OT: What's a pre-amp?

BrianStanding wrote on 7/27/2004, 12:22 PM
O.K., please forgive my plodding ignorance, and be kind to a video guy.

What, exactly, is a pre-amp and what is it used for?

How does a portable pre-amp differ from a microphone mixer?

Why would I want to use one with my Sony PD-150 camera, as opposed to plugging mikes directly into the XLR inputs of the camera? Doesn't this just create more gear in the signal path?

Comments

bgc wrote on 7/27/2004, 1:05 PM
Hi Brian,
A microphone pre-amp is an amplifier that boosts the very low level signal output by a microphone so that it can interface to other audio equipment which requires larger signal levels. A simple example is the line level type input to a cassette deck or other recording device.

Most microphone mixers have a built-in microphone preamp to provide this functionality. The difference between a microphone preamp built into a mixer, and a portable or decicated pre-amp is sometimes that the dedicated preamp provides a higher quality boost in gain (this is why audio geeks like myself will spend long hours talking about and arguing about different types of preamps and which "sound" better).

For your Sony PD-150 (although I'm not completely familiar with this model) I believe the XLR inputs are expecting a microphone level signal and so having an external preamp doesn't make much sense. If the camera has a line level input, which I think it does, you could use an external preamp (which would most likely be a higher quality preamp than the one built into the camera) to interface the microphone to the camera. This would most likely improve the quality of the audio, but you will have to decide whether that's a big concern of yours for your applications.

Hopefully this helps and makes sense. I've left out all of the technical nitty gritty because it doesn't sound like your really interested in that level of detail.

Cheers,
B.
BrianStanding wrote on 7/27/2004, 2:05 PM
Thanks for the response. So, a pre-amp basically takes a mike-level signal and converts it to a line-level signal? That's it?

I have the option of both a line-level or mike-level inputs on my camera. Being an advocate of the KISS principle (Keep It Simple, Stupid), I'm inclined to keep going directly into my mike-level inputs unless I'd realize some spectacular increase in sound quality or some other benefit.

Thanks for enlightening me.
drbam wrote on 7/27/2004, 2:32 PM
You can easily experience a "spectacular increase in sound quality" if you're willing to spend the $ for a good preamp. Even a "decent" micpre ($250-500 range) will probably give you dramatically better results compared to what the built in pres on your camera can provide. That being said, only you can determine whether the work you're doing actually "needs" the improvement.

drbam
bgc wrote on 7/27/2004, 3:31 PM
Hi Brian,
Yes, basically you got it :)
If you wanted to see if the benefit of an external pre-amp is worth it, you could borrow one from someone and give it a shot and compare the difference. Maybe you could "buy" one from Guitar Center or another music store that allows returns and see if it helps. If not, just return it.
VegUser wrote on 7/28/2004, 12:26 AM
a nice small (portable) preamp out there is the RNP (really nice preamp) from FMR audio
( http://www.fmraudio.com/ ).
Pretty flexible, 2-chnls, small, sounds GREAT, and not a bad price for what you get.
They also make one of the better lower cost compressors to go with it (RNC).

Not battery operated though.
BrianStanding wrote on 7/28/2004, 7:55 AM
Thanks for the tips. I may see if I can borrow one somewhere.....
CorporateSound wrote on 7/28/2004, 1:56 PM
What mics are you using with the camera? Generally speaking the PD-150's mic inputs have a poor reputation, but plenty of people use them satisfactorily. They do benefit from using a preamp, but an easy way to make an improvement without adding a preamp is to use a mic with a very high output. This allows you to use the MicATT switch and keep the manual gain on the camera fairly low, thus improving your noise floor.
farss wrote on 7/29/2004, 3:09 AM
The audio on te 150 is wierd. It seems switching to manual gain brings the noise floor UP. The mic input seems to have some form of mild compression with a limiter in full auto. But I suspect even with a decent preamp and going into it at line level the built in A/D converters may be holding you back.
Maybe we've all become a bit too picky, compared to what video cameras had to offer a decade ago or what's on the cheap consummer ones the 150 isn't too bad. At least you get balanced inputs.

Bob.
BrianStanding wrote on 7/29/2004, 8:55 AM
For run 'n' gun documentary work, I generally use a Sennheiser K6 with the ME-66 short shotgun capsule. CorporateSound, I often do exactly what you suggest and use the Mic-ATT setting, since the ME-66 is a wee bit hot. Lately, I've been using the ME-66 in Channel 1 and the stock PD-150 mike in Channel 2 as a backup/ambience mike.

I also use a Sennheiser Evolution 100 UHF wireless lavalliere on one channel & the ME-66 in the other if I'm focusing on a particular subject.

At other times, I will use a pair of ElectroVoice EV635 dynamic mikes, if I'm shooting a live music event or a stationary lecturer at a podium or table. I also have a Sony ECM-44B and a ECM-55B wired lavallieres, and an assortment of cardioid and omni dynamic mikes I use for special situations.

Been pretty happy so far, although I've lately been running into situations where I could really use a battery-powered, portable 3-channel mixer. My budget is pretty limited, though, so about the only thing in my price range is the Rolls MX54.
CorporateSound wrote on 7/29/2004, 1:17 PM
Have you looked at the PSC DVPromix3? At $450 it is more expensive than the Rolls, but it's the lowest priced 3x2 mixer around with professional quality and features. If you've been satisfied so far, then a mixer would help you most with control, routing and adding more inputs, rather than helping out with the PD-150's mic inputs. As was already suggested, only you can tell for sure after doing a test with a borrowed or rented item whether it's worth it.
The ME66 is very hot. The AT4073a short shotgun is even hotter and a much better mic in total, although it's well over $500. At some vendors it's over $600. Neither of these mics would benefit much from preamping.
You would probably notice the biggest difference when using lower sensitivity mics in quieter situations. Any good preamp will be better than the camera's built-in preamps for low-sensitivity mics.