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How to calculate system SNR?

post #1 of 8
Thread Starter 
Hi Friends,
I wanted to calculate total SNR of my system? Is it an avg. SNR or lowest SNR component prevails or it is even lower then that

Source:
SNR: 120 db
THD: 0.00005%

Preamp:
SNR: 100db
THD: 0.01%

Amp:
SNR: 120 db
THD: 0.008%

Will the effective SNR of my system will be 100 db or lower?
post #2 of 8
Quote:
Originally Posted by aazeez1975 View Post

Hi Friends,
I wanted to calculate total SNR of my system? Is it an avg. SNR or lowest SNR component prevails or it is even lower then that
Source:
SNR: 120 db
THD: 0.00005%
Preamp:
SNR: 100db
THD: 0.01%
Amp:
SNR: 120 db
THD: 0.008%
Will the effective SNR of my system will be 100 db or lower?

The SNR of your system is whatever it actually is. In general measured SNR's tend to be far less than might be predicted from manufacturer specs. This is because manufacturer specs are usually based on an ideal system, and in the real world and as you know, nothing is ideal.

The SNR of your system might be measured by measuring the voltage across your main speaker's terminals when the system is playing a recording that is essentially silence. You would compare that to the voltage that corresponds to the rated output of the power amp that usually drives that speaker. This ignores things like loudspeaker driver compression, which is usually on the order of several to a dozen or more dB.
post #3 of 8
Quote:
Originally Posted by aazeez1975 View Post

I wanted to calculate total SNR of my system

Arny gave you good advice, and I can add a little:

The s/n is almost always limited by the original recording. Many recordings have an s/n of 60 dB or even less, and occasionally 70 dB. So all your gear, even if it's not as quiet as the specs claim, will still be far quieter than the source.

For gear having the same s/n that's wired in series, each time you double the number of devices the s/n is reduced by 3 dB. You have two devices with 120 and one with 100, so obviously the 100 will dominate. It's possible to calculate how much that 100 dB will be reduced by, but it's not worth the effort. I'll guess you'll end up with something around 99 dB.

Audio gear specifies s/n using a method called A-Weighting, which takes into account the ear's sensitivity at difference frequencies.

--Ethan
post #4 of 8
Thread Starter 
Awesome guys, thank you for your reply. I understand that manufacturer's spec are for ideal environment, but just for sake of argument lets take that on its face value.

I have Magnepans 1.7 with stereo sub. Room is treated enough to be able to produce good measurements on REW. Now, when I connect source directly to the amp level matched, it sounds better on treble region even with digital attenuation. But, when I put preamp in the chain the sound is not as crisp. I can detect that even in streaming media like pandora etc. and much more on high res files.
post #5 of 8
Quote:
Originally Posted by aazeez1975 View Post

when I put preamp in the chain the sound is not as crisp.

Lack of "crispness" is not related to the noise floor of your gear. That's usually due to a poor high frequency response or audible distortion. However, the specs you showed are good. I can tell you for certain that you need to compare with and without quickly, and with the levels exactly the same. Do you have an SPL meter? Simply having one playback 1 or 2 dB softer than the other will definitely sound like it's less crisp.

--Ethan
post #6 of 8
Quote:
Originally Posted by Ethan Winer View Post

Lack of "crispness" is not related to the noise floor of your gear.

Agreed. Yes, background noise can reduce sound quality, but only when it is as bad or worse than we were forced to tolerate in the days when vinyl and analog FM were all we had. Those days are thankfully gone for most of us.
Quote:
That's usually due to a poor high frequency response or audible distortion. However, the specs you showed are good.

The probable source of a lack of crispness has not been talked about that much in this thread, but it is talked about all over this forum.

The most probable cause of a lack of crispness is the room and the speakers. The recordings have a lot to do with that as well but there isn't that much that we can do about them.
post #7 of 8
Thread Starter 
Good points, so what should I do?

1) Sell the preamp and connect the source directly to the amp? (Live with digital attenuation?)

2) Use the preamp and not worry about SNR and THD
post #8 of 8
Quote:
Originally Posted by aazeez1975 View Post

Good points, so what should I do?
1) Sell the preamp and connect the source directly to the amp? (Live with digital attenuation?)

Digital attenuation done right is not a problem. It has the same noise and distortion characteristics as analog, only the noise floor can be moved down as low as you would like to put it by adding bits to the processing.
Quote:
2) Use the preamp and not worry about SNR and THD

Sorta. Your problems with lack of crispness are probably coming from the other influences we described. Fix them, and see what you want to do.

These is some possibility that the preamp has a flaw due to partial parts failure since it was built. You might want to implement the configuration change in step one, and see what happens.
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