SLC data Interference on speaker system.

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AlarmSystemTech
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Fri Nov 02, 2018 2:31 pm

Recently I have upgraded to a Notifier NFW-100 for my home system serving as a full life safety system protecting lives and property.The NFW-100 is tied into a FireLite ACC-25/50 voice panel. If you put your ear up next to any of the speakers in the house you'll here a rapid high pitch tone with stuttering every time a smoke detector blinks. It's not a huge deal right now but when ever I want to plug into a music source to play background music it amplifies that same sound and then its really obvious. Is this SLC interference and would I need to run shielded FPL cable for the speaker circuit?
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Fri Nov 02, 2018 3:11 pm

The stuttering is SLC noise. The high pitched tone could be the amplifier supervision tone or could be something else. You may need to run TSP for the speakers. However, there are some things that can be tried before that.

What kind of wire is used on the SLC? If it is TSP, where is the shield landed?

You can try making the negative side of the power supplies in the two panels common together. If the SLC shield is landed on chassis ground move it to power supply negative. The noise situation could get better, get worse, or stay the same.

On connecting background music, keep in mind that in fire alarm equipment the power supply negative is not common to chassis ground. That is for the required ground fault sensing. Typical audio equipment has the power supply negative common with chassis ground. You may need a 1to1 audio isolation transformer between the equipment to isolate the grounding difference. Those are available from various sources including car stereo retailers.
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Sat Nov 03, 2018 3:26 pm

The wire is unshielded FPL. I noticed there is a shield terminal for both the amplifier and the SLC. Is shielding one better than the other or both? Could I jump a negative off of the negative terminal and land it on the shield for now?
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Sat Nov 03, 2018 5:47 pm

Jumping the power supply negative to the SLC shield terminal will not do anything to help. That SLC shield terminal might already be at zero volts. It could be at chassis ground which would give a ground fault trouble if connected to power supply negative. Your problem is coming from having unshielded cable on the SLC and speaker circuit. In addition, you have two panels so there is not a unified single point zero volt common.

I dealt with Simplex audio systems for many years. Best results were achieved when the SLC and speaker circuits used twisted shielded pair cable. Then land the cable shields on zero volts (power supply negative). In these cases the SLC and the speaker circuits were in the same panel. Even though they had several power supplies, all the power supplies had their negative side common together.

Because you have two separate panels I also suggest connecting the power supply negative on each panel together. That establishes the same (unified) zero volts for both the SLC shield and the speaker circuit shield. That gives the best noise immunity.
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Sat Nov 03, 2018 6:59 pm

Which power supply are you referring to?
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Sat Nov 03, 2018 8:33 pm

The power supply negative on the Notifier NFW-100 and the power supply negative on the FireLite ACC-25/50. That way there is no potential difference between the SLC negative and the speaker circuit negative. Then they have the same reference and might reduce the noise enough. It is worth a try before rewiring everything.
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Sat Nov 03, 2018 9:38 pm

So jumper a wire from SLC negative and speaker amplifier negative? Wouldn't this create a ground fault?
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Sat Nov 03, 2018 10:20 pm

I would try the connection between the AUX power terminal on each panel as shown below. Be sure to set JP4 to nonresetable as shown. I don't think the connection can cause any damage. This connection is required on some Simplex systems like connecting an original 4003 voice panel to a 4020 or other FA panel. The original 4009 also requires this 0V connection.

It is possible a ground fault could be generated. However, this is just a test to see if the noise can be reduced. This might not make the noise situation any better. If there is a problem or it doesn't work, disconnect the wire.

If you are nervous about doing this don't do it.
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AlarmSystemTech
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Sun Nov 04, 2018 12:50 pm

Tried this and it just created a temporary ground fault. Noise did not change.
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Sun Nov 04, 2018 3:08 pm

It was worth a try. Sometimes these little tricks work and sometimes they don't. Remember, you are not dealing with a HiFi audio system. There may always be some slight noise if you get your ear right next to a speaker.
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Sun Nov 04, 2018 4:00 pm

I'm going to run a bit of shielded FPL. I believe the shield goes into a dedicated terminal on the amplifier but where does it terminate at the EOL and other speakers on the same circuit?
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Sun Nov 04, 2018 4:45 pm

If the devices have a shield terminal use that. If not connect the shield from the incoming cable to the shield in the out going cable. Use good wiring practice to be sure the shield does not touch any other wires or ground. The shield does not connect at the EOL. The shield dead ends at the last device. Shields terminate at the signal source end only. That would be the shield terminal on the amplifier.
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