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Old 11th January 2009, 11:36 AM   #1
Ziggy is offline Ziggy  Australia
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Default Ptc Resistors

Hi guys. Have a question :

I am experimenting with NXT drivers over in the planar loudspeaker forum.

Each of the six drivers I am using in array form have a 1.0A PTC resistor soldered in series to their positive terminal.

My question is : I have the drivers wired in paralell for each of the three 8 ohm drivers (2.7 ohm) and then these pair of three drivers are wired together in SERIES to get 5.4 ohms total inpedence. This utilises all six drivers.

I need to know : do I only need one PTC resistor on the input to this array instead of leaving all the six connected?

Also, can I increase the AMP RATING of the PTC in the above wiring configuration? If so, what amp rating PTC should I use?

Any help is appreciated.

Cheers,
Rob.
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Old 11th January 2009, 07:07 PM   #2
djk is offline djk
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PTCs have a very long time constant. A 1A PTC will pass 200% of rated current for 10s of seconds before opening.

I woudn't trust the NXT devices to handle more than 10W average power, 60W for all six. 60/5.4= 11.11 (I^2), or 3.33A RMS. This would suggest no larger than a 1.6A PTC device. A 1A device will offer plenty of protection, and probably not open on musical program with peaks of up to 56V (levels found in a 120W/8R amplifier), unless driven into clipping.

If you want to guild the lilly, add a 2R2/25W resistor in series with a #1156 automotive lamp, and place this combination in parallel with the PTC. This will cause a gain reduction of about 3dB when the PTC opens (rather than a complete silence), and still protect the drivers. Bose, JBL, and others use various lightbulb tricks similar to this on their smaller speakers.

In normal operation, the cold (closed) PTC device shorts out the lightbulb, effectively removing it from the signal path.
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Old 12th January 2009, 05:45 AM   #3
Ziggy is offline Ziggy  Australia
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Thanks for your kind response djk.

Oddly enough I have managed to to have a sudden sound drop (but not complete silence) for a few seconds until I reduced the volume on my amp and then raised it back up which brought the sound level back up also. Never suspected that the amp was clipping for this to happen as the sound was not really that loud or at deafening levels.

Even though there is nothing in the amp's manual, could it be some form of protection built in to the amplifier(a Pioneer VSXLX60 Audiovisual receiver : 170 watts in to 6 ohms)? Or, is the amplifier possibly faulty as I have had problems with the power supply locking up all controls, refusing to stay in standby mode and the odd shutdown in the middle of listening. However, all these problems seem to have disappeared and the problem rectified itself........weird.

This has only happened with my CD player though where I need to turn up the volume control due to it's rather low output. Can't say that I have noticed this with FM radio which I also play loud occasionally.

There is still the question though : do I need a PTC on each of the drivers or only one at the speaker array's input terminal?
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Old 12th January 2009, 09:30 AM   #4
djk is offline djk
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As discussed, only one PTC would be required for your series-parallel of six drivers.
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Old 13th January 2009, 02:31 AM   #5
Ziggy is offline Ziggy  Australia
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Thanks Again djk...........killed too many brain cells over Christmas/New year
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