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ATC SCM 45 A Pro - Input Sensitivity

patient 67

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Sep 3, 2022
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Hi together.

I own a pair of ATC SCM 45 Pro Speakers.

the input sensitivity of the speakers amp, can be adjusted from 1V to 2V.

The speakers are connected to an Interface (UA Apollo 16) with Output of 20dBU.

Volume control is analog, passiv.

Changing the setting at the ATC from 1V /2.2dB to 2V / 8.2dBu makes a not subtle change in sound.

I did some research, but did not find a clear answer so I want to try it in this forum.

With the 1V setting the bass sounds "mellow" and everthing else is somehow undefinied. But the bass is more prominent.

With the 2V setting everything comes to live, bass is less prominent audible, the whole thing sounds more cohesive.

And I really do not mean subtle changes. The intern, customers and even my wife can hear a clear difference.

I measured the frequency response at the listening position.
The curve is the same with both settings.

What is happening here? Is the phase differnt, is it a differnt level of distortion?

I would be really intersted in what is going on here on the technical side.

I say thank you in advance to everybody!
 
OP
P

patient 67

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Simple: It's not level matched.
thank you for your answer.

I do not really understand why one should be a better match than the other.

The reality is, as far as I understand it, that the amp sees always less power than it should get on the paper.

let me explain the signal path.

the music is coming from an audio interface. this is also the D/A converter.
I can choose to set the audio / music level digital before the converter, or analog after the converter.

For the difference in sound i am hearing by switching from 1V to 2V at the active Speaker, I can do both things, analog gain reduction after converter or digital gain reduction before the converter - the difference in sound is the same. so it does not matter here.

fact is, for a comfortable listening (or working level) the signal has to be turned down be around 35 dB before it gets to the active speaker. No one listens tpo active speakers at full level all the time - they blow you ears off.

Turning the amp at the active speaker from input senstivity 1V to 2V results in a drop of output level of around 6dB -> at the amp of the speaker.

To sum it up.

I have the input sensitivity at the active speaker set to 1V. The music that hits the amp is peaking at around - 35 dBFS - it sounds not good.

I switch the input sensitivity to 2V at the active speaker. The Music that hits the amp is now peaking at around - 29 or -30 dBFS - it sounds better.

But this is only 6 dB more of digital gain at the 2V setting + the amp expecting 6 dB more gain at the same time.

I have no clue why it sounds so different.
 

HarmonicTHD

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As dfuller said. You need to level match the output when comparing. Meaning measuring the voltage at your speakers input using a test tone and make sure it is the same in both your scenarios. Otherwise a difference in loudness will occur which is perceived as sounding different by you.
 

FeddyLost

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The higher signal goes from interface (below clipping), the better SNR and all distortion numbers.
It's a fact that you can't ignore: just check local tests of audio sources.
So, most probably you have some audiable signal degradation when you have to attenuate it at interface instead of speaker.
To be sure if this really present, I'd
1) make a DBT with precision level matching (if you want, because it will be really not easy to do with fast switching) ,
2) exchange the cables (maybe your current cables are more supsceptible to noise than could be) and
3) try out this experiment with interface with lower nominal output.

PS To have correct volume regulation, I had to trim input on my speakers to -20 dB.
 

Darkman

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The problem is in pre.
It fails like most modern drivers.
 
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