Film-Tech Forum ARCHIVE: How to interface line-level audio to amps 40 ft away

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Posted by Phil Hill (Member # 371) on 07-05-2008, 03:32 PM:

I want to interface a CP-650 to high-impedance, 10k input, amplifiers located about 40ft away. Not especially interested in active or transformer 10k to 600 ohm balanced cable solutions.

Does anyone have experience with the cat5 interface devices listed in the below link?

(Note: Link is a pdf page)

http://www.calrad.com/download/Audio%20Passive%20Baluns.pdf

Posted by Louis Bornwasser (Member # 3063) on 07-05-2008, 04:23 PM:

One possibility is to use the "computer link" as espoused by Crown or maybe QSC.

I believe there is also a "digital snake" available through pro audio stores.

If level is high enough, you might be able to just "do it" using a conventional balanced line snake as well. I have done so in non-cinema applications. HF rolloff, if present, can well be dealt with during normal EQ. Louis

Posted by Mark Gulbrandsen (Member # 72) on 07-05-2008, 04:29 PM:

Phil,
I recently used the CAT 5 to VGA devices for a remotely located D-Cinema monitor but can't speak for the audio stuff, I suppose you try it and if it bombs or not. If it does send it back for a refund. The VGA stuff worked pretty good though.

I rescued a case of(100)Jensen trannys someone left by the trash a couple of years ago... Would be happy to send you 6 of them if you want to go this route. Will dig em out and see what P/N they are.

Mark

Posted by Richard Fowler (Member # 893) on 07-05-2008, 05:00 PM:

I used the Gefen brand equal for running audio and video for many rentals and visiting road company shows. They work well with the weak point being low quality Cat5 and connection point from those "phone plugs on steroids".
I prefer snakes but I have had issues with some poorly contructed or maintained breakout units. + or - to everything [Wink]

Posted by Gordon McLeod (Member # 33) on 07-05-2008, 05:27 PM:

Just because it is high impedance balancing isn't an issue most pro amps have balanced inputs and the cp650 has balanced outputs so I would use some star/quad type cable arrangement

Posted by Tim Reed (Member # 137) on 07-05-2008, 05:46 PM:

quote: Mark Gulbrandsen
Would be happy to send you 6 of them

He's probably wanting to get it done THIS year...

Posted by Phil Hill (Member # 371) on 07-05-2008, 05:52 PM:

Yes Gordie, I'm well aware of all that...

The amps have ONLY 10k inputs. Pioneer VSX-92TXH.

AND, there is NO option for another amp! HA!

quote: Tim Reed
He's probably wanting to get it done THIS year...

Oh Boy! I can't wait to see what our Grand-Poopie-BAH has to say about that... [uhoh]

Posted by Bruce Hansen (Member # 281) on 07-05-2008, 08:04 PM:

This should not be a problem. I would not bother with the cat5 stuff, BTW the cat5 interfaces are transformers. Good old 8451 should be OK. You could put a 604 ohm resistor (nearest 1% value) across the input of the amp.

Posted by Steve Guttag (Member # 268) on 07-05-2008, 10:00 PM:

Oh Phil...connecting professional equipment to consumer crap....

Foget the Cat 5 stuff...as another has pointed out...all you are doing using a transformer to use crappy twisted pair cable.

The CP650 is low impedence active balanced out and they have a properly balanced output stage (not just a stupid inverting op amp that would be shorted out if you connected unbalanced). If you have 1V on the output of the CP650 and tie "-" to ground...you still have 1V of output just now unbalanced as the entire signal will be above ground.

There will be nothing wrong with the signal coming from the CP650 and it will make it much farther than a mere 45-feet without any help. The problem lies with your consumer A/V receiver. The problems include...unbalanced inputs (not so good for noise immunity)...no good ground reference (note the lack of a third prong on the AC line). The CP650 will ground via the AC input and the signal on its output will travel just fine using the CP650 as the ground reference.

The smartest solution is to use a bridging transformers on the inputs of the consumer input (10K - 10K)...this will present a balanced input and isolate the CP650's real ground and the multitude of grounds that the A/V receiver may be receivng (via antennas, cables and what ever else will contaminate the ground system...including the display device).

However, you may get lucky in that your grounds will be close enough in potential such that you will be able to connect the CP650 straight up. I would run a ground reference line while pulling the cable (not one of the shields...let them do their jobs of noise immunity) of 18awg or bigger so you may bond the A/V receiver to the CP650 to ensure they are at the same ground point. If you get hums (video or audio), the transformers are the solution that WILL allow a very stable and good performing system.

Again, the Cat 5 solution is a bad one that will not do anything more than what good transformers will do and actually will likely do worse. But first, you can try a straight connection...and then fix it from there, if you find you need to.

Steve

Posted by Brad Miller (Member # 2) on 07-05-2008, 11:19 PM:

Phil, for that room you could probably get away with a pair of 4 channel DCA amps from QSC. (I say that assuming you will use "consumer" speakers with a passive crossover built in.)

BTW, yes the baluns will work, but the man with the cheese is absolutely correct...just run good quality shielded cable and don't jack with the balun crap. The end result will be better...and cheaper.

I wouldn't run out and buy any of the DCA amps just yet. You won't be needing them until about the time QSC changes their amp line and can then get them at a discount! [Razz]

Posted by Louis Bornwasser (Member # 3063) on 07-06-2008, 06:45 AM:

Amplifying on Steve's post: Run an extension cord to power the amps and use the same outlet as the cp650. This will eliminate multiple ground reference points in the building. Louis

Posted by Jack Ondracek (Member # 1466) on 07-06-2008, 07:31 AM:

Just tossing an aside in here...

Phil started his question out by eliminating most options, then focusing in on what he probably was really asking in the first place... how to hook his amplifiers up with Cat-5.

When Paul Allen's Cinerama was remodeled, they installed a new sound system over the original. Much of the old 'booth-to-stage' wiring was kept, and they maintained separation between the day-to-day sound system and the Cinerama electronics. The new setup links the booth to the rack rooms with Cat-5, and the channels are multiplexed, not merely transformer balanced.

Don't remember the brand of equipment they used, and it's now something like 15 year-old technology, but I've never heard anyone mention problems with it.

I suppose you'd have to study the internal latency of that kind of equipment, especially with the analog sound. The digital stuff you could adjust out with the delay settings.

The other question would be how prepared you'd be if something went wrong with the link equipment. Seems a lot of technical overhead, just to replace good-old shielded pair.

Posted by Steve Guttag (Member # 268) on 07-06-2008, 07:52 AM:

Jack, don't confuse any old Cat-5 solution. It is likely that if they were doing DSP processing and extended runs, they were running "Cobranet". That is something entirely different. It is a relatively old standard and very stable. BUT you are not running analog audio on it. You are running propritary digital audio and quite a few channels at that.

Since Cobranet is a standard, of sorts, you will find multiple vendors that support it. In the Cinema industry, QSC is using it for their "Basis" systems.

Since it is a standard, you can even mix and match manufacturers boxes at each end to suit your needs.

Again, you are really doing a A/D conversion at one end and an D/A at the other. Generally speaking, people using Cobranet are doing more complex things than just sending analog audio a mere 45-feet. One thing about Cobranet...it is pretty darn reliable and you can get quite a few channels moving about over very small cable (or fiber).

Steve

Posted by Brad Miller (Member # 2) on 07-06-2008, 04:33 PM:

I didn't suggest the Cobranet option to Phil since he is a cheapass (except to the pizza delivery boy) and I knew he wouldn't want to throw down the extra money. I have been playing more and more with QSC's BASIS system and I am liking it more and more over a conventional CP650/amplifier setup. The setup and calibration possibilities are far beyond what you will find in the processor.

Posted by Bruce Hansen (Member # 281) on 07-06-2008, 05:05 PM:

Does that piece of "consumer crap" (thank you Steve) even have a ground pin on it's power cord? If not, you may not have ground loop problems. Of course, you could always cut off the ground pin, but that is not the "safe" way to do it.

BTW, if you check the link Phil provided, these cat5 interfaces are the passive ones that are just transformers.

Posted by John Hawkinson (Member # 1135) on 07-06-2008, 06:09 PM:

Remember that UL-listed devices with 2-prong plugs are frequently better devices than those with 3 prongs. The UL standard for leakage current on 2-prong devices is significantly more strict. It's only when the leakage current exceeds some threshold that manufacturers add the third prong.

Phil's constraints seems really weird. He wants to do it on-the-cheap, but doesn't want to use transformers and is considering much more expensive CAT5 transports? Is this for the home screening room, Phil?

--jhawk

Posted by Steve Guttag (Member # 268) on 07-06-2008, 08:35 PM:

The point John, is that with consumer grade equipment (or crap) is that they ARE 2 prong devices. Everything that touches their low-side can affect the reference...including a Cinema processor, that will be grounded, 45-feet away.

UL is concerned with safety...we are talking about noise immunity over the extended run for an unbalanced input.

Steve

Posted by Phil Hill (Member # 371) on 07-06-2008, 08:57 PM:

Yes jhawk, this is for my home screening room.

Let me clarify the issue from my POV:

My 1st choice was to "do it right" and use Jensen Transformers and Belden 8451/9451 cables. But:

DIN-PC $135 x 6 = $810 ...way too expensive!

JT-11P-1 $70 x 6 = $420 ...still too expensive!

JT-10KB-D open frame 4:1 xmfr $60 x 6 = $360 Not sure about the open frame thingie...might be the best solution.

The Calrad baluns are transformers and would provide ground/hum isolation. The specs look like they have respectable performance and are about $60 pr.

But JFC! Almost $400 for the interface? At that point, I may as well do as Brad suggested and buy the QSC amps.

Side note: There is a 1.5" steel flex conduit between the locations and a #12AWG insulated common ground wire.

In some pile of stuff around this joint I call home, I have 10ea of Jensen-in-a-can input xmfrs 600-ohm to 10k...just don't know where.

Oh, and FWIW, Brad is right... I am a cheapass. And BTW: No matter how cute, I've NEVER tipped ANY pizza boy more than $60 on a $18 pizza! [Wink]

Posted by John Hawkinson (Member # 1135) on 07-06-2008, 09:24 PM:

I probably should have made a more narrowly-scoped statement.

In UL-listed devices, in a 2-prong device (Class II), leakage current is limited 0.75 mA (aka 750 MICRO-amps). For a 3-prong device (Class I), the limit is 5 mA (6x as much). Three-prong devices are frequently those that cannot meet the strict requirements for two-prong devices. So it's a misconception that "three-prong devices are better."

Is this directly applicable to the question of "Which crummy unbalanced consumer device is crummiest?" I'm not sure. Yes, everything that affects the low side of the signal can affect the noise floor, but just because there is a third prong does not mean that there is no connection between chassis ground and neutral (yes, there is no direct connection. But noise can run across caps, etc., etc.), or that the ground of an unbalanced signal wire is less noisy.

Bill Whitlock (at Jensen) gives a good explanation of this in his talk...it does not seem to be in the notes associated with that talk, alas.

(There is some peripheral discussion around page 9 of his Understanding, Finding, & Elminating AV Ground Loops, which is quite a good discusson of grounding-related issues.)

------------------

So Phil: What happens if you Just Do It? Do you actually have a problem?
--jhawk

Posted by Steve Guttag (Member # 268) on 07-06-2008, 10:38 PM:

Phil, the BEST "bang-for-the-buck" audio transformers are by RDL. Not necessarily the "best" but when one weighs price and performance...they more than do the job.

For your application, there is the TX-1A (which is quite handy for one to lower a +4dBu to -10dBV as it has a trim on it)...the TX-10B is a straight bridging 10K transformer. These are on the "cheap" side of things but are flat 20-20KHz with only a +/- .5dB. THD is less than .05%. In short, they are good.

If you want the frequency response down to .1dB 20-20KHz then they have that too with the TX-AFC1F. However, they are going to creep you back up into the Jensen price range again as they are direct competitors.

If interested...visit www.rdlnet.com

We do sell them so I can speak to their quality.

Steve

Posted by Tim Reed (Member # 137) on 07-06-2008, 10:55 PM:

I'll second the RDL suggestion. I've used their "hum-killers" to great satisfaction.

Posted by Phil Hill (Member # 371) on 07-07-2008, 05:36 PM:

Steve,
I totally forgot about RDL's stuff even though I've used their products many times in the past.

I will take your suggestion to use the RDL TX-1A. It looks like a good solution at $30 ea.

Thanks!

Posted by Brad Miller (Member # 2) on 07-07-2008, 07:06 PM:

Told you he was cheap!

Posted by Phil Hill (Member # 371) on 07-07-2008, 07:17 PM:

Hey Grand-Poopie-BAH, NOT cheap but frugal! HA! [fu]

Posted by Kenneth Wuepper (Member # 1174) on 07-07-2008, 08:59 PM:

NO! Parsimonious!

That's it.