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Old 2nd December 2008, 11:37 AM   #1
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Default PLH Question R6 P1 HELP!

Hi,
i've finally got all the bits together to build the PLH amplifier, but just noticed a potential problem.

looking through the circuit diagram, I noticed that P2 and R4 are high wattage types ~3W
but P1 and R6 are just 1/4w how come???
as far as i can see the same current must flow through R6 as R4?
they are the same value ~220 ohm.
actually R6 isn't so much of a problem i have 3w 220 here. but P1?? i found it really hard to get high wattage 500R pots. i only have 2 for P2 (L+R)

have i missed something?
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Old 2nd December 2008, 06:27 PM   #2
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Go back and read the article again. Specifically the paragraph below figure 8. Those two values are called out as requiring higher values.
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Old 3rd December 2008, 07:03 AM   #3
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I've just re-read that paragraph, it says "most resistors are 1/4w types but I recommend a 3 watt value for R4 and R8 and R9" and "p2 is best chosen as a higher wattage type. you can get by with 2 watts, but 5 watts is preferred."

from this I read that R6 and P1 are 1/4w.
R6 must carry the same current as R4, and they are both ~220 ohms. therefore the same power dissapation????
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Old 3rd December 2008, 09:01 AM   #4
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Fair Radio offers 4 gang pot 1k 2 watts each gang for $3.75. As the setting is not ultra critical you could parallel the sections and adjust both amps at once . Go to Fair Radio.com and type potentiometer in the search box.
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Old 3rd December 2008, 09:14 AM   #5
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shipping to the uk could be problematic.

i'm just really looking for clarification as the article seems to say that these 2 parts are 1/4w. but i don't understand how they can be?
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Old 8th December 2008, 11:28 AM   #6
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anyone???

those who have built this amp sucessfully, what ratings did you use?
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Old 8th December 2008, 12:43 PM   #7
FYC is offline FYC  Hong Kong
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I built the PLH also, and were a bit puzzled for a while on this also. I finally chose to set fixed resistors instead of using pot on this as I did not intent to play around.

My view on this is that - yes those 1/4W are OK. It is related to AC vs DC operating conditions. For the pot and the related R4 specified for high wattage, they will see rather high current in AC (coming via C2 bootstrap C). While R6 only take the DC portion, so can be at a lower wattage as the bias current of IRF610 is about 10mA only. This is how I see the requirement.
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Old 8th December 2008, 07:14 PM   #8
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Bingo.

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