• Registered: 2016-06-14
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Topic: How to understand preamp specifications right…?

Hi all
Can someone help me with this:

ramses wrote:
oli77sch wrote:

And the mic gain even is ok to use some ribbon mics.

I need some of this extra gain when doing silent recording with levels around 70-75 dB.

That’s from another thread. My statement is concerned to the Babyface pro preamps, ramses talks about UFX III. In the meantime I was looking for something different initially, but came across the preamp gain specs again and discovered this:
UFX III
- Maximum input level, Gain 0 dB: +18 dBu
- Maximum input level, Gain 75 dB: -57 dBu
UCX II
- Maximum input level, Gain 0 dB: +18 dBu
- Maximum input level, Gain 75 dB: -57 dBu
UCX
- Maximum input level, Gain 0 dB: +10 dBu
- Maximum input level, Gain 65 dB: -55 dBu
Babyface pro FS
- Maximum input level, Gain 0 dB: +8 dBu, PAD +19dBu
- Maximum input level, Gain 65 dB: -57 dBu, PAD -46dBu

Now my question is: how to understand this numbers correctly? The maximum input level at highest gain setting is (almost) the same on this four devices. But what happens now with the amplification of, let’s say, dynamic mics with low outputs (SM7B, ribbons…) where I have to set very high gain values? Is it 10dB higher on the 'better' preamps on UCX II and UFX III?

UCX - FF 400 - Babyface pro - Digiface USB - ADI-2 (original)
Mac mini M1 - Macbook pro - iPad Air2

2 Reply by ramses (edited by ramses 2023-10-01 14:05:50)

  • ramses
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Re: How to understand preamp specifications right…?

If a preamp offers more gain, then microphones with low output power can be amplified correspondingly higher.
I have to record here (as already noted) because of the neighbors at quiet 70–80 dB,
then I need for the two SM57 a gain between 61–63 dB to get a sufficient level (how I like/need it for mixing).

With the large-diaphragm mics with their tube preamp, it is only 28 and 35 dB (also on the 12Mic).

The other aspect is how resilient the inputs are in terms of levels.

Regarding PAD: if they already support an input level of +18 dBu (e.g., UFX III), you can skip a PAD circuit or use a passive switchable attenuator in front of your Mic. Same like those that you can use to cure also level mismatches (see https://forum.rme-audio.de/viewtopic.php?id=25399 … e.g., JTS Mic attenuator MA-123).

When you activate maximum gain on an input, then the input becomes more "sensitive".
In the case of an UFX III, the input signal is amplified by 75 dB.
With such an amplification, you can't no longer send +18 dBu into the input.
You need to subtract this maximum gain of 75 dB from the maximum possible input level: 18 dBu - 75 = -57 dBu.
This is how you arrive at -57 dBu as the maximum input level for the UFX III if the gain is turned to 75 dB gain.

The same calculation applies for the other interfaces.

BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub14
  • Registered: 2016-06-14
  • Posts: 1,142

Re: How to understand preamp specifications right…?

Thank you ramses, you confirm what I already assumed. There was a point I was a bit confused because the max. input levels at max. gain are more or less the same on all the four preamps. But of course the output level at a certain point of 'sensitivity' differs by around 10dB.

UCX - FF 400 - Babyface pro - Digiface USB - ADI-2 (original)
Mac mini M1 - Macbook pro - iPad Air2
  • ramses
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Re: How to understand preamp specifications right…?

You're welcome Oli wink

BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub14