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Author Topic: Chasing loudspeaker-level Crosstalk  (Read 6180 times)

Hayden J. Nebus

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Chasing loudspeaker-level Crosstalk
« on: February 09, 2015, 04:42:22 pm »

I seem to be observing some crosstalk between Left and Right, on the order of -30dB. Confirmed it is downstream of amplification. Low impedance loudspeakers on multichannel class D amps - L and R live in discrete amp chassis and are driven on discrete AES3 drive lines. Iso Power and rack rails are shared.

 All runs are #10AWG THHN, physically parallel and tight bundled for ~30ft before hitting discrete EMT pipes per side.     

No change when driving only one side via analog straight into amp; acoustic output is still present at the undriven side.

Link to Transfer function of this behavior, measured acoustically at the undriven side in near field, referenced to the driven side : https://drive.google.com/file/d/0B0pQLbAUdKDhVUVuUzFac1luQ0E/view?usp=sharing 

Have looked for swapped neg conductors, found none. Have checked for drive voltage on (seperately solidly grounded) loudspeaker conduit grounds, none present. Powering off amplification for the undriven side eliminates undesired output. Perhaps signal is leaking into the noise floor of the undriven amps via G/N/Chassis-to-rails??

Would certainly welcome some input where to look next!

 
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Luke Geis

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Re: Chasing loudspeaker-level Crosstalk
« Reply #1 on: February 09, 2015, 06:26:12 pm »

Are you certain the crosstalk is not present from the mixer?

Have you independently sent signal into each amp from a more direct source ( such as a small mixer or ipod located at the amp rack )?

It sounds as if there is one amp per side, or am I misunderstanding?

I find it hard to believe that there is any coupling in the speaker cables. There is either a leak of signal that is getting into amps or the mixer itself.
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Hayden J. Nebus

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Re: Chasing loudspeaker-level Crosstalkt
« Reply #2 on: February 09, 2015, 10:48:35 pm »

Are you certain the crosstalk is not present from the mixer?

Have you independently sent signal into each amp from a more direct source ( such as a small mixer or ipod located at the amp rack )?

It sounds as if there is one amp per side, or am I misunderstanding?

I find it hard to believe that there is any coupling in the speaker cables. There is either a leak of signal that is getting into amps or the mixer itself.

Yes, driving analog from a PC interface direct into one side only yields same result.

There are 2 x 4 channel amps per side.

I can unplug the load and plug a monitor wedge straight into the rear of the amps. There is nothing audible above the amp noise floor with no input present . There is low level audible signal on the R side loudspeaker lines with L side driven-  even when not loaded onto an amplifier.










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Josh Millward

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Re: Chasing loudspeaker-level Crosstalk
« Reply #3 on: February 10, 2015, 04:51:00 pm »

I seem to be observing some crosstalk between Left and Right, on the order of -30dB. Confirmed it is downstream of amplification. Low impedance loudspeakers on multichannel class D amps - L and R live in discrete amp chassis and are driven on discrete AES3 drive lines. Iso Power and rack rails are shared.

Interesting...

All runs are #10AWG THHN, physically parallel and tight bundled for ~30ft before hitting discrete EMT pipes per side.

Saying this makes me think that all the individual conductors are literally laid parallel to one another in a large bundle for 30 feet. Then they split into separate bundles to go into the pipes to the various loudspeaker locations.

This could absolutely cause what you are hearing.

What you need to do is cut that bundle apart and find the pairs of wire. Then you need to twist together the individual wires to form a two conductor cable. Twist them tight using a drill and they will not come apart. Honestly, you should probably pull all the wire out of the conduits and twist each pair over their full lengths. After all, you don't really want your LF signal leaking into the HF driver, even if it is way down in level.

Have a look at the attached photo for an example of where I had twisted pairs together using a drill. Granted, the wires you are looking at in this photo are only 16 AWG, not 10 AWG, but you can get the point. I have used a small cordless drill to do the same thing to wires up to 14 AWG, if I were planning to do this with long runs of 10 AWG, I would definitely use a corded 1/2 drill. Keep in mind, you are going to lose some length by twisting the wires, how much you will lose depends on how tightly you twist the wires. Put the far end securely in a bench vise, chuck up the other end in the drill, and go to town. Then you can pull the cables back into the pipes and bundle them back up. You will notice that now the cables will take up a lot more room than they used to since they are wrapped around one another.
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Josh Millward
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Luke Geis

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Re: Chasing loudspeaker-level Crosstalk
« Reply #4 on: February 10, 2015, 07:31:41 pm »

30' isn't very long and capacitive coupling should not be that stark, at least not very audibly. The only other way to test this out would be to run new independent speaker runs in as direct a path as possible to the amps and speakers, but you say that even bringing a monitor near the amp and connecting directly to it has the same results.

The above mentioned method will nearly eliminate any coupling due to the nature of the twist in the wire. It will cause it to naturally cancel out common modes. It will also require a longer length of cable as the twists will cause the cable to reduce length.
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Lee Buckalew

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Re: Chasing loudspeaker-level Crosstalk
« Reply #5 on: February 10, 2015, 10:30:50 pm »

30' isn't very long and capacitive coupling ...

This is an example of inductive coupling, not capacitive.

Lee
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Hayden J. Nebus

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Re: Chasing loudspeaker-level Crosstalk
« Reply #6 on: February 11, 2015, 09:48:14 am »

This is an example of inductive coupling, not capacitive.

Lee

Wouldn't there be more low pass slope to the crosstalk spectrum, or could it just be happening above my measurement nyquist?
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John Roberts {JR}

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Re: Chasing loudspeaker-level Crosstalk
« Reply #7 on: February 11, 2015, 10:27:03 am »

is there a common contaminated ground between the two sides?

Is a speaker quiet when connected at the amp?

For test purposes if you run a separate ground lead from a clean ground at the amp out to the speakers and see if the speaker with crosstalk gets quieter when connected to a clean ground.

Speakers are differential devices so they can't tell the difference between signal at their + or - input.  If the grounds are common or ground currents get co-mingled it will degrade separation due to IxR voltage drop in the ground run.

JR

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Josh Millward

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Re: Chasing loudspeaker-level Crosstalk
« Reply #8 on: February 11, 2015, 10:39:12 am »

...also, I would be interested to know exactly what power amplifiers you are using in this application. You said they are four channel amps. Let's name some brands and models.
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Josh Millward
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Jason Lavoie

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Re: Chasing loudspeaker-level Crosstalk
« Reply #9 on: February 11, 2015, 03:19:16 pm »

Powering off amplification for the undriven side eliminates undesired output.

Since we would expect that having the amp on but with no signal to be a load on the cable and theoretically reduce the crosstalk on the wire I would be suspecting either a fault in the amp or that the crosstalk is upstream of the amp.

does the problem go away if you turn down the input gain to the quiet channel?
you say it is being fed by AES but at some point in the amp that becomes low level analog and may be susceptible to a bad ground or bad design.
I'm also wondering if maybe the analog input is still hot while using AES (amp model would help)

Jason
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Re: Chasing loudspeaker-level Crosstalk
« Reply #9 on: February 11, 2015, 03:19:16 pm »


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