[ProAudio] Microphones question--Mic preamp gain and optimum mic output level

Richard L. Hess lists at richardhess.com
Sun Jun 20 13:46:53 PDT 2021

Hi, Dan,

As someone who has been involved with the design of some large 
facilities, this type of approach occurred to me. We had two big 
concerns: Maintaining the quality of the microphone sound, and not 
having one operator shoot another in the foot by adjusting gain at the 
wrong time.

My solution was to have dual-gain mic preamps at the connection panel 
and run an intermediate level to Radio broadcast, TV broadcast, 
recording, FOH, and Monitors. The mic preamps would switch between 
condenser and ribbon (for sake of argument)--and maybe have a "line" 
input. It didn't fly.

Another engineer I know currently does designs for large venues/shows 
and in his world, the inputs are all bridged across the raw mic output.

I fear this type of thing is like pushing a string.

Good luck!



On 2021-06-14 3:49 p.m., Dan Mills via ProAudio wrote:

> The interesting takeaway from that calculation is that actually given 
> a reasonable ADC (-97dBu self noise or so), and a reasonable mic 
> (-120dBu self noise or so) we only need about 30dB of front end gain 
> going into a modern ADC to make the mic self noise completely dominant 
> (At least for a condenser mic, a ribbon I am unconvinced about, I 
> would need to do sums).
> If that is in fact the case, then all the rest of any required gain 
> can be done digitally (post capture if need be) without in any way 
> impacting the noise floor, such a system would be almost immune to 
> clipping anything except the mic (-90dBu self noise from mic + preamp 
> at ADC, ADC clips at +20dBu or thereabouts, so you have a theoretical 
> 110dB of dynamic range), assuming the mic can swing to -10dBu and the 
> preamp can hit +20dBu....
> All of a sudden those preamps with 60dB of gain range on the pot start 
> to seem rather pointless, just do a fixed +30dB and hit the ADC, the 
> mic is dominant, and a simple multiply in the digital domain gets you 
> any level you like with little risk of managing to clip anything?
> In fact, given that that +20dBu to drive an ADC is purely an artefact 
> of pro audio expectations (The actual ADC chips usually want about 
> 2V), so you really only need about +12dB (Which I will grant is hard 
> to make really quiet). I am wondering about one of those opamp
> What have I missed?
> On the subject of Ein, does nobody discuss Iin, particularly for a 
> bipolar input stage it is often a more significant noise source then 
> Johnson once the source gets above a few k.
> Regards, another Dan.
Richard L. Hess                   email: richard at richardhess.com
Aurora, Ontario, Canada           http://www.richardhess.com/
Quality tape transfers -- even from hard-to-play tapes.

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