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Offline Rairun

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Question about preamps
« on: February 14, 2024, 12:08:00 PM »
I've been taping for nearly a decade with a pair of CA-11s and an old CA STC-9000 preamp. I know the gear pretty well at this point, but I'm starting to dabble in field recording (mostly to use in my game development project) and the CA-11s just aren't sensitive enough for this type of recording.

I ordered a pair of clippys (with Primo EM272 caps), which are considerably more sensitive than the CA-11s (-28 dB +/- 3dB at 1 kHz), and also seem to have a better signal to noise ratio (80 dB). My question is: how hot a signal would be too hot for my CA preamp? And does it make a difference whether I use the +15 dB or the +35 dB setting when it comes to the preamp itself overloading? I ask this because the clippys' can handle up to 122 dB SPL (typ) at 1 kHz, so I'm curious to see what they sound like for live music as well.

I know the clippys with +35 dB gain at 122 dB is going to generate a really hot signal that can overload the recorder. I just need to know whether the preamp's gain setting affects whether the preamp itself will overload (i.e. noise aside, is the +35 dB setting with the attenuation pot dialed down more likely to overload the preamp than the +15 dB setting with no attenuation? Or is all that matters the signal that the mics produce?)

I've never overloaded my STC-9000 preamp, but I rarely ever use the +35 dB setting, and as I said, the CA-11s are way less sensitive. Is this something I actually need to worry about, or is the bottleneck in practice always the recorder?
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Online fanofjam

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Re: Question about preamps
« Reply #1 on: February 14, 2024, 03:03:24 PM »
I also never totally understood how to interpret the tech specs when it comes to your question.  What I think I remember though is that mic sensitivity as a measure of when the mic overloads in a completely independent thing from the signal chain overload.  In other words, if your mic has a 122db rating, make sure the SPL at the show doesn't exceed 122db and your mic won't cause any audio distortion on your recording.

When it comes to the recorder - preamp interface, there have been numerous discussions in forum history about how to identify unity gain on, say a handheld, and then setting your recorder to a level that doesn't exceed unity gain.  With that done, adjust your preamp to get the levels you want to see.  This way, you can rest easy that you will never overload the recorder.  From there, always use the preamp if you need to adjust levels upwards.  However, if levels need to go down, then either the recorder or the preamp would work.  I'm not referring you to history to be lazy...it's just the I don't remember all the technical mumbo jumbo.  I'd probably start with a search on the term 'unity gain'.

But regarding your last question, I'd say that for handhelds, yes it's usually the recorder that is the bottleneck and what you need to pay attention to the most as far as your level setting.

Now others can please chime in and elaborate and/or correct any misinformation I might have provided with this response.  I'm 65 and my memory just sucks.
« Last Edit: February 14, 2024, 03:09:59 PM by fanofjam »

Offline Gutbucket

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Re: Question about preamps
« Reply #2 on: February 14, 2024, 06:17:36 PM »
The TL;DR is that I don't expect you will have problems with those mics through the CA preamp, but it may depend on the SPL of what you are actually recording. 

Quote
I just need to know whether the preamp's gain setting affects whether the preamp itself will overload (i.e. noise aside, is the +35 dB setting with the attenuation pot dialed down more likely to overload the preamp than the +15 dB setting with no attenuation?)

I don't know the answer to that.  But you mention needing more sensitive mics for the content you are collecting, which implies you are recording quieter stuff, and for that reason you are not likely to overload the preamp.. even if you otherwise could overload it with something really loud.  Its more complicated if the content is highly dynamic, with both very low SPL and high SPL content.

The hottest (most sensitive) mics I've used through CA preamps are DPA 4060.. which I've used for decades without problems.. as long as the battery powering it is fresh / has a good state of charge, which tends to be the variable most effecting good preamp performance.  4060 is less sensitive than the mics you mention, at 20 mV/Pa; -34 dB re. 1 V/Pa, ±3 dB at 1 kHz, but has a higher max SPL of 134dB at 10% THD, and a relatively wide dynamic range for a miniature at 100dB.  I can successfully record a quite wide dynamic range with them through CA preamps, the widest probably being large symphonic performances where the venue's noise floor defines the bottom of the range (when the audience is church mouse quiet) and the loudest full symphonic crescendo from a close position immediately behind the conductor the top of it.

The problem you are most likely to encounter recording quiet material and/or highly dynamic material is noise floor.  Either the noise floor of the recording environment or the self-noise of the microphones.
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Offline Gutbucket

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Re: Question about preamps
« Reply #3 on: February 14, 2024, 06:23:15 PM »
In reply to some of what fanofjam posted-

Unity gain is relevant with things like preamps, mixers, outboard gear.. anything that features both an input and output, the levels of which can be compared against each other.  In that case, when the output level from the device is the same as the input level into the device, that's unity gain through the device.  Unity gain isn't really relevant in a recorder where the input is analog and the output is a recorded file on an SD card.  What is relevant is determining the "do not go below" input gain setting which avoids "brickwalling".  That's the point at which lowering the input gain farther to accommodate high SPL material will no longer prevent the recorder from overloading.  Stay above that point and the recorder is good.  If you have to reduce gain, turn down the external preamp or attenuate ahead of the recorder.

Generally, to achieve the best dynamic range possible with the equipment via good gain-staging, its best to use the earliest amplification stage in the chain for the majority of the total gain required.  That's because each additional amplification stage amplifies the noise of all previous amplification stages along with the signal.  Unity gain then becomes a useful concept for the following devices in the chain.  In practice with the gear we are discussing here, that usually means setting a comfortable input gain on the recorder that's above its "do not go below" point, and adjusting gain on the preamp as necessary to achieve good recording levels.

The max SPL specification of a microphone will always be at some specific distortion level, although its not always noted.  In the case of the 4060, the 134dB max SPL I mentioned above is at 10% THD.  The specification at 1% THD is 120dB SPL RMS, 123dB SPL peak.
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Offline grawk

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Re: Question about preamps
« Reply #4 on: February 14, 2024, 08:46:50 PM »
i recommend finding somewhere loud you can try it out. Stereo recording is usually fairly straight forward and you can spend thousands of hours studying something 10 minutes of trial and error can answer.
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Offline rocksuitcase

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Re: Question about preamps
« Reply #5 on: February 14, 2024, 11:22:40 PM »
i recommend finding somewhere loud you can try it out. Stereo recording is usually fairly straight forward and you can spend thousands of hours studying something 10 minutes of trial and error can answer.
This is it in a nutshell. But, for a more "formal" experiment: Use your stereo, or Go to a loud place, concert, train station, etc. Record with varying gear/levels. have a notebook where you write down timings of said samples. Listen at home. decide what works and what doesn't.
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Offline DSatz

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Re: Question about preamps
« Reply #6 on: April 05, 2024, 01:24:39 AM »
rairun, the specifications given in your message aren't enough to answer your questions. If you don't have measuring equipment available, then the advice that people gave you (to experiment) was the only way to find out what will work and what won't.

For future reference, mike preamps can generally be overloaded at their inputs OR at their outputs. Some can even overload in between; I've seen that in some cheap, portable tube-based preamps. Their manufacturers consider it a feature rather than a bug, though, because some customers think that tube distortion is a good thing by definition--and then they listen and find out that it's not necessarily so (and then it's generally too late to go back and do the recording over).

Anyway depending on how the preamp is designed, its input overload voltage may depend on the gain setting or not. That's one of those things that you need to measure (or experiment) and find out. The output overload point is usually constant--but whether you reach it or not DOES depend on the gain setting. (The higher the gain, the less the input voltage has to be to reach the fixed limit at the output.)

Finally, "sensitivity" in microphones doesn't mean what most people would think based on common sense. If it weren't such a long-established term, I'd say that it should be changed--but it's way too late by now. The common sense assumption would be that it means: What's the softest sound that the mike can pick up? But sensitivity means, instead: How much signal (voltage) does the mike put out for a given sound pressure level at 1 kHz? And the standard SPL for the sensitivity specification for professional microphones is 1 Pascal, or approximately 94 dB SPL. That's loud, but certainly not the loudest thing you've ever heard or will ever have to record. So your preamp input had better be able to handle considerably higher voltages than whatever the sensitivity spec of your microphones says. Like, at least 10 times as high, and preferably 40 or 50.

btw, the spec that tells you "what's the softest sound that the mike can pick up" is the "equivalent noise level". If that isn't specified, you can figure it out from knowing the mike's signal-to-noise ratio, which (like sensitivity) is based on a 1 Pa sound pressure level. So if the signal-to-noise ratio is 75 dB, the equivalent noise level must be 94 - 75 = 19 dB SPL--which implies (and this is horribly oversimplified) that a sound occurring at 19 dB SPL will be right at the noise floor of the microphone. You can normally hear such sounds in a recording if you turn the playback volume up high enough (amateur radio operators routinely "read" Morse code that's >10 dB below the noise floor; I've done it myself). But it's usually a lot nicer if the noise floor is at least 6 to 10 dB below the level of the quietest sounds that you want to pick up distinctly.

--best regards
« Last Edit: April 05, 2024, 01:33:47 AM by DSatz »
music > microphones > a recorder of some sort

 

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