Interference-like hiss when using the minijack connector

[Ron Cooper]

Sorry if I scared you Pietro !

Shorting inputs (I/P's), is never a problem apart from losing sound temporarily, but it causes no equipment harm. However, shorting outputs (O/P's), is not recommended, although hopefully most low level outputs, - line, headphone etc. won't be harmed either, but don't try it on loudspeaker outputs.

I/P - Input. O/P - Output.

The input of an amplifier is basically a high impedance so that it does not cause loading on the preceding output which is connected to it. However, this high impedance, if left this way, will exhibit more noise than if it were shorted across.

If you have nothing connected & turn the gain up you will hear "some" noise which is the inherent thermal noise of the input devices, - transistors, resistors etc. This noise is present in ALL devices but it is the level of the noise that concerns us here.

In well designed input stages the noise will be much lower for a given amount of gain (amplification) than poorly designed equipment. My experience is that most video camera inputs are in the latter category regardless of brand. Frustrating, as it would cost less than a buck to rectify it. (Don't get me started on this one as noise is just the tip of the iceberg!).

However, in well designed equipment I/P noise is miniscule & in most cases inaudible if used correctly. BUT, as this noise is affected by the input impedance connected across it, it is desirable to feed the input from a LOW impedance. - Ideally the best noise figure would be if we could generate a signal across a short circuit and connect this to our input.

So in other words it is desirable NOT to have strict impedance matching in the interest of best noise performance. In other words, feed a low output impedance into a high input impedance. This is why most good (Pro) microphones will have an output impedance of around 100 - 200 ohms so they present a low impedance termination to the following input. Further if you try to "Match" the microphone output impedance to a similar following input impedance, you will "load down" the signal and it will need more gain to bring it back up, thus increasing your noise further. In low level audio such as this we are simply dealing with voltage NOT POWER, so hopefully from the above, you will see that matching is not necessary or desirable.

Re the 47uf BP, (Bi-polar, - non-polarised capacitor). I was merely referring to some consumer cameras that have a DC bias on their minijack inputs to power electret mics and also auto sense mono switching if only one channel is connected. Incidentally, don't fuss over "ferrite rings" as thay have no relevance here.

R.C.

[Pietro Impagliazzo]

So the ultimatum is...

These cams have lousy audio circuitry and you have nothing to do besides:

a) Buying a better cam
b) Completely replacing the cam audio parts
b) Going with a separate audio recorder

Right?

[Michael Liebergot]

Originally Posted by Pietro Impagliazzo

So the ultimatum is...

These cams have lousy audio circuitry and you have nothing to do besides:

a) Buying a better cam
b) Completely replacing the cam audio parts
b) Going with a separate audio recorder

Right?

Not necessarily so Pietro.

If you want to bypass the cameras loud internal circuitry then it's best to knock the signal down before it gets to the camera, either via Mixer or XLR adapter box with active pre amps (like Juidelink box that you have).

As I said earlier, since you have a Juicedlink adapter, then use it on either the Medium or Low pre amp setting. This will knock the noise floor down and should give you considerably quieter audio.If you are running running your Juicedlink box on High pre amp, or using a straight mic feed into the FX7's mic port, then you will get noisy audio. Oh and also if you are using the JL box and only running one XLR input instead of two XLR feeds, then make sure that you turn down the unused XLR port all the way to '0'. As you mihgt still pickup some extra noise in the JL box, because you are knocking your audio well below the noise floor, lower then the internal pre amps.

But as stated another way to capture great audio is off camera using some of the excellent digital audio recorders available today. I use a Marant PMD620, Edirol R09, Zoom H2, and Edirol R-44 (4-track field recorder), in various situations. I tend to like to use my on camer audio for sync backup purposes only, but also like quiet audio, whcih is why I use the JL boxes on my FX1's.

But for my final video edits, I find that I mainly use my audio recorded from my recorders and dump them on my timeline in my NLE and sync up and edit the video as needed. Using teh captured audio in camera for sync purposes.

[Pietro Impagliazzo]

Michael, the problem is some electronic rambling noise that is only present when I hit record.

My guess is that the tape transport mechanism and the recording process interferes with the audio in some way.

However this "electronic high pitched rambling" is only perceivable in total silent.

And in total silent I can easily remove it by editing the spectral view on Adobe Audition.

Thanks to everybody in this thread.

[Pietro Impagliazzo]

Originally Posted by Ty Ford

OK, Wait a minute.

1. I question that a transformer is needed here. In fact, that could be part of the problem. You don't need to match impedances as long as you are plugging low into high. By that, I mean a low impedance mic into a preamp with a higher impedance mic input is OK.

OTOH, high impedance into a lower impedance input is a problem. Want to keep that theory in your brain? Try this plumbing analogy. Small pipe (high impedance) feeding big pipe (low impedance) lots of slop, not a good match. Big pipe (low Z) feeding small pipe (high Z), works great.

2. There are "regular" cables that are built to connect a balanced mono source into an unbalanced stereo input.

3. There are special cables that have a blocking capacitor to prevent the DC voltage that some camcorders have at their mic input for powering some mics from getting into mics that don't want to see that voltage.

Without knowing the circuits' and mic's technical descriptions, it's impossible to tell which would work best.

Regards,

Ty Ford

Ty, I missed this statement of yours.

Since the FX7 mini-jack has this so-called PLUG-IN POWER, I think that might the problem.

Doing a search on B&H under camcorder XLR adapters I couldn't find anything but beachteks and the Hosa MIT-156 (XLR to Mini) that I already have and also gives me this problem.

So, could you point me to a cable with such a blocking capacitor?

[Ty Ford]

Ciao Pietro,

Trew Audio - Cables, Connectors & Adapters - DV Cam Cables - DV Cam Mono Input Cable

Regards,

Ty Ford