Topic: the difference of 20db

[spcyrfc]

this may be a silly question to some, but its something ive never really wrapped my head around...

ive been looking at getting a new pre, the aerco mp2 has options of 0-50 or 20-70 gain settings.

is there a way for me to quantify this difference in terms that i can understand? 
i used to run some marantz machines and they had a -20db pad... i can imagine what my levels look like with the pad on and pad off, but how do i translate that image into the 0-50 / 20-70 difference?

i have recorded in the past (with modded 660) in situations where with the pad off the signal was too hot, but with the pad on, it was to soft.  dont want to run into that situation after buying a new pre.
thanks
lb

[rustoleum]

for every increase of 10db, the human ear will perceive the sound to be twice as loud.  Thus, adding 20Db to your signal (assuming you can add that much before reaching full digital scale) would be perceived as 4 times louder than not.  Not sure if I answered your question, but that's certainly one difference between adding 20db and not.

[spcyrfc]

 thank you for the quantification.

so a pre with gain settings of 0-50 could handle a signal four times as loud as a pre with 20-70?

is the right way of looking at it?

and what is full digital scale, when is it a concern?

[Patrick]

thank you for the quantification.

so a pre with gain settings of 0-50 could handle a signal four times as loud as a pre with 20-70?

Microphone preamps are rated in the amount of amplification that it gives to a mic level (very low) signal.  70db amplification is hefty- you most likely will not be using that much for any type of audience recording.  So, to answer your question, a preamp that can amplify up to 70db can theoretically AMPLIFY a signal 4x as loud compared to preamp that is rated at 50db of gain max. 

I believe some of the Aerco's out there have a fixed 20db input gain stage.  I know the AD-20's have something like 17db as well. 

and what is full digital scale, when is it a concern?

Digital Full Scale refers to when you have used all of the available values given by a certain bit depth.  If you attempt to futher increase the level of the signal at DFS, you will get digital clipping.  This is (obviously) a concern when using digital systems, and avoiding artifacts as a result of digi clipping.

[SmokinJoe]

The real question is "how much gain do you need?"

I have a V3 and know that usually the gain on the V3 is set to something between 25-35 db for most shows that I tape.  It goes to 70db, and I can envision using 50db if I were micing an unamped solo acoustic situation (my friend's bluegrass band), but I can't envision using more that that, unless someday possibly to capture the loons at camp.

I tape mostly average rock bands, no super loud death-metal stuff.  If you do, I can see a situation where the minimum of 20db might be too much.

So... Most of the time for concert recording either will do.  But where do your occasional tendencies lie? 
- Do you see yourself at the superloud death metal show?  If so, go for the 0-50 model.
- Do you see yourself at the very quiet acoustic stuff, and maybe capturing nature sounds?  If so, go for the 20-70 model.

[Gutbucket]

The real question is "how much gain do you need?"

The loudness of what you are recording is the first factor in how much gain you'll need, but the second and just as important is the sensitivity of the mics you use.  Most of us are using powered condensers which are relatively sensitive (some more than others of course) and those don't require huge amounts of preamp gain to get good level to the ADC stage.  If you were to use dynamic mics, which are typically a good bit less sensitive than the average condenser, you'd need more preamp gain to achieve the same level.  One extreme example that applies to concert recording would be using ribbon mics to record something that isn't very loud like a string quartet, as ribbon mics are lower in sensitivity than most other dynamic mics and string quartets aren't very loud.  Yet that's a reasonable application that would probably require 50-70db of preamp gain.

If you are using condensers like most do here you'll probably be fine with the 0-50db range.

[DSatz]

spcyrfc, a preamp acts as an intermediary between your microphones and some kind of recording device. The inputs of any recording device will require a certain voltage in order to record at the levels you'd like to reach for the highest-level (generally, the loudest) sounds in the performance that you're recording.

You probably don't want to have to turn the input controls on your recorder ALL the way up, since that usually adds some amount of unnecessary noise. So the voltage that you want to send in to the recorder's inputs at the moments of maximum sound pressure level will depend on the design of the particular recorder, on where you choose to set the input level controls on that recorder, and on your personal choice of a maximum recording level relative to 0 dB (full scale).

Let's just pretend that you have recorder "X," and that its line inputs (when its record level controls have been set to some comfortable middle level) require exactly 1 Volt to reach a recording level of 1.5 dB below digital full scale. The recorder doesn't care how you produce that 1 Volt of signal--you can drive it from the line outputs of a stereo receiver, or the analog line outputs of another recorder, OR the outputs of a mike preamp among other possibilities. But given that recorder, that record level setting, and your desire to reach 1.5 dB below digital full scale, your goal is to feed a 1-Volt signal to the recorder during the absolute peak of the music you want to record. That will get you the record levels that you want, without adding undue noise.

OK. Now, the preamp is your tool for boosting the signals from your microphones exactly enough--not less, and not more--so that the level of those signals will be 1 Volt during the loudest sound(s) in the concert. Typically, a microphone might put out (say) 150 mV during loud sounds, and your preamp's mission is to boost that to 1 Volt. Thus the amount of gain you need from the preamp is whatever factor is required to boost what your microphones are actually putting out up to (in this case) 1 Volt.

At any given gain setting, the preamp will multiply the varying voltage at its input by some constant factor. If your microphones are sensitive enough to put out, say, 50 mV at the peak of the sound pressure during the concert, and your requirement is to get 1 Volt of signal out of the preamp at that same moment, then the preamp has to multiply the input voltage by a constant factor of 20, no? OK, a factor of 20 in voltage = about 26 dB of gain. If the mikes only put out 25 mV, then that multiplication of the voltage has to be by a constant factor of 40, or about 32 dB. If you set the preamp's gain up to 40 dB then it will multiply the (varying) input voltage by a constant factor of 100--it would raise a 10 mV signal to 1 Volt.

So it all depends on how much voltage is actually coming out of your microphones during the moments of the highest SPL, and how much voltage you would like to send in to the recorder at those moments. You set the preamp so that it supplies the needed gain for that situation. Do you see how this works? Your microphones have a constant sensitivity--for a certain sound pressure level, they put out a certain voltage. Your preamp then raises that voltage until it's suitable for your recorder's line inputs.

--best regards

[aaronji]

Stupid question, but:  you get those dB values from 20*log (ratio of voltages), right?  Thanks...

[DSatz]

aaronji, right. For the "voltage factor of 20" example that I used, log 20 = 1.3; for "voltage factor of 40," log 40 = 1.6; for "voltage factor of 100," log 100 = 2.0. Multiply each logarithm by 20 and you get 26, 32 and 40 dB respectively. (These are base-10 logarithms, rounded off of course.)

I mostly use an integrated preamp + A/D converter, which simplifies the picture considerably. For most of the live music recording that I do, with professional condenser microphones (sensitivity in the range of 10 mV/Pa), I find myself almost always setting my preamp gains in the range of 30 - 40 dB. It's remarkable how consistent that is.

With other types of microphones, of course, the range would be different, and if I had to feed different A/Ds or recorders with varying input sensitivities, that would add an additional variable to the equation. But due to my laziness and my fondness for ultra-quick, simple setups, I'm spared all that.

--best regards

[BC]

So... Most of the time for concert recording either will do.  But where do your occasional tendencies lie? 
- Do you see yourself at the superloud death metal show?  If so, go for the 0-50 model.
- Do you see yourself at the very quiet acoustic stuff, and maybe capturing nature sounds?  If so, go for the 20-70 model.

agree, with my mics I would run ~+35 at rock shows with a PA but for classical chamber music +55 which gave conservative levels. Not sure how the sensitivity of your mics compares though, the 4022's are relatively insensitive, I think 7mV/Pa off the top of my head.

[spcyrfc]

the Senns have a sensitivity of 20 mV/Pa (-34 dBV/Pa)
 
if the dpas have 7, wow, what a difference.

[DSatz]

spcyrfc, the difference between 20 mV/Pa and 7 mV/Pa is about 9.1 dB. If you're using these Sennheiser microphones, you'd select a preamp gain setting that's about 9.1 dB lower than what you would use with the DPA microphones for the same placement and sound pressure levels.

There's no free lunch, though, since each preamp has a limit on how high a voltage its input circuit can take without overload. Not every preamp will indicate (via LED or whatever) that its input circuit is being overdriven. Most such indicators are attached either to the output stage of the preamp, or to the stage that drives the output. A few years ago I tested one well-known portable preamp that (depending on various settings) could put out over 25% THD before its "Over" light came on. The distortion was due to input stage overload, as its output stage was faithfully amplifying all the distortion coming from the input stage.

The more sensitive your microphones are, the more vulnerable you are to that type of preamp overload. If you're using microphones with 20 mV/Pa sensitivity and you're recording loud music, you should check your preamp's overload limits very carefully--you may need to use resistive pads at the inputs of your preamp.

--best regards

[Myco]

DSatz always has good info to add. Good to have him around here.

Thanks guy! 

[spcyrfc]

DSatz always has good info to add. Good to have him around here.

Thanks guy! 

too true. still trying to find time to dissect/digest it all.

[carpa]

Thank you Dsatz for your clear explanation!
I thought that the more sensitive a mic is, the better for the preamp in order to use not too much gain (expecially for not-so-good preamps).
Today I received an Edirol R09HR; I played around a little bit and I think it is very good.
For better recordings I know I have to add something external (which I'll buy in the future), and I was leaning towards a phantom supply and something like:
a couple of AT's like 4021 (19,9 mV) or
Beyerdynamics (30 mV) or
New AT stereo BP 4025 (25 mV)
What I had in mind was using any of these sensitive mics in order to save gain of the internal Edirol preamp, or even trying to run them directly line-in for close miking (of course feeding them with phantom). I've read good things about the rode nt4 also, but I thought:less sensitive=not so good at the pourpose.

Now, what you said about overloading preamp input makes me wonder if I would have any problems with such a rig (I think that a 10 db pad would not do much...).
Can you ( or everybody else, of course)tell me your opinion, please? It would prove disappointing to choose mics, buy them
in order to have low noise and eventually have a "very low-noise distorted stuff!!!
c