Topic: Do I need to use my mic's pads to protect my 32-bit float recorder?

[Gutbucket]

Real world example-

I run relatively sensitive mics into a first generation Zoom F8.  Using those sensitive mics on stage I found I would occasionally illuminate the peak indicator.  Was never a problem out in the room where peaks are limited through the PA.  Unlike later generations of the F8 recorder, first gen only allows for use of phantom power in mic-in mode via the XLR inputs. Line-in is always via the TRS inputs without the option of phantom power.  Ordinarily I would switch to Line-in mode with phantom engaged to lower input sensitivity sufficiently and achieve sufficient headroom, but because phantom power is not available I couldn't do that. 

Instead I use mic-in and engage the hybrid limiter.  The hybrid limiter works by first engaging a pad on the input which  lowers levels enough to prevent any overs I'd normally encounter.  The limiting is applied with a variable ratio that engages progressively - the few peaks that would have gone over when recording on stage are caught and just slightly limited with a low compression ratio.  However, if an unusually high SPL event were to occur the limiter will engage more and more aggressively as the signal approaches 0dbFS.  Using this arrangement, the mics will now clip before the recorder.

At first I was disappointed that engaging the limiter in this way was necessary, and considered upgrading to a later gen F8.  However in testing I found the hybrid limiter well implemented and unusually transparent sounding, and since it rarely needs to do anything and basically acts only as a transparent safety mechanism, I have no problem keeping it engaged across all channels all the time. 

The limiters are disabled in 32bit float mode, for obvious reasons.

No. The marketers imply that its is obvious, but its not. The analog front end of the recorder is separate from its ADC architecture and file storage format.  If an analog input to ANY recorder is hot enough to overload the recorders analog input circuitry, the recorder's input sensitivity will need to be adjusted, a pad-engaged, or limiting applied to prevent it from overloading.   If the current version of the Zoom F8 (the F8 N Pro, which does feature 32-bit float recording) did not have the option to provide phantom when in line-in mode and if I did not have the hybrid limiter engaged, I would still encounter the same problem with high-peaking SPL sources on stage causing overs even with the recorder set to write 32bit float files.

That problem is more likely to be encountered with inexpensive, smaller 32-bit float recorders than it is with F8, MixPreII, etc.

[DSatz]

> The analog front end of the recorder is separate from its ADC architecture and file storage format.  If an analog input to ANY recorder is hot enough to overload the recorders analog input circuitry, the recorder's input sensitivity will need to be adjusted, a pad-engaged, or limiting applied to prevent it from overloading.

This is a key point (aside from the fact that past a certain point, limiting can't prevent overload in an analog front end; it protects only the stages that come after it, such as the A/D. But the possible benefit of a limiter is, uh, limited since it is just as easy to overload the input of an analog limiter as any other analog circuit--and limiting changes something essential about the sound of a recording, and requires judicious application).

-- Just wanted to add that throwing the pad switch on a condenser microphone reduces the signal/noise ratio of the recording by the amount of the pad, since it reduces the signal levels without lowering the noise floor of the electronics. Those switches are for emergency use ONLY. If you reasonably anticipate that a microphone's output levels could overload your recorder or preamp's inputs, get some in-line resistive pads (typical values are 10 and 20 dB) and put them at those inputs when setting up for a loud event. They knock down noise exactly as much as they knock down signals, thus preserving your s/n ratio. They're cheap, they're rugged, and they don't interfere with phantom powering.

Or depending on your recorder, it may or may not have settings for input sensitivity that work like the resistive pads that I'm talking about.

There's a ~40 dB difference in sensitivity between the most and least sensitive microphones on the professional market, and the maximum SPL of live sound itself varies by (say) another 40 dB from situation to situation. The gain setting of an analog mike preamp MUST be at least approximately correct to avoid clipping and/or the addition of extra, potentially audible noise. That's regardless of whether the rest of the recorder is 16, 24 or 32-bit.

[bonghitwillie]

i recorded a show last sunday with my h1e using the internal mics. i was 5 feet away from the amplifiers. it was so loud that i wore earplugs. i was worried that it would be distorted, but it came out fine and real good stereo separation.

[Craig T]

-- Just wanted to add that throwing the pad switch on a condenser microphone reduces the signal/noise ratio of the recording by the amount of the pad, since it reduces the signal levels without lowering the noise floor of the electronics. Those switches are for emergency use ONLY. If you reasonably anticipate that a microphone's output levels could overload your recorder or preamp's inputs, get some in-line resistive pads (typical values are 10 and 20 dB) and put them at those inputs when setting up for a loud event. They knock down noise exactly as much as they knock down signals, thus preserving your s/n ratio. They're cheap, they're rugged, and they don't interfere with phantom powering.

Great info here.  I've been using the pad switch on my mics for some time not realizing there was a better option.  Any "ideal" attenuator value?  Or just whatever is needed to give some headroom on the input?

[morst]

-- Just wanted to add that throwing the pad switch on a condenser microphone reduces the signal/noise ratio of the recording by the amount of the pad, since it reduces the signal levels without lowering the noise floor of the electronics. Those switches are for emergency use ONLY. If you reasonably anticipate that a microphone's output levels could overload your recorder or preamp's inputs, get some in-line resistive pads (typical values are 10 and 20 dB) and put them at those inputs when setting up for a loud event. They knock down noise exactly as much as they knock down signals, thus preserving your s/n ratio. They're cheap, they're rugged, and they don't interfere with phantom powering.

Let me know if this is a red herring, but I think my question applies.
I've been at this a long time and still don't quite have an intuitive understanding of the reasons for, and effects of impedance matching.
I think it's so that energy at all frequencies can transfer from one part of a circuit to the next, and not load down components to the point where they can't deliver the goods.


Does adding inline resistive pads effectively raise the output impedance of the mics in a linear fashion?
(by linear, I mean the same effect at all frequencies)


OK, I hope that was on topic. I know that the reason for using pads is so that large signals can fit in small holes, but I would like to have a better understanding of the relationship of voltage and current in this dance.


Thanks as always for the wisdom and clarification, DSatz! You rock!