Output resistors for parellell 4780

jaste · 16th January 2007, 10:59 AM

Is there any problem using 0.1 ohm 2w dale resistors as output resistors instead of 0.1 ohm 3w when driving the llm4780 in parallell config?

Leolabs · 17th January 2007, 01:08 AM

It depends on much output power is needed.

jaste · 17th January 2007, 07:05 AM

so how restricted am I if I use the 2w instead of the 3w?

tlmadsen · 17th January 2007, 08:51 AM

You should be fine under normal use.

Remember: P(W)=I*I*R

So in your case you can pull around 4,5 amp before it should get to hot.
H
Have fun

Thomas

sixtek · 17th January 2007, 11:36 AM

No-no, Tlmadsen you aren't right.
A 2W rated resistor will get to hot even at 0,5-0,7W. At 1-1,5W it will be very, very hot.

AndrewT · 17th January 2007, 11:54 AM

Hi,
how long will it take for them to get hot?
and very very hot?

How long will the current be at or exceed the value required to get the emitter resistors to either of these temperatures?

If the time constant of the signal is shorter than the time constant of the thermal capacity of the resistor then provided your are not running with maximum continuous ouput power nor a severely clipped output then the resistor will never get up to these temperatures.

What is needed is surviveability at the peak current output into low loads and reasonable running temperatures at normal to loud music level.

Keep in mind that each emitter resistor carries halfwave rms current when sending unclipped sinewave to the output. Power dissipation will be related to the half wave Isquared R loss.

jaste · 17th January 2007, 01:30 PM

alright, sp if I drive a pair of 4 ohms speakers and dont go too loud for too long then I should be ok with my 2w resistors?

AndrewT · 17th January 2007, 02:07 PM

Hi,
2w will easily cope with normal music reproducing operation.

But will they survive power testing or accidental shorting of output?

16th January 2007, 10:59 AM	#1
jaste diyAudio Member Join Date: Dec 2005 Location: Stockholm	Output resistors for parellell 4780 Is there any problem using 0.1 ohm 2w dale resistors as output resistors instead of 0.1 ohm 3w when driving the llm4780 in parallell config?

17th January 2007, 11:54 AM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Output Emitter Resistors Hi, how long will it take for them to get hot? and very very hot? How long will the current be at or exceed the value required to get the emitter resistors to either of these temperatures? If the time constant of the signal is shorter than the time constant of the thermal capacity of the resistor then provided your are not running with maximum continuous ouput power nor a severely clipped output then the resistor will never get up to these temperatures. What is needed is surviveability at the peak current output into low loads and reasonable running temperatures at normal to loud music level. Keep in mind that each emitter resistor carries halfwave rms current when sending unclipped sinewave to the output. Power dissipation will be related to the half wave Isquared R loss. __________________ regards Andrew T.

17th January 2007, 02:07 PM	#8
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, 2w will easily cope with normal music reproducing operation. But will they survive power testing or accidental shorting of output? __________________ regards Andrew T.

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17th January 2007, 01:08 AM	#2
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	It depends on much output power is needed.

17th January 2007, 07:05 AM	#3
jaste diyAudio Member Join Date: Dec 2005 Location: Stockholm	so how restricted am I if I use the 2w instead of the 3w?

17th January 2007, 08:51 AM	#4
tlmadsen diyAudio Member Join Date: Aug 2003 Location: Roskilde - The vikingships	You should be fine under normal use. Remember: P(W)=IIR So in your case you can pull around 4,5 amp before it should get to hot. H Have fun Thomas

17th January 2007, 11:36 AM	#5
sixtek diyAudio Member Join Date: Jan 2007	No-no, Tlmadsen you aren't right. A 2W rated resistor will get to hot even at 0,5-0,7W. At 1-1,5W it will be very, very hot.

17th January 2007, 01:30 PM	#7
jaste diyAudio Member Join Date: Dec 2005 Location: Stockholm	alright, sp if I drive a pair of 4 ohms speakers and dont go too loud for too long then I should be ok with my 2w resistors?

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