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[Barry Hufker]

Guys, you've just gone into algebra -- and I was never any good at that!  No matter if I plug "cable A" into "cable B" to learn "sound C" at some point I'm going to have to solve for "X".  Every algebra problem involves solving for "X" and that frightens me.

Barry

[Ralf Kleemann]

Hi Barry,

When you used SpectraFoo ("Complete" version) for creating the frequency response charts, did you also take a look at the Transfer Function feature (part of the instrument list)? This allows for a "power vs frequency" as well as "phase vs frequency" chart, which is most interesting for this type of experiment, especially when combined with the built-in function generator. Try using short bursts/sweeps and you will get a nice impulse response graph for the system you are examining.

Best regards,

/Ralf

___________
P.S.:

Barry Hufker wrote on Fri, 29 June 2007 17:11

Every algebra problem involves solving for "X" and that frightens me.

Why? It's so easy:

[Klaus Heyne]

Ralf Kleemann wrote on Sat, 30 June 2007 13:02

...Try using short bursts/sweeps and you will get a nice impulse response graph for the system you are examining.

..but how does this examination help me understand why one cable sounds better than the other?

[mdemeyer]

Hi Klaus,

Would you elaborate on how you terminate the GAC3 cable you favor?  Wondering how you are using the extra conductor and shields.

Thanks,

Michael

[Barry Hufker]

Hi Ralf,

No I didn't look at that, but will.

Thanks!

Barry

P.S.  Thanks also for finding X.  I was worried it would be under my bed tonight while I'm sleeping.

[Ralf Kleemann]

Dear Klaus,
Sorry if I didn't elaborate in my previous message.

With the power/frequency and phase/frequency transfer functions (no matter if you use the one provided in SpectraFoo or create one yourself), you have a very good tool to see the overall response of a system, or how it changes the original signal. I wrote "see", not "hear", because while this transfer function gives you a clue about the most vital parameters (power, frequency, phase [as in: signal over time]), it is never meant to replace an actual listening test or a direct comparison.

However, I believe that applying this methodology can yield useful results and goes beyond the simple frequency response chart. Definitely worth a try, especially since Barry is using an application that has this functionality built-in.


Having said this, I would never base a final claim about a cable's musical capabilities on such an experiment alone.

Best regards,
Ralf

[Klaus Heyne]

mdemeyer wrote on Sat, 30 June 2007 20:27

Hi Klaus, Would you elaborate on how you terminate the GAC3 cable you favor?  Wondering how you are using the extra conductor and shields.

Gladly.
I use the same termination scheme that Neumann has used in the past, and is still using.

Both connectors on a three conductor + shield mic cable, like Gotham GAC 3, are terminated like this:

Ground conductor and shield go to pin 1 of XLR and to the metal connector's solder lug provided for this purpose. On most connectors this ground/shield lug is a metal loop which has conductance to the connector's shell.

Where there is no dedicated third conductor for ground return, I still will connect the shield (which in these cases also acts as ground wire) the same way as above.

I start out the cable/connector terminations in line level applications between pres, processors, or recorders the same way, but may have to remove the shield termination on one end of the cable if I detect ground loops.

[maarvold]

Klaus Heyne wrote on Sun, 01 July 2007 16:41

Ground conductor and shield go to pin 1 of XLR and to the metal connector's solder lug provided for this purpose... but may have to remove the shield termination on one end of the cable if I detect ground loops.

This was my experience when I made my own cables with Gotham GAC-4/1.  I had some mics that exhibited RF-related problems in several studios in Los Angeles so I decided to try Gotham's cable with double-Reussen shielding.  If I connected the shield to both Pin 1 AND the connector's shell at both ends it created ground loop problems in some studios.  When I disconnected the shield from the connector shell at the male XLR end,  the grounding problems disappeared; the RF-related problems still were cured with this configuration.  On an impulse, I removed the RF traps (I would call them)--simple inductor & capacitor circuits in the microphones--and the purity of the mic's sound improved as well.  It was as if there was a tiny [audible] 'hitch' that had been in every waveform that was no longer there.  FWIW, GAC-4/1 has even 15 dB better RF rejection than GAC-3.  GAC-4/1 features both double-Reussen shielding and a star-quad configuration in one cable.  Quoting from the Gotham USA website:
"Gotham "Star-Quad" cables are the most advanced microphone cables presently available. "

[Barry Hufker]

Mike,

I agree with you.  I believe grounding connectors has been largely disapproved of for years just for the reasons you mention.

Barry

[Klaus Heyne]

maarvold wrote on Mon, 02 July 2007 11:14

...If I connected the shield to both Pin 1 AND the connector's shell at both ends it created ground loop problems in some studios.

Quote:

Mike, I agree with you. I believe grounding connectors has been largely disapproved of for years just for the reasons you mention.

Your reasoning is flawed, in my opinion, and keeps contributing to the perpetuation of an unexamined myth that seems impossible to get rid of.

How can there be a ground loop in a microphone line, when the microphone is the beginning of the recording chain? Unless the mic's body or its connector touch another electrical device directly (which it never should!), there is no opportunity for a ground loop.

This is of course in stark difference to interconnecting self-powered units (mic pre/processor, etc.), where, as mentioned before, ground loops may occur, and removal of one of the two cable shields from pin one or the connector case may be necessary.

I have equipped "properly" terminated mic cables for more than 20 years for probably more than 1000 clients. Never once did I get a call with a ground loop issue afterwards.
But I have gone to numerous studio sites after hearing "ground loop" complaints that were supposedly microphone-related. Never once was the proper termination of the connectors implicated (although RF- issues stemming from improper terminations could often be improved upon) but either improper wiring to the mic or other components downstream were the cause of the problem.

P.S.: Does anyone really think that Neumann, which equips all of its premium mic packages with a properly terminated 25 footer of GAC3 would choose a termination which, right out of the box, would spell trouble?

[Schallfeldnebel]

Klaus you are right if the microphone is connected directly into the mixing desk or pre-amp.

But in many cases the microphone is connected to a stage box going into some patchpanel, and then sent to the desk. It is that problems occur because of wrong earthing in those patch panels.

Erik Sikkema
Schallfeldwebel

[Klaus Heyne]

Schallfeldwebel wrote on Mon, 02 July 2007 14:13

...It is that problems occur because of wrong earthing in those patch panels.

Then I suggest to fix those secondary problems, instead of sacrificing the audio performance of multi-thousand dollar mics.

[maarvold]

Klaus,

This is a nice idea in theory.  But, using the Eastwood Stage (aka The Warner Brothers Scoring Stage in Burbank, CA) as the example where I encountered this specific problem, I sense reluctance to change a wiring scheme that has been used to generate hundreds of thousands of dollars successfully just because a couple of people want to bring their own cables--ESPECIALLY when lifting the connector's connection at the male end solves the problem: both ground hum AND RF.  

The problem, to me, implies that the mic panel is grounded and 'sitting' at a different ground potential than the audio ground and the difference 'resolves' itself through the audio portion of the line, but  am not a 'big enough fish' to feel comfortable asking them to change.  

[Barry Hufker]

I wrote two sources, which I've yet to hear back from.  It may be the "vacation" week.  So to provide a speedier -- and still expert -- answer I posted the question in Bruno's forum here.  His answer is as follows:

IMNSHO, balanced cable connectors should've never had three pins to begin with. This is what caused the confusion in the first place. Equipment is supposed to respond only to the difference voltage between the hot and cold pins. The ground pin should be used only roughly to equalise the ground/chassis potentials of the interconnected equipment. Had XLR connectors had only two pins, the function of the shell would have obviously been to connect the shield to the chassis.

Unfortunately, a profusion of different wiring practices has spawned quite a few boxes whose performance degrades when current flows into pin 1. In a typical bad case, pin 1 is wired to the circuit board, allowing all current flowing through the cable shield to make its way into the internal ground of the circuit. This is why some people have resorted to the practice of leaving the shield open on the receiving end or connecting it only through a capacitor. At least this cuts out hum in pin 1 sensitive gear, although RFI is still free to come in through the front door.

A new standard, AES48, finally normalises the situation (unfortunately compliance is optional) by requiring pin 1 to be directly bonded to the chassis and the XLR shell. Two pieces of AES48 compliant equipment will always interface correctly because no amount of current into pin 1 can cause currents anywhere in the audio circuit. Ground loops are a fact of life in any sufficiently complex setup and AES48 compliance insures that they are allowed to exist without impacting audio performance.

So, if all equipment were correctly designed, the answer to your question would've been a definitive "both pin 1 and shell". Pin 1 would suffice for low frequencies, the shell would help shield the inevitable pig-tail ground connection. (For best results, Pin 1 ought to be connected to chassis ground rather than signal ground.) But lacking AES48 compliance across the board, the answer is, unfortunately, "depends".

I believe this then to be the answer the people in this thread arrived at.  It would be nice to connect to the shield but for those of us who have to work with what we find, connecting to the shell isn't practical.

Barry

[Klaus Heyne]

Thanks, Barry, for your investigative work.

The poster confirms my termination scheme and gives good technical explanations why.

However, I assume from his advocating to eliminate ground in a balanced line that he refers primarily to audio equipment downstream from the mic - you couldn't run phantom powering for mics that way.
Additionally, deleting shield termination at the beginning of the audio chain would bring you the "antenna" effect, nice, hissy RF.

His final comment that one may not find the ideal terminations in the field and in some unknown audio equipment it is connected to, is largely irrelevant in the case of microphone set ups, because of the fact that nothing is fed to the mic (first link in the the chain!) and except for the couple of extreme situations that were mentioned earlier, I cannot imagine any variation from the grounding/shielding scheme I advocated.

Conclusion: always start by terminating mic cables as I described earlier in this thread.
(soon to be a stickie)