Balanced/Unbalanced audio

[Aaron Koolen]

I intend to get mic like the Sennheiser ME66 which has XLR plugs on it and normally people get an XLR adapter for their camera for it to go into. Can someone tell me why I can't just get a converter plug from XLR to Mic jack and just go straight into that and use the cameras's audio controls for mixing? Is there not enough preampage on it or something?

Cheers

[Andre De Clercq]

There is no problem to do so, if the signal levels are OK, if the mike is fixed on the camera and the mini(micro)jack matches with yr camcorder's internal configuration. Some inputs are DC sensitive in order to switch to mono on both channels.

[Aaron Koolen]

Ok, I wort of understand that. How can I found out if this can be done with a Senn Me66 and Gl2 then? Will the camera and microphone's specifications tell me enough?

Cheers

[Andre De Clercq]

Yes, but the input specs for external mikes(minijack configuration and levels, are often not available in the user manual.

[Don Palomaki]

You should be able to go directly from a Senn ME66/K6 combo to the GL2 if you use a XLR-to-Mini Phone adapter, Pins 1 and 3 to ground, pin 2 to the hot leads (the tip and ring if using a stereo plug) of the mini-phone jack.

Note: If you connect pins 2 & 3 to left and right channel, you will have channels recorded out of phase, a potential problems if you make VHS copies that are played on mono equioment.

Be aware that you will be losing the noise cancelling benefit of balanced audio connections, especially important if you are using long runs of wire.

You can find matching transformers that will go from balnaced XLR to unbalanced (often 1/4" phone) that you can also use. Then use a 1/4" phone to 1/8" mini-phone patch cord to connect to the GL.

The GL mic input specs are in the manal. The ME66/K6 is a nigh output mic, and you may find that insome situations using the MIC ATT setting rather than the MIC setting is approriate.

[Andre De Clercq]

Very good and specific info. One remark however. The "floating mode" (2 and 3 to left and right) which indeed results in channel phase reversal is not allowable in stereo equip either. Most stereo amps don't have speaker phase reversal anymore. So...low frequencies are gone and stereo effects are deteriorated. In addition, floating connections are very sensitive to common mode signals (hum...).

[Aaron Koolen]

Hi Don, thanks for that. You said I'd be loosing the noise cancelling benefits of balances audio and that brings me back to waht I was thinking when I asked this question. When you have an XLR adapter, it eventually converts the sound from balanced to an unbalances mic jack so you'd lose that too, wouldn't you? If your adapter was on the end of your XLR cable then I'd only have a short distance to the camera. Does this matter? Is there someone I can learn about balanced/unbalance and audio stuff in general (What MIC att, and line level and all that stuff is)?

[Andre De Clercq]

I see that some of yr questions have been answered recently (line level, mic att..). A short overview on the way unwanted signals penetrate the mike wiring will maybe clarify some of yr other questions. F.Y.I, keeping the symmetrical wires and just unbalance close to the cam will not be troubled by (medium strong) induction fields (see hereunder) but will still be affected by egalisation currents difference in case of inhomgenious electric fields. Hopefully what follows is not too technical...
There are three known external conditions that can result in unwanted interferences like hum, small band noise… in low signal level audio interconnections. Line level interconnections are more robust because of the higher signal voltages but are potentially affected by the same mechanisms. The first problem is the presence of alternating fields (magnetic or electromagnetic) which induce a voltage component (EMF) in the interconnection wires. The second situation, is the presence of non homogeneous electric fields in non or single end grounded equipment resulting in induced voltage differences between the two parts (e.g. camera and mike). A third situation is the result of double sided grounding, where the ground potentials (voltages) are different. This last situation is somewhat more complicated and not relevant for mobile camcorder/mike use and will not be covered in this post
In the whole story, what must be remembered, is that the mike input only “hears” voltage differences between the input points.
-Magnetic/electromagnetic fields:
In case of balanced (symmetrical…XRL) interconnection, both wires get the same amount of induced voltage with the same phase. So there is no voltage difference, hence nothing is “heard” by the camera. However if in balanced mode, very high (common mode ) voltages are being induced, which can happen in the three conditions mentioned above, the input amplifiers have to be able to handle these voltages, and this property is limited and set by the maximum CM voltage. Common mode rejection factor (CMR) on the other hand determines how good those unwanted signals are suppressed in balanced modes. An almost ideal (w.r.t. CMR) solution is the use of a small transformer that blocks all these (unwanted) symmetrical signals, and also transforms into unbalanced mode with the correct impedance. Active (using transistors..) balance to unbalance devices are available too. If a camcorder has an XLR input, the conversion is done internally, because all signals have to be further processed as unbalanced voltages. In the asymmetric/unbalanced interconnection (coax/screened wires) the voltages induced ( especially in long wire runs) are known as unequal (higher on the screened part than on the inner part), resulting in unwanted voltage differences which are captured at the mike input together with the (wanted) audio signals.
-Inhomogeneous electric fields :
When both devices (or persons holding them) operate in different electric field ambients(close to TV screens, in power plants..), legalization currents travel through conductors. In the unbalanced mode both conductors have (“see”) different resistances (screen vs. inner conductor) which result in non canceled unwanted voltages. In balanced mode all this voltage are canceled of course, with the above mentioned limitations (CMR factor and limits) taking into account.
All these strange effects are non existent if the mike is (with a short lead) directly on the camera with a balance to unbalance connector..

[Don Palomaki]

Aaron: In short, the longer the unbalanced leads, the greater the likelihood of unwanted or intrusive noise being introduced. In general, if the leads are short (thus the XLR adapter close to the mic input jack) there is a low risk of intrusive noise being introduced. Most users find this works for them. However, there are be exceptions where even short leads may present a problem; e.g., a strong noise source very close to the camcorder.

Be sure the XLR adapter is one that does balanced-to-unbalanced conversion, not just simple pin mapping. Simple min-mapping adapters are typically rather low cost (under $10).

The (stereo out-of-phase) pin 2&3 to left and right (pin 1 to ground) connection does result in a strange stereo image, and can have adverse effects on low frequency sound. The low frequency effects depends in part on speaker locations. How noticeable the results are will depend on the quality of the sound system use for playback.

[Aaron Koolen]

Ok, thanks guys. That answers it I'm sure ;) Will need to read over dre's post a few times to understand that.

Thanks again!