DPA 4023 on long cable - problels

Posted: Sun Mar 25, 2007 1:57 pm

I am wondering if these mics are suceptable to long cables. I tried to use a pair for the first time in a Cathedral yesterday for a Choral recording but they were unusable because of hum and other noise.

A pair of DPA4006 and a Neuman SM69 were fine on the same cables.

I was using the cable network in the Cathedral that was installed for Radio Broadcast - length possibly 45m

The 4023 were also fine when plugged directly into the preamp.

Would be interested from comments from others that uses these mics

Thanks
Larry Elliott

Posted: Mon Mar 26, 2007 2:00 am

lell010 wrote:

I was using the cable network in the Cathedral that was installed for Radio Broadcast - length possibly 45m

That's strange... We have used an ORTF pair of 4023s on cables up to 300m long with no problems at all. The specs say they should be able to drive up to 100m.

I would suspect there is something wrong with the wiring of those cathedral cables, like, perhaps one leg tied to ground or similar. 45m is not particularly long, well within the 4023's design spec, and your tests prove that the 4023s worked without the cable in the circuit. So the cathedral cables are the only variable...

But why did the other mics work? Do they have transformer-balanced outputs? If so, they'd be resilient to a problem such as one leg shorted on the cable, but the transformerless 4023s would not.

Or perhaps there is a problem with the voltage, and perhaps the 4023s are more susceptible to it than the others? Were you able to measure the voltage at the end of the cable?

What preamp were you using?

Just suggestions...

Posted: Mon Mar 26, 2007 2:39 am

Simmosonic wrote:

But why did the other mics work? Do they have transformer-balanced outputs? If so, they'd be resilient to a problem such as one leg shorted on the cable, but the transformerless 4023s would not.

Or perhaps there is a problem with the voltage, and perhaps the 4023s are more susceptible to it than the others? Were you able to measure the voltage at the end of the cable?

What preamp were you using?

Just suggestions...

Certainly my 4006’s are the transformer version. I cant find specific details on the SM69, but given that the polar pattern selection box was in the line perhaps there are transformers in that.

I didnt get the chance to check the voltage...

I was using DAV BG1 preamps.

Thanks for your suggestions - I too suspected the in built cabling.

So off to by a multiway cable of my own...

Regards
Larry

Posted: Mon Mar 26, 2007 4:30 am

I too have used 4023s down some long cables (about 170m) with no problems apart from a little bit of HF loss due to the loadings of the starquad multicores.

When you had the problem, did you try swapping the 4023s with a couple of mics which were working ok, or try plugging them into different tie lines?

Also, did you test the installed lines before trying to use them?

Whenever I use someone else's tie lines, especially things installed in cathedrals I make a point of testing any lines that I want to use, preferably far enough ahead of the job for me to make any other arrangements necessary, and again as early as possible on show day - just in case something's happened between between my line test and the day of the job. (If the line test date is more than a few days ahead of the job, it can be worth checking the day before that nothing's changed. I did one job where I tested the lines a week before the job and arrived on the day only to find that builders had started work in the room adjacent to the termination panels and they'd sawn through a 14" diameter bundle of cables, and removed a 12' section, killing all the tie lines! Shocked

)

Many larger venues/cathedrals over here have tie lines fitted by the BBC over the last 60 years or so. These often go to useful places that the house PA/audio lines don't go, and end up somewhere sensible either a room in which to set up a carry-in rig or somewhere handy for parking a mobile. If working, they can be a huge time saver, in some venues making the difference between a 6-8 hour rig for four people, or even a rigging day (or two) before the job, and a comfortable two or three hour setup for two people on the day. However, given that some of these are rather old and very infrequently used, it's always a good idea either to test them yourself or speak to someone who's used them recently for a usability update.

Line testing, especially in large venues like cathedrals, is obviously much easier with more than one person but for testing on my own, I have a few small 8-way boxes which contain pairs of green LEDs indicating phantom presence on each pair together with little phantom powered 400Hz oscillators with mic level outputs on short XLR tails. I also have some XLR barrels containing the same. (Between the two types I can set up about 30 lines at a time.) For an initial test, I'll set up the oscillator boxes/barrels at the mic end of whatever lines I want to test, then go to the control room/patchbay/termination panel end and check each line in turn with an RTW 1255 line tester (it has a mic pre with phantom power, a switchable frequency/level oscillator - good for loop back tests - and a headphone output). If the line is good I should hear a nice clean 400Hz tone. (This can obviously be done by patching in a mixer and listening to each line in turn but the RTW is pocketable and runs on batteries!) If the line has a fault which could stop a phantom powered mic from working there's no tone. I can then plug the suspect line(s) into a phantom power source and go and check the other end to see whether any 'phantom ok' lights are out; if so, it's a safe bet that either a screen is off/the line is cut somewhere (both lights out) or it's one legged (either light out). If I really need the line (not enough working spares available) I can replace the oscillator with the remote barrel of a Canford Audio cable checker which reliably will identify almost any fault (at least, more or less all common faults). Then I have the choice of trying to locate the fault or, on very large installations, just marking it and informing the owner, and making other arrangements for my job.

A few installations I know have common 'faults' in that someone thought it'd be a good idea to leave all the screens terminated at one end only 'to avoid ground loops' Rolling Eyes

which is a bit frustrating for phantom! The cheat way I've found around this, to enable me to use the lines, is to bring along outboard phantom power supplies which I can set up at the stage end of the lines and phantom the mics locally (They're also quite useful to ensure stable, full voltage/current phantom power is available on very long cable runs.) Alternatively, using remote mic amps is an even better way to go but I don't have that many channels of those available.

If there's more than one person available, then two-way radios and one person walking around with the XLR barrel version of the tester or, if you don't have one of those, a phantom powered mic (nothing too expensive!) plugging it into each line in turn whilst you stay at the other end and work through the lines is a lot quicker. If you have a cable tester with fault display and they remember to take the remote barrel with them (!) they can do all the main tests pretty quickly.

On show day, I try to make the first job (once all the gear is in/truck is parked up and ready) plugging up all the necessary tie lines and patching. I can then run the phantom/oscillator barrel tester through everything just to reassure me that all's well (or has a good chance of being well!) before we break for the first cup of tea Wink

At least that way, I have a bit of extra time to fix/find a way around the problem and I reduce the likelihood of any nasty 'faulty line' surprises later in the day when there's a bit more pressure and less time to fault find/repair things.

Posted: Mon Mar 26, 2007 5:31 am

A lot of cable manufactures are using the crimped version of XLR connectors to save cost in manufacturing. They simply don't work after a couple of times being uncoiled and coiled. We purchased about 25 - 25 and 50 foot cables a couple of years ago from our local music retailer. We used them for a couple of gigs and they started going bad. LUCKILY the music store was able to get them replaced with soldered connectors (but we still had to spend an additional $250.00 to "upgrade") also a lot of cable manufactures are, for some strange reason, case grounding the XLRs at both ends which can also cause some problems.

Best if you have the time and necessary skills to make up your own cables.

We too try and check out all installed snakes BEFORE we use them. A couple of years ago we had a problem in a college auditorium with a snake and out of 24 cables we found 6 that were problematic on top of the 3 or 4 that had tape on them saying DON'T USE so almost 1/2 the snake was bad. YUCK!

I personally have never had a problem with the DPA 4006 microphones that we used on long runs. The SM-69 does indeed have transformers on the output and we never had a problem with that microphone with a couple of hundred feet of cable on it. I have never used the 4023s so I can't comment but I would be willing to bet that it was a cable problem and NOT a microphone problem.

One caveat I don't really like star quad cables and think that you could do much better by looking around for good cables on the net.

Posted: Mon Mar 26, 2007 1:44 pm

Thanks so much for your comments.

Larry Elliott
Auckland, New Zealand

Posted: Mon Mar 26, 2007 1:51 pm

Thomas W. Bethel wrote:

also a lot of cable manufactures are, for some strange reason, case grounding the XLRs at both ends which can also cause some problems.

One caveat I don't really like star quad cables and think that you could do much better by looking around for good cables on the net.

The ground shouldn't make a difference on the XLR cables as:
1 - it's usually the drain wire which is coupled to the shield throughout the cable anyway. (Gotham cable is an exception...)
2 - other than the 2 connectors, there's no other connection for this ground - in other words, you shouldn't get a ground loop out of it.

In general though...I agree, no need to ground the connectors so long as the cable is wired right (not that it would fix it if it weren't.)

I also agree - I generally don't like star quad cables - at least for location recording. In addition to having capacitance issues over long distances, it's generally stiffer with greater internal mass causing issues when needing to quickly roll and unroll. Good ol' fashioned 2 lead with shield is fine.

Posted: Tue Mar 27, 2007 12:40 am

Thomas W. Bethel wrote:

...also a lot of cable manufactures are, for some strange reason, case grounding the XLRs at both ends which can also cause some problems.

As I understand it, this approach has benefits for mic level signals by extending the shield *through* the connection to the chassis. It shouldn't be a problem with microphones because (with the exception of microphones that have an external mains-powered PSU), the microphone itself is not connected to any other ground.

However, it causes problems when connecting mains-powered units together. Case grounding via the shield means that the audio 0V is now connected to the chassis 0V, and that's something to avoid!

This was discussed in considerable detail many moons ago on either the Pro Audio newsgroup or SynAudCon. Historically, the original balanced audio connection was supposed to have four connections: +, - , signal OV and chassis ground. With this system the chassis of each device, the shells of the XLRs and the shields of the cables were all connected to provide a complete shield around the audio signal at all times, while remaining electrically isolated from it. But somewhere in history the industry lost its way and starting combining the chassis ground with the audio 0V, introducing ground loops and so on. It is referred to as the "pin one problem". Some manufacturers have made a return to keeping the audio 0V isolated from the chassis (e.g. Rane). There are still some balanced cables that support the four-wire system, and all XLRs still have the four connections available.

Thomas W. Bethel wrote:

I don't really like star quad cables and think that you could do much better by looking around for good cables on the net.

I agree. I went through my star quad phase, glad I did, but gladder that I abandoned it. Unless you are working in electrically hostile environments, its cons outweigh its benefits...

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