1 Topic by Data (edited by Data 2023-01-04 12:11:54)

  • Data
  • ADAT user
  • Offline
  • From: Germany
  • Registered: 2021-04-13
  • Posts: 29

Topic: Mic preamps in the digital domain

This maybe a stupid question... what is the purpose of having mic preamps when a line level output is never used? I am thinking of products like the RME 12Mic where the signal goes directly into the AD converter, but the same goes for many interfaces with in-built mic preamps. Surely a mic signal can be AD converted directly. Maybe AD converters require a line level signal for some reason? Or does it have to do with optimising SNR?

UCX II, Quadmic II, ADI-2 DAC FS, MacBook Pro (M1 Pro), Logic Pro

2 Reply by ramses (edited by ramses 2023-01-04 12:50:03)

  • ramses
  • DAW Mastermind
  • Offline
  • From: UTC+1
  • Registered: 2012-10-14
  • Posts: 10,778

Re: Mic preamps in the digital domain

Sorry, I do not get the point of your question, please rephrase.

The task of a Mic preamp is, to bring the low output of a mic to line level.

The 12Mic performs an A/D conversion which is needed to be able to send the data to the recording interface, either through MADI or through ADAT.

Some Mic Preamps have
- either exclusively analog outputs (QuadMic II) or
- ADAT and additionally analog outputs (OctaMic II)
so that you can connect the preamps also analog to another AD converter which are located
- either directly at the recording interface
- or located at an AD converter which is connected to the recording interface via ADAT, MADI, AVB, ...

BR Ramses - UFX+, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1650v4, Win10Pro22H2, Cub12Pro

3 Reply by Data (edited by Data 2023-01-04 13:13:37)

  • Data
  • ADAT user
  • Offline
  • From: Germany
  • Registered: 2021-04-13
  • Posts: 29

Re: Mic preamps in the digital domain

The 12Mic does not have any analog outputs (not counting the Phone out), so why would a line level voltage ever be required? The mic level signal could go directly into the ADC for conversion. In the subsequent digital domain "line level" does not have any meaning, to my knowledge.

UCX II, Quadmic II, ADI-2 DAC FS, MacBook Pro (M1 Pro), Logic Pro
  • ramses
  • DAW Mastermind
  • Offline
  • From: UTC+1
  • Registered: 2012-10-14
  • Posts: 10,778

Re: Mic preamps in the digital domain

Wait a minute, I have the feeling that everything is getting more and more confused wink

First. Since I didn't understand your initial question, I just summarized some basic things about the topic in the hope that it helps you to collect your thoughts and to formulate your question understandably.
TBH .. I still don't know what you are even getting at or what your problem is.

> The 12Mic does not have any analog outputs (not counting the Phone out)
Has no one claimed

> so why would a line level voltage ever be required?
Has no one claimed, maybe there is a misunderstanding from your side

> The mic level signal could go directly into the ADC for conversion
Thats what I told: "The 12Mic performs an A/D conversion which is needed to be able to send the data to the recording interface, either through MADI or through ADAT"

> In the subsequent digital domain "line level" does not have any meaning, to my knowledge.
Of course not and has no one claimed.

BR Ramses - UFX+, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1650v4, Win10Pro22H2, Cub12Pro

5 Reply by ramses (edited by ramses 2023-01-04 13:47:04)

  • ramses
  • DAW Mastermind
  • Offline
  • From: UTC+1
  • Registered: 2012-10-14
  • Posts: 10,778

Re: Mic preamps in the digital domain

Maybe I should rephrase my sentence here:

"The task of a Mic preamp is, to bring the low output of a mic to line level."

The output signal of a microphone is so low that a normal ADC could not convert it.
You need a preamp stage to bring it to a signal level where the ADC conversion can take place.

Analog converter have different reference levels as you have not one common signal level regarding connected devices.
From consumer level up to +24 dBu.

I do not know exactly what level is needed by the pure AD converter chip. In a data sheet of the AD9213 I found values in the range of 1,2V which is American studio level (+4 dBu).

The mic signal alone is definitively too low for AD conversion, as it is in the range of 0.001 - 0.01 V.

BR Ramses - UFX+, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1650v4, Win10Pro22H2, Cub12Pro

6 Reply by Data (edited by Data 2023-01-04 13:52:44)

  • Data
  • ADAT user
  • Offline
  • From: Germany
  • Registered: 2021-04-13
  • Posts: 29

Re: Mic preamps in the digital domain

ramses wrote:

Maybe I should rephrase my sentence here:

"The task of a Mic preamp is, to bring the low output of a mic to line level."

The output signal of a microphone is so low that a normal ADC could not convert it.
You need a preamp stage to bring it to a signal level where the ADC conversion can take place.

Analog converter have different reference levels as you have not one common signal level regarding connected devices.
From consumer level up to +24 dBu.
I do not know by heart what exact level is needed by the pure AD converter chip.
But .. the mic signal is definitively too low, it needs an amplification in the range of factor 100 if I remember right.

Thanks, this is the answer I was looking for. So ADC chips are designed to require a certain input voltage (line level, meaning a few volts) to work properly, and if it is too low (mV range) it cannot do a conversion. Didn't know that...

UCX II, Quadmic II, ADI-2 DAC FS, MacBook Pro (M1 Pro), Logic Pro
  • ramses
  • DAW Mastermind
  • Offline
  • From: UTC+1
  • Registered: 2012-10-14
  • Posts: 10,778

Re: Mic preamps in the digital domain

You're welcome, btw I added more specific information to my last posting.

BR Ramses - UFX+, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1650v4, Win10Pro22H2, Cub12Pro
  • jusu
  • ADAT user
  • Offline
  • Registered: 2023-02-11
  • Posts: 15

Re: Mic preamps in the digital domain

One addy to the discussion:  I prefer those recording interface which do not only  provide MIC inputs ( with internal preamp) but also a line output of these signals have taken the analog way. I use those outputs to attach a second recording unit for both redundancy and peak protection (recording at a lower amplification stage).

  • From: Germany
  • Registered: 2007-04-10
  • Posts: 9,385

Re: Mic preamps in the digital domain

Data wrote:

Thanks, this is the answer I was looking for. So ADC chips are designed to require a certain input voltage (line level, meaning a few volts) to work properly, and if it is too low (mV range) it cannot do a conversion. Didn't know that...

It can - but the signal will be noisier. In practice (depending on the signal source and microphone), most mic signals won't need more than about 20-50 dB of preamplification. A good 24 bit converter would also be able to do that without gain, but soft passages would be too close to the converter's noise floor... Also, the mic inputs provide phantom power where needed.

So yes, you were on the right track here already...

Data wrote:

Or does it have to do with optimising SNR?

Regards
Daniel Fuchs
RME

RME Support's Website

  • From: Houston, TX
  • Registered: 2007-05-11
  • Posts: 1,189

Re: Mic preamps in the digital domain

I'm pretty sure some of the digital mics (Neumann?) use minimal gain before hitting the AD circuit inside the mic.  But those AD's will never see anything other than what's hardwired inside the mic.  So there's no need to handle line level into them, or to hand-off line level to downstream devices.

MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8
  • cyrano
  • DAW Mastermind
  • Offline
  • From: Belgium
  • Registered: 2015-04-28
  • Posts: 676

Re: Mic preamps in the digital domain

The +4 dB line level is a standard. Over time some manufacturers offered +14 dB or even more. A mic preamp needs a +/- 12V DC supply to get to 0 dB. Want more? You need a +/- 17 VDC. Some preamps even use +/- 24 VDC.

That was fine in analog land.

ADCs usually have a 5V power supply. Sometimes even just 3,3 VDC.

That results in a bit less than half of that as input level. So the analog standard is a bit too hot. That's why most ADCs attenuate the line level signal before entering the converter.

MB Pro - FireFace 400 & DigiFace USB
ADAT gear: Korg, Behri, Fostex, Alesis
Appsys.ch ADAT over UTP

cyrano's Website