pads, driving mic pre's harder

[fenix]

Okay, this may be a difficult question to answer, i'm not sure. Here's my question:

I've read several places (other recording forums, the rnp website) that people will activate a pad for the sole purpose that they can "drive the mic preamp hotter". What is the point, disadvantages, and advantages of this? Will this increase or decrease noise, thd, etc?

[Tonio]

IMO it would depend on the pre and mic. Of course padding the pre would neccesitate jacking up the pre , so it could raise the op level to possibly running into THD. But some pre's sound better that way .
Especially it it has iron in the input stage. In a solid state chipped based unit, you prolly wouldn't want to do that.
Some opinions say that padding could add unecessary filtering artifacts, and should be avoided. I think you will get a two sided answer.
Its just my opinion, and I'm sure some others will give you more
to go with. But it sounds like something to experiment with.

[chessrock]

A pad is used for safety purposes to avoid clipping.

And engaging one seems kind of counter-productive to the idea of "driving something," where the goal would be to get it as hot as possible at the input.

[McQ]

Originally posted by fenix
Okay, this may be a difficult question to answer, i'm not sure. Here's my question:

I've read several places (other recording forums, the rnp website) that people will activate a pad for the sole purpose that they can "drive the mic preamp hotter". What is the point, disadvantages, and advantages of this? Will this increase or decrease noise, thd, etc?

Preamps can have at least two distinct dynamic ranges: the signal handling capability of the "front end" (i.e., pre-gain stage) and the dynamic range of the circuits after gain is applied. Quick example: let's put a +28dBu signal into the front end of a preamp whose gain is 0 and whose clip points for both the front-end and post-gain stages are the same (let's say +30dBu). With no gain and a +28dBu signal, neither the front-end or post-gain stages will "clip". However, if we add 3dB of gain to the same input signal, while the input is not clipping, the output will be.

As I said, there are AT LEAST the two pre and post-gain stages with possibly distinct dynamic ranges. It's possible that additional gain stages can be added after the first, each with their own dynamic range (example: ART Tube MP...it's possible to clip the crap out of the front end, while the last output gain-stage is fine...and vice-versa). "Proper" gain-staging (usually taken to mean that we maximize a given signal path's signal-to-noise ratio), requires that we maximize the input signal level before clipping of each stage so that additional "downstream" stages aren't configured to raise signal level ('cause it's too low) along with the noise injected by each successive gain block.

Front-end pads can passively range the input signal so that the preamp front-end is not overloaded. Assuming that the signal coming in is large enough, then a pad is a good idea. Otherwise, padding the signal only to add gain downstream will unduly raise the noise floor...

Also, FWIW, many folks claim that pads "sound different" from the unpadded source...

In general, to maximize SNR with a preamp, add as much gain as close to the source signal as possible. If the mic/source can overload the micpre front end and you can't control the sound source levels, pad the input to the preamp and judiciously add post-source preamp gain to make up level deficiencies...


Does this make sense?
McQ

[chessrock]

Originally posted by McQ
Also, FWIW, many folks claim that pads "sound different" from the unpadded source...

Because it alters the load the mic sees (raises the input impedence), right?

[McQ]

Originally posted by chessrock
Because it alters the load the mic sees (raises the input impedence), right?

As always, the answer is "it depends". It doesn't have to. It can. Usually, it's a choice the designer makes...