SNR/THD With Pad vs. Without Pad

Just finished doing some crude experiments taking SNR and THD readings from wave files recorded into pro tools through different mic pre's.
The test tones were ran through the pre's at 1khz and also at 20khz.
I never realized how much a mic input pad can effect your SNR.
You can get a drop of about 9-12 points depending on the pre,conditions,etc..


Does an attenuator on the output of a mic pre have the same effect as a balanced pad on the input?
Do you still get a big drop in SNR and dynamic range when an output attenuator is used in place of the input pad?Or can you use a input attenuator instead of the pad with better results?
I know all pre's are different so this question is aimed towards ic based pre's with ic based balanced output.or any pre I guess.

Attenuating the output of a mic pre would definitly improve SNR compared to attenuating the input.

But you attenuate the input because the source/mic is to much for the first gain stage to handle without overloading. with the pad you have to turn up the output more and then you hear the opamp's self noise more.

I don't think there is a way around this, or rather a way to build a quieter pad. Its not the Pad causing the drop in SNR.

I think thats what your asking about?

Its not the Pad causing the drop in SNR

Click to expand...

I have an Altec 1588c that I've made a mic pre from and this is the only pre I have that does not seem to be effected by an input pad.

The main difference between the pre's that loose SNR and the Altec is an input transformer.

An input transformer seems to keep the SNR ratio the same even when an input pad is applied.

Well thats weird. Theoretically if you lower the source signal (with pad)you will have to increase the gain of the whole system to achive the same output signal level. It seems it would be impossible to turn up a mic pre and not have more noise.

Maybe I'm missing something here.

It seems it would be impossible to turn up a mic pre and not have more noise

Click to expand...

Actually the SNR stays very close, or closer than the ic based pre's.
Yes there is still more noise when the pad is applied.

So surely the pad is just attenuating the signal? So signal drops, preamp noise remains the same., S/N therefore drops.

[Edit - I think I'm just restating what Bluebird said]

If the preamp is VERY quiet in terms of noise then you may not notice the difference.