External mic pre-amp with AF100?

K

Ken Hull

Well-known member
So I had been using my AT897 plugged in directly to my AF100, and things were going well..... until we were shooting a scene in which our lead actress had to speak softly. We couldn't get her to speak with enough volume, while still having a "soft voice". Our director/editor had to jump through hoops to hide the hiss when he edited the scene.

Well, I'd be thinking of getting an external mic pre-amp for some time, so this was the time. I got a Sound Devices MP-1. My first tests were disappointing. The noise seemed slightly worse with the MP-1. How could this be?! :shocked: Sound Devices is supposed to be top notch equipment. So I experimented and found that I got better results when I turned the camera's gain controls way up, allowing me to put the MP-1's gain down to 36dB or 28dB, and getting a fairly clean signal.

Now, I'm not complaining, but I am a bit puzzled. Since I had the camera's input switched to "line", I didn't think the camera's gain controls would be in the loop. (Aren't those controls part of the camera's pre-amp?) So ... is it "normal" to set a camera's audio gain to max when using the line input? :huh:

-- Ken
 
The pots still adjust line level in. You're still using the pre-amps, but just padded down - via the line level in switch - to accept a (hotter) line-level signal.

If you're running the pots right up high, you've got a line level mis-match and you're likely to introduce noise/hiss.

I'm not familiar with the MP-1, but a quick look at the specs suggests to me it should do what you want. You're running the MP-1 line output from the XLR, yes?

One other question - are you using the internal battery on the AT897 or the Phantom Power from the MP-1? If it's the former, is it a fresh battery?

One last point - at the moment, you've got no way of matching the MP-1 to the AF100, with potential for mis-match in the signal chain from mic to AF100. I think you'd be better with a small mixer (eg Sound Devices Mix Pre D) which has a built-in oscillator/test tone. Run that at a given level (eg -12dBfs) and then match that to the AF100's audio meters, which have markers for -12dBfs and 0dBfs. That way, you know your signal chain is running at "unity gain" (ie set up to pass the signal from one to the other without adding or subtracting.) You then use your mixer to adjust levels on it's superior meters (and better quality limiter), safe in the knowledge that it's passing through to the clip recording at exactly the same level.

Ben.
 
Thanks Ben,
I'm using phantom power for the AT897, and I've set the MP-1's phantom voltage to 12v, per Sound Devices instructions. And yes, the MP-1's batteries are new.
It seems strange to me that switching from MIC input to LINE input would still route the signal through the camera's pre-amp. That means the signal is now going through 2 pre-amps .... seems like an invitation for even more hiss.

You point out that there's no test tone in the MP-1, to calibrate camera to pre-amp. Yeah, I was concerned about that. The MixPre-D sure looks tempting, but $899 is a lot for this cameraman to spend. If I keep the MP-1, then I need to find a constant-level sound source, and experiment to find the lowest camera pre-amp setting that still produces a high S/N ratio when the MP-1 gain is set to produce a given level on the camera's meters. Meanwhile, I keep nagging the producer to get a real audio person, with his own gear (not some 18-year-old intern, who doesn't know how to hold a mic boom). :)

-- Ken
 
Ken Hull said:
I keep nagging the producer to get a real audio person, with his own gear (not some 18-year-old intern, who doesn't know how to hold a mic boom). :)

-- Ken
Click to expand...

Sound - 80% of the picture and so undervalued, especially so - I find - since the DSLR video revolution. If you can borrow a field mixer, you could run a -12dBfs tone out of that from the the mic level outputs and set your "window" up with that - I'd try with the MP-1 pot set at the 12 'o clock position.

Then just note your input level on the AF100, perhaps with a small dot of paint or nail varnish.

It will still be one pre-amp, by the way, just with a resistor/s in line, via the switch, to pad down to accept line level.

Ben.
 
On the AF100, do you find that the on-camera volume settings are untra sensitive?. Too low and the signal suffers. Increase the settings a few clicks and you are bordering on red-line territory.

This is my experience on AG-AC160A. Wondering if the same applies to the AF-100. No point using the volume limiter on this camara, All it does is clip off the top. When I get a peak I have to edit out one or two frames in front/back of the peak which, for music concert recordings is not a great thing.

The other thing is I have Sony Vegas Pro 12 and there seem to be two volume boosts in this NLE, one per track segment and one per entire track.

Neither boost to where you get hissing so it would be nice to find out if there are any other ways to increase volume.

Would MixPre-D help me in any way? I mostly do stage recordings using an XLR feed from a sound board.
 
kwkeirstead said:
On the AF100, do you find that the on-camera volume settings are untra sensitive?. Too low and the signal suffers. Increase the settings a few clicks and you are bordering on red-line territory.

This is my experience on AG-AC160A. Wondering if the same applies to the AF-100. No point using the volume limiter on this camara, All it does is clip off the top. When I get a peak I have to edit out one or two frames in front/back of the peak which, for music concert recordings is not a great thing.

The other thing is I have Sony Vegas Pro 12 and there seem to be two volume boosts in this NLE, one per track segment and one per entire track.

Neither boost to where you get hissing so it would be nice to find out if there are any other ways to increase volume.

Would MixPre-D help me in any way? I mostly do stage recordings using an XLR feed from a sound board.
Click to expand...

No - the AF100 has a good audio dynamic range and reasonable pre-amps.

I think it's possible that your issues may be
kwkeirstead said:
I mostly do stage recordings using an XLR feed from a sound board.
Click to expand...

From the symptoms you're describing, I'd guess you're being fed a line-level signal from the desk and you've set the camera input to mic level. Is that possible? If not, maybe the line level aux feed you're getting from the desk is just too hot. You should be looking to get a healthy level on the meters, with a bit of space or headroom to catch peaks, at somewhere around half way up the input pot scale (so around the 12 'o clock position.) Turning the pot up and down should result in a gradual increase or decrease in meter reading - not a huge exponential jump when you go above, say, 2 or 3 on the pot.

The Mix Pre-D (or any other mixer) is useful for matching varying input sources to the optimum "window" of dynamic range the camera requires. But your camera has the necessary pad to switch between mic level or line-level - so, adding a mixer won't necessarily help you in the situation you're describing. Set the AF100 meters with some headroom to catch peaks and you should be able to rely on your limiter to save anything from clipping.

In post production, you can fatten the mix with a gentle compressor/limiter - this effectively reduces the dynamic range and pushes all the levels a little higher and flattens the peaks at a given point - so, less differential between high levels and low levels - but a more immediate and "punchy" presentation. Don't overdo it, or it can become fatiguing on the ears.

I'm grossly oversimplifying in places - but it's difficult to download the whole context in the time and space I've got. Hope it's still useful.

Generally on live stuff, we do a multitrack record and mix it down afterwards to straighten things out - it can be quite challenging for the sole operator.

Ben.
 
bengiles said:
Sound - 80% of the picture and so undervalued, especially so - I find - since the DSLR video revolution.
Click to expand...

I find that extremely accurate.

Everyone is after that certain look when it's just as important to pursue that proper clean sound.

to add to what Bengiles said - we've been recording through a mix pre d to a HMC and as long as you've got unity gain I never encountered any bad hiss in my audio. Of course, it helps if you record a bit hot and dial the levels down in post to hide the noise in the audio.
 
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Perhaps a different mic choice would be in order ... I would have chosen either a Countryman B6 pin lav or perhaps a parabolic if
that wouldn't work.

The MP-1 is a pretty quiet with a EIN of -128dbv and THD of 0.05 % .. that's studio grade. As Bengiles suggests, making sure the inputs on the
AF100 are set to Line level and that you aren't applying post-gain from the MP-1 and you should get a pretty quiet sound.

We rarely use the on-board audio on the AF100 and employ a field recorder instead . . which might be another option to consider.

jeff
 
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