Microphone cables....

[KA1ZGC]

My point of the post was and still is simple.. You can have XLR to XLR cables in balanced and unbalanced configuration. My microphone requires a balanced cable. I was using an unbalanced XLR to XLR cable.  I had a huge difference in sound when switching to what the microphone required. \

Whats the wiring of that 'unbalanced XLR - XLR  is it:

Only two conductors used,

or is it one of the signal lines bonded to ground

Or something else?

I've only every seen such things as hacks for specific pieces of equipment - though its not as if I spend my life connecting bits of audio gear.

(Although I have seen a lot of XLR cables with the GROUND/SHIELD lifted at one end.)

I got the impression the old cable was a coaxial-style cable with XLR connectors on it.

Clark, it sounds as though you may have had RF issues with the previous cable. Stu's explanation makes a lot of sense, and I experienced a similar situation as you without any phantom power on the line.

I have a cheap radio shack microphone that I've had since I was a teenager. It's a hunk of plastic crap but it's a decent-sounding 600 ohm dynamic. If you axe-murder the 200 Hz energy it actually sounds quite nice.

The stock wiring was a fairly chintzy piece of audio coax with a formed 1/4" phono plug on the end. The Arrakis 12000 I'm using for a console already had some holes drilled in one channel blank for 1/4" plugs, so I installed one and wired it to one of the mic inputs.

No matter how I tied the mic in, I had RF problems. I could abate it somewhat if I used the base of my D-104 to key the transmitter with the mic in the other hand, but otherwise it was a no-go. Even then, it still had a noticable effect on my audio.

I eventually broke down and replaced the mic cable with ordinary shielded balanced audio cable, also from radio shack, which actually isn't that bad. I tied the element to the two inner conductors and let the shield float (plastic case, nothing to tie it to). At the console end, the shield is soldered to a copper ground rail at the input. The input ground is also tied to the rail, and the + and - are direct from the mic element.

RF issues eliminated, problem solved. I won't say with authority whether it was better shielding or common-mode rejection or both (does it matter, really?), but that same mic is now boom-mounted and the station is stomp-to-talk.

I suspect you were experiencing pretty much the same deficiency manifesting itself a slightly different way, and the presence of the phantom power added another variable.

Not much point in comparing the cables themselves, because you've changed the way the circuit is established, and that's ultimately what solved your problem.

[ke7trp]

YEs.. The old cable was coaxial style cable with XLRs on it. 

I never had any feedback issues with the old cable. What I mean is that it did not squeal or feedback.. However, RFI might have effected this setup in other ways. I agree.  It could have been an RFI issue.  I think its really just like you said.. its now hooked up differently and probably has better shielding. I have alot more output from the mic to the mixer now. I have backed the PAD down alot to compensate.

I guess my point of the post was so that people would inspect the cables they use. I think there are people out there running along with the cable type I had before not knowing that with a simple and cheap cable swap, they could get better audio. 

But to say its not true and the change is not happening is just horseSh*t. It clearly has more mid and treble and and more output with the balanced heavy Cable.  I got lucky and got these cables for free so that is something right there!

Clark

[DMOD]

YEs.. The old cable was coaxial style cable with XLRs on it. 
I never had any feedback issues with the old cable. What I mean is that it did not squeal or feedback.. However, RFI might have effected this setup in other ways. I agree.  It could have been an RFI issue.  I think its really just like you said.. its now hooked up differently and probably has better shielding. I have alot more output from the mic to the mixer now. I have backed the PAD down alot to compensate.

I guess my point of the post was so that people would inspect the cables they use. I think there are people out there running along with the cable type I had before not knowing that with a simple and cheap cable swap, they could get better audio. 

But to say its not true and the change is not happening is just horseSh*t. It clearly has more mid and treble and and more output with the balanced heavy Cable.  I got lucky and got these cables for free so that is something right there!

Clark,

We were not trying to say you had no changes in audio. What we were trying to get from you was specific information about your cables and connectors, so we could analyze the situation and learn from it for posterity. If you had done that in the beginning, as requested, we wouldn't have spun wheels.

You have people here with many years of experience in Audio and RF and our natural inclination is to help out and solve problems.

I suspected from the beginning you were using simple coax-type, single conductor cable which would have high capacitance and high frequency rolloff, and would also give you lower audio voltages due to resistive losses.

In addition, if the mic element was designed for balanced operation, and you were using single-ended, then all types of problems would/could ensue.

But Clark, you didn't seem to give us any specific details in the beginning that we could chew on, and your answers seemed vague, IMHO.

See below for another diswcussion on mic circuits:

http://forums.qrz.com/showthread.php?t=194888&page=2

[AB2EZ]

Perhaps this will help a bit:

I found this article on line. It is an English translation of and article written in German... but it is readable.

www.neumann.com/download.php?download=lect0008.PDF

Note that I once disassembled a Marshall MXL2003 condenser microphone, and I reverse-engineered the schematic. I can't find my file with the reverse-engineered schematic, but it was essentially the same as the "transformerless" Neumann TLM-type microphone described in this article.

This article starts out by reviewing their traditional (using transformers) approach to designing a balanced output microphone using 48 volt phantom powering... and then it goes on to describe their transformerless approach.

Note that the output of a transformerless condenser microphone like a Neuman TLM-series microphone (and also a Marshall MXL2003 or a Behringer B1, etc.) includes two, low-output-impedance transistor output amplifiers in a balanced (push pull) configuration.

This is the reason why using a coaxial cable (1 wire + ground) produces 6dB less output than a balanced (2 wires + ground) cable. Each of the push pull transistor output amplifiers is referenced to ground (they do not float with respect to ground). Therefore, if you only use one of the push pull outputs (i.e. connecting the cable to one output and ground) you will only have half the voltage driving the input to your preamplifier (which has balanced, high impedance inputs). The result is a 6dB increase in "output from the microphone" when you switch to a balanced cable that uses both push-pull outputs.

Likewise, if you connect the coaxial cable (center conductor and shield)  between the two balanced outputs, and also ground the shield, you will not only lose 6 dB of output, but you will likely cause some disturbance of the circuitry in the microphone because one of the outputs is shorted to ground. This may explain both the change in "output from the microphone", as well as the change in the frequency response, etc. 

Returning the the block diagram of the classic transformer-based design... refer to the right side of the diagram which shows the transformer in the preamplifier that feeds the phantom power.  Note that if only one of the two balanced wires were used, half of the phantom current would flow through the cable side winding of the transformer (even if the microphone on the left side of the diagram was a modern transformerless design). This would cause some saturation of the transformer, and would impact on the quality of the audio (e.g. frequency response and distortion products).

It is possible that the preamplifier that is being used with this Marshall microphone is the type that has a transformer at its input (i.e., it is not a transformerless microphone preamplifier such as those which are used in most, but not all, modern audio equipment). If that is the case, and depending on some of the details of how the unbalanced cable was wired on the preamplifier end, you could end up with both a gain reduction and the transformer saturation effect. I.e., if there were a transformer present and you connected one side of the transformer to ground, that would correspond to a shorted winding on the transformer... and depending upon the resistance of the winding, would result in a significant increase in the load that the microphone is looking into.

Stu

[KA1ZGC]

Likewise, if you connect the coaxial cable (center conductor and shield)  between the two balanced outputs, and also ground the shield, you will not only lose 6 dB of output, but you will likely cause some disturbance of the circuitry in the microphone because one of the outputs is shorted to ground. This may explain both the change in "output from the microphone", as well as the change in the frequency response, etc. 

Most reputable manufacturers of devices with "active-balanced" outputs advise against this. My Arrakis is a good example, even though the output devices can theoretically drive a dead short, they can't do it forever and will eventually fail if tied to ground.

Given the mix of transformer-coupled and active-balanced devices in the world today, I habitually leave the unused phase untied if I'm straight-wiring between balanced and unbalanced devices.

[KA1ZGC]

It clearly has more mid and treble and and more output with the balanced heavy Cable.  I got lucky and got these cables for free so that is something right there!

It clearly has more mid, treble, and output with balanced cable. Heavy may not enter into it at all.

The point I'm making here is that by changing cables, the biggest, most fundamental change you made is to the very nature of the circuit between the mic element and the first amplifying device. You changed that from an unbalanced DC-biased circuit to a balanced DC-biased circuit.

That change is so electrically significant that it's almost impossible to compare any other properites of the two cables. The two circuits they complete exhibit different characteristics.

Here's an idea: go to your friendly, local radio shack (snicker) and pick up a small spool of their audio cable. It's gray, two conductors with a shield. Take it home, cut off a chunk, slobber some XLR connectors on the end, and try it. Compare it to the old coaxial cable, and to the cable you're using now.

There'll be differences in all three, but I'll bet you dollars to doughnut holes that the old coaxial cable will be the one that really sticks out.

Nobody's saying you didn't get a good deal out of this, or that none of it's happening. A few folks are suggesting that it's happening for reasons you weren't expecting, that's all.

What matters is that it's happening, and you've improved your station as a result.

[ke7trp]

Ok. This makes sense. Thanks for posting the links and explaining this to me. I think it comes down to the fact that I used the wrong type of cable with my MXL mic.  I accidently used the wrong one and then the correct type and noticed the difference. I would believe the 6DB difference. I was going to suggest it was 10 as that is the indication on the PAD adjustment that I used but close enough. 

With the old cable. I had to crank the output of the mixer up to get the VU meter to move in the normal range. Now, with the control at mid scale, the VU meter can peak up near max on voice peaks.

I just wonder how many people out there are running the wrong cable and not realizing it. I sure learned something and for that I am greatfull!

I now have nice wide clean audio that people love to listen too. Thats what its all about!

Clark

[K1DEU]

     Also; "never boring is the fact that RF entering audio stages in small amounts, in phase and out of phase can
cause audible EQ changes.
     If mostly regenerative, a slight hallow sound well before oscillation in band around max gain area can remove our muddy gain reduction notch around 250-350 Cycles for Male speech, confusing us into thinking we should adjust our grafic equilizer instead of improving our RF shielding!
   How many of us test our transmitters on 20 meters or shorter wavelengths, for good RF shielding?

    Never skimp on RF decoupling networks with short leads to the grid, gate etc. in several (not just the first) low level audio stages.   Regards  John

[KA1ZGC]

I've been thinking about this some more over the last few days, and just have a few followup questions and opinions.

I would believe the 6DB difference. I was going to suggest it was 10 as that is the indication on the PAD adjustment that I used but close enough. 

That's quite possible, but you can allocate 6 dB of your sum loss to the fact that you were running in an unbalanced configuration.* So let's say that sum really is 10 dB, you had another 4 dB in there from some other cause.

That's where things like capacitive and resistive qualities come into play, like some folks were referring to. It could also mean the mic's output was being somewhat abusively coupled into.

Now I'm curious as to how the cable was wired, if the unused pins were tied to anything else or left afloat. If you get bored sometime, I'd like to know what pins have continuity to what sockets, and vice versa, on the old cable.

With the old cable. I had to crank the output of the mixer up to get the VU meter to move in the normal range. Now, with the control at mid scale, the VU meter can peak up near max on voice peaks.

Some of that (as well as the aforementioned 4 dB) may have been the result of phase cancellation from RF. Your previous descriptions make it sound as though you were picking up a slight amount of RF in your audio with that old cable. The fact that you were running phantom power on it just exascerbated that vulnerability.*

--Thom
*Per AB2EZ's discussions earlier this thread

[K4TAX]

One common error with cables and mikes requiring phantom power is the fact that a ground and a shield is correctly required.  The shield should be connected at one end only and the wiring, being 3 conductors plus shield, such that the 3rd conductor is indeed ground and the path for the phantorm power return.  Color coding is normally RED, BLACK & WHITE.  Red = + audio polarity, Black = - audio polarity and White = is ground.  Shield usually connects to the load end only being the transmitter input or mixer or mike preamp input.

Then there's the configurations of balanced above ground, balanced to ground and unbalanced above ground.  A whole different can of worms.

Good quality cable is worth it.  They are easy to make using good connectors such a Cannon, Amphenol or Neutrik.  I like Mogami because it is nice and flexable.  Good cables already made in 25 ft lengths should cost about $15 to $25.

73
Bob, K4TAX

[k4kyv]

Shield usually connects to the load end only being the transmitter input or mixer or mike preamp input.

That is the way I interconnect two units with balanced 500/600Ω line, such as mic preamp and equaliser or compressor/limiter.

But between microphone and pre-amp?  I have always connected the shield at both ends, to the microphone shell and to ground at the mike preamp input.  Wouldn't leaving the mic case floating (ungrounded) make it vulnerable to rf pick-up, especially if it is a hand-held mic?

There should be no ground loop problem with a desk mic or hand-held mic.  If the mic is mounted on a boom, wouldn't it be better to leave the boom ungrounded, and let the whole thing be grounded through the mic cable shield?  Or better still, experiment by trial-and-error and see what works best.

To further complicate matters, the configuration that results in minimum hum is not always the same as the configuration that results in minimum rf pick-up.  And what results in minimum rf pick-up on one band may not be the same for another band.

[AB2EZ]

Don
et al.

I find that it is easier to understand what the tradeoffs are with respect to rf shielding by keeping in mind that shields and ground wires are not perfect conductors:

1. The rf fields, in the "shack", that fall on the microphone cable, will travel on the outside of the shield (outer surface). For a decent cable (not anything special), provided the cable is short compared to the rf wavelength, the conductors inside the cable will not "see" any significant amount of rf. This is true whether the shield is "grounded" at both ends, or at one end. The shield acts as a "Faraday cage" that shields the conductors inside the cable from rf outside the cable. This is also true whether or not the cable is an unbalanced cable (center conductor and shield) or a balanced cable.

2. The ground wire inside the cable (if present) will not "see" any rf (see item 1 above), and can be used as an independent conductor to carry audio and lower frequencies.

3. If the shield is "grounded" (connected to the equipment) at both ends then the shield (acting as an antenna) will conduct rf current between the two pieces of equipment. Since the shield is not a perfect conductor (resistance due to skin effect, and inductance), there will be some voltage drop, end-to-end, across the shield when this current flows. This will cause an end-to-end potential difference between the pieces of equipment (at the rf frequency). Generally, this is undesirable because it usually leads to rf currents trying to flow through the circuitry inside the equipment ... which, in turn, is rectified to create "rf in the audio" distortion.

4. Grounding the shield at the high impedance end of the cable (i.e., the end at which the equipment has the highest audio frequency impedance) is (in my opinion) a good idea. In modern audio equipment, the source is usually low impedance, and the sink is usually high impedance (but not always). Thus I ground the shield at the sink end, but not the source end.

6. With respect to a microphone, grounding the shield at the microphone end is important, because there are other objects that are connected to the microphone, and which act as antennas (including the operator, if the microphone is held or touched)... and you want all of those objects (antennas) to be connected together at the microphone so as not to create potential differences across the components inside the microphone. For example, you want to connect the shell of the microphone to the shield.

One could, to some extent,  "deal" with the issue of minimizing hum v. minimizing rfi by using ferrite cores to block rf, while letting audio frequencies pass.

Stu

[k4kyv]

With respect to a microphone, grounding the shield at the microphone end is important, because there are other objects that are connected to the microphone, and which act as antennas (including the operator, if the microphone is held or touched)... and you want all of those objects (antennas) to be connected together at the microphone so as not to create potential differences across the components inside the microphone. For example, you want to connect the shell of the microphone to the shield.

I agree.  If it is a balanced cable, rf will enter the mic pre-amp as a common mode voltage.  The balanced input may not reject 100% of the unwanted signal, particularly since we are dealing with the unamplified output of a microphone; at such an extremely low signal level, it doesn't take much stray rf to cause mischief.  If the shield is connected to the microphone shell but left ungrounded at the pre-amp end, then the entire microphone and cable assembly becomes an even more effective antenna.

[AB2EZ]

Don

Yes...

As you point out, the things that are connected to the microphone (directly or capacitively coupled) are not grounded, and therefore provide no path for shield currents to flow, which means the conductors inside the shield have a common mode end-to-end rf potential across them. Therefore connecting both ends of the shield of the microphone cable (one end to the shell of the microphone, the other to the chassis ground of the preamplifier) is necessary to keep longitudinal r.f. current from trying to flow on the conductors inside the shield.

I guess you could say that every shield should have one most direct path to ground, and every extraneous antenna that is connected to a piece of equipment (microphone or otherwise) should have as direct a path to ground as possible (and that most direct path should  never go through the signal-carrying conductors).

As an aside, when the shielded audio cable running between two pieces of equipment gets longer than about 1/8 wavelength, it may be best to ground both ends, because each end of the cable begins to act independently of the other end with respect to the currents flowing though the shield to ground.

Stu

[ka3zlr]

Always..To Provide a Path Of Least Resistance Yes......For RF....It can be a Swindler Tracking those little things down ...

73
Jack.