output transistor resistors

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i was wondering if i used a lower resistance resistor for on the output of the transistors then would it produce less heat?

my reasoning is i need a 1% or better tollerance resistor that is 5w or more at 0.1Ohm, if i used a 0.05 or lower resistor could it work at a lower (say 1 W) rating

the reason being that 1% 5w resistors are very hard to find for me (no i dont really need help finding them - i know i can get them just it is complicated as im limiting my self to a few suppliers as possable)

thnaks
 
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Well, all other things remaining the same (bias current, load, etc) halving the resistor value would cut the dissipation in half. Think of the .1 Ohms as two .05 in series, then take one away.

BUT, you probably need to re-adjust the bias current. According to Self, the ideal bias current depends on a specific voltage across the resistors. If you halve the resistors, you need to double bias current to get the same ideal bias. So the dissipation from the bias current actually increases (same voltage, lower resistance = more dissipation). But the load dependent dissipation decreases.

Bottom line: Halving the resistor values increases dissipation due to bias, and decreases the dissipation due to load current.
So the end result will depend on how hard you drive the amp. My gut feeling is that it won't bring you a whole lot.

Jan Didden
 
Hi, just a quick pointer here, as you say these are for the output of a bridged gain clone. I belive that somewhere in the data sheet ( I thik the one from national on building bridged/paralle amps) it says that if you use a value much less than about 0.25 ohm for the output resistor on each chip you will have problems. I believ that this will occur because the resistors are not high enough in value to properly balance the current draw from the paralled chip, and you will get on chip supplying and one sinking current even if the amp is at standstill. This will lead to the activation of the thermal/curent protection in the chips and it won't work.

So I think (I could very well be talking rubish here, it's been a while since I actual read the data sheet) that to to this you would need to use at the very least 0.22 ohm resistors, however the tolerence for these parts is not too critical as they are only there to balance the current between the two (or more chips). as far as the power rating goes (for the 0.22 ohm resistors), you would need a rating of only 1.5W (assuming you are using a bridged/paralled configuration, with a 200W max RMS into an 8 ohm load). However, you would need to increase this if you wanted to drive lower impedance loads.

Andrew.
 
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