'd channel(s) but still in some context with everything else. The balance of solo'd channel to the rest is usually adjustable.
User avatar
Hugh Robjohns
Moderator
Posts: 30631
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound
In my world, things get less strange when I read the manual... 

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

Postby The Elf » Thu Mar 04, 2021 12:36 pm

Hugh Robjohns wrote:Some Solo systems get even more clever, and include a 'Solo-in-Front' mode where, rather than muting everything else, they just turn everything else down a bit... so this allows you to hear the solo'd channel(s) but still in some context with everything else. The balance of solo'd channel to the rest is usually adjustable.
One of the features of Cubase I use most often. Steinberg call it 'Listen'.
User avatar
The Elf
Jedi Poster
Posts: 16547
Joined: Tue Aug 14, 2001 12:00 am
Location: Sheffield, UK
An Eagle for an Emperor, A Kestrel for a Knave.

PreviousNext